Transcriber’s Note
When italics were used in the original book, the corresponding text has
been surrounded by _underscores_ Mixed fractions have been displayed
with a hyphen between whole number and fraction for clarity.
Superscripted characters are preceded by ^ and when more than one
character is superscripted, they are surrounded by {}. Subscripted
characters are preceded by an underscore and surrounded by {}.
Some corrections have been made to the printed text. These are listed in
a second transcriber’s note at the end of the text.
ON
MOLECULAR AND MICROSCOPIC
SCIENCE
VOLUME THE FIRST
_By the same Author._
ON the CONNECTION of the PHYSICAL SCIENCES. 9th Edition. Portrait. Post
8vo. 9_s._
PHYSICAL GEOGRAPHY. New Edition, thoroughly revised. Portrait. Post
8vo. [_In the press._
[Illustration:
_Fig. 89, p. 20._
EUCYRTIDIUM CRANOIDES.
[_Frontispiece to Vol. I._
]
ON
MOLECULAR
AND
MICROSCOPIC SCIENCE
BY MARY SOMERVILLE
AUTHOR OF ‘THE MECHANISM OF THE HEAVENS’ ‘PHYSICAL GEOGRAPHY’
‘CONNECTION OF THE PHYSICAL SCIENCES’ ETC.
_Deus magnus in magnis, maximus in minimis_—St. Augustine
IN TWO VOLUMES—VOL. I.
WITH ILLUSTRATIONS
LONDON
JOHN MURRAY, ALBEMARLE STREET
1869
_The right of translation is reserved_
LONDON: PRINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
AND PARLIAMENT STREET
PREFACE.
MICROSCOPIC INVESTIGATION of organic and inorganic matter is so
peculiarly characteristic of the actual state of science, that the
Author has ventured to give a sketch of some of the most prominent
discoveries in the life and structure of the lower vegetable and marine
animals in addition to a few of those regarding inert matter.
The Author feels bound to return her best thanks to kind friends—Sir
John Herschel, Mr. Huggins, Mr. Gwyn Jeffrey, Prof. Tyndall, and Mr. T.
Moore of Chelsea, who have aided in revising some of the sheets for
press, and have thus counteracted the disadvantage under which she
labours, of a residence abroad, and at a distance from libraries of
reference.
CONTENTS
OF
THE FIRST VOLUME.
PART I.
ATOMS AND MOLECULES OF MATTER.
SECT. PAGE
I. ELEMENTARY CONSTITUTION OF MATTER 1
II. ON FORCE, AND THE RELATIONS BETWEEN FORCE AND 23
MATTER
III. ATOMIC THEORY, ANALYSIS AND SYNTHESIS OF MATTER, 93
UTILITY OF WASTE SUBSTANCES—COAL-TAR COLOURS, ETC.
IV. THE SOLAR SPECTRUM, SPECTRUM ANALYSIS, SPECTRA OF 129
GASES AND VOLATILIZED MATTER, INVERSION OF
COLOURED LINES, CONSTITUTION OF SUN AND STARS
PART II.
VEGETABLE ORGANISMS.
I. MICROSCOPIC STRUCTURE OF THE VEGETABLE WORLD 167
II. ALGÆ 179
III. FUNGI 260
IV. LICHENS 298
V. CHARACEÆ 312
VI. HEPATICÆ, OR LIVERWORTS 316
VII. MUSCI, OR MOSSES 323
VIII. FILICES, OR FERNS 335
IX. EQUISETACEÆ, OR HORSETAILS 367
X. MARSILEACEÆ, OR RHIZOSPERMÆ 371
XI. LYCOPODIACEÆ, OR CLUB MOSSES 373
XII. GENERAL STRUCTURE OF FLOWERING PLANTS 378
XIII. MONOCOTYLEDONOUS, OR ENDOGENOUS PLANTS 383
XIV. DICOTYLEDONOUS, OR EXOGENOUS PLANTS 404
ILLUSTRATIONS
TO
THE FIRST VOLUME.
(_The Author is indebted to the works of Dr. Carpenter, Rev. M. J.
Berkeley, Mr.
Gosse, and Mr. Darwin, for the larger part of these Illustrations._)
FIG. PAGE
89. Eucyrtidium cranoides _frontispiece_
1. Form of stratified discharge in a vacuum tube 79
2. Form of stratified discharge in a vacuum tube, as 81
affected by an electro-magnet
3. Development of Ulva 171
4. Vertical section of the cuticle of Iris germanica 173
5. Longitudinal section of stem of Italian reed 176
6. Palmoglœa macrococca 182
7. Protococcus pluvialis 184
8. Volvox globator 189
9. Various species of Staurastrum 192
10. Economy of Closterium Lunula 193
11. Diatoma vulgare and Grammatophora serpentina 197
12. Biddulphia pulchella 198
13. Pleurosigma angulatum 199
14. Actinocyclus undulatus 200
15. Meridion circulare 201
16. Bacillaria paradoxa 203
17. Cell multiplication in Conferva glomerata 207
18. Zoospores 208
19. Threads of Rivularia nitida 215
20. Trichodesmium erythræum 216
21. Conjugation of Zygnema quininum 217
22. Ulva latissima 225
23. Polyides rotundus and Furcellaria fastigiata 229
24. Vertical sections of conceptacles of Gracilaria 230
armata, Grinnelia americana, and Corallina
officinalis
25. Callithamnion corymbosum 232
26. Rhabdonia Coulteri, Sphærococcus coronopifolius, 236
Wrangelia penicillata, and Cruoria pellita
27. Dictyurus purpurascens 242
28. Polyzonia cuneifolia 243
29. Fruit of various species of Ectocarpus 245
30. Dictyota dichotoma 247
31. Vertical section of receptacle of Fucus 254
platycarpus
32. Various species of Pucciniæi 276
33. Puccinia Graminis 280
34. Various species of Mucedines 286
35. Torula Cerevisiæ, showing successive stages of 287
cell-multiplication
36. Various species of Sphæriacei 294
37. Various species of Lichens 299
38. Sporopodium Leprieurii, Coccocarpia smaragdina, 300
and Lecanora affinis
39. Paulia perforata, Calicium tympanellum, and 301
Graphis Leprevostei
40. Nitella flexilis 302
41. Antheridia of Chara fragilis 314
42. Further development of antheridia of Chara 315
fragilis
43. Marchantia polymorpha 317
44. Anatomy of frond of Marchantia polymorpha 318
45. Archegonia of Marchantia polymorpha 319
46. Elater and spores of Marchantia 319
47. Funaria hygrometrica 324
48. Polytrichum commune, group of antheridia 325
49. Polytrichum commune, development of spermatozoids 325
50. Microscopic structure of leaves of mosses 330
51. Development of spores of Pteris serrulata 336
52. Antheridium and spermatozoids of Pteris serrulata 338
53. Archegonium of Pteris serrulata 338
54. Section of footstalk of fern frond 341
55. Pinnule of Polypodium bearing sori 342
56. Sporangia of Polypodiaceous ferns 343
57. Pinnule of Lastrea Filix-mas with sori 346
58. Sorus and indusium of Polystichum or Aspidium 347
59. Pinna of Polystichum Lonchitis 348
60. Sorus and cup-shaped indusium of Deparia prolifera 350
61. Scolopendrium vulgare 351
62. Athyrium Filix-fœmina 353
63. Asplenium Ruta-muraria 353
64. Ceterach officinarum 355
65. Blechnum Spicant 357
66. Pteris aquilina 357
67. Adiantum Capillus-Veneris 359
68. Trichomanes radicans 361
69. Hymenophyllum tunbridgense 362
70. Equisetum giganteum 368
71. Pilularia minuta 371
72. Orchis mascula, side view of flower 389
73. Orchis mascula, front view of flower 390
74. Orchis mascula, pollinium 391
75. Orchis mascula, pollen grains 391
76. Orchis mascula, pollinia 392
77. Orchis pyramidalis, front view of flower 393
78. Orchis pyramidalis, side view of flower 394
79. Orchis pyramidalis, disc with one pollinium 395
80. Orchis pyramidalis, pollinia, attached to disc 395
81. Orchis pyramidalis, pollinia, with disc contracted 395
82. Orchis pyramidalis, pollinia, withdrawn 395
83. Epipactis palustris, side views of flower 397
84. Epipactis palustris, side view and dissection of 398
flower
85. Listera ovata, side view of flower 399
_Errata._
Page 3, _line_ 5 _from bottom, insert_ the _before_ earth
59, _line_ 19, _dele_ the
59, _line_ 21, _dele_ the
59, _lines_ 20 _and_ 22, _for_ part _read_ parts
100, _In the Table of Atomic Weights, read_ Copper 32; Zinc 32·5;
Rubidium 86; Cæsium 133
101, _lines_ 10 _and_ 33, _for_ 32 _read_ 32·5
104, _line_ 12 _from bottom, for_ 29 _read_ 32
MOLECULAR AND MICROSCOPIC
SCIENCE.
PART I.
ATOMS AND MOLECULES OF MATTER.
SECTION I.
ELEMENTARY CONSTITUTION OF MATTER.
THE INVESTIGATIONS which have revealed the most refined and wonderful
relations between light, heat, electricity, and highly elastic media;
the relation of these powers to the particles of solid and liquid
matter, new methods of analysis, and the microscopic examination of that
marvellous creation, animal and vegetable, which is invisible to the
unaided eye of man, have brought a new accession to the indefinitely
small within the limits of modern science.
Wherever the astronomer has penetrated into the depths of space,
luminous points are visible; and since light merely consists in the
undulations of the ethereal medium, matter must exist in every part of
the universe of which man is cognizant, for although the luminiferous
ether is so attenuated that its very existence is almost an hypothesis,
its atoms are not more inconceivably small than those of highly elastic
ponderable matter on earth. Atoms are the ultimate constituents of
homogeneous simple substances; molecules, or groups of heterogeneous
atoms united in definite proportions, constitute such as are compound.
High pressure steam is invisible as it issues from the boiler, yet each
of its molecules contains two atoms of hydrogen and one of oxygen. The
perfume of a flower is a compound invisible substance formed of
molecules.
We know nothing of the forms either of atoms or of those groups of atoms
which we call molecules; but we cannot suppose them otherwise than as
excessively hard, since conceive them how we will, we are sure that an
atom, whatever be its form or nature, is ever the same. It never wears,
it never changes, though it may have formed part of thousands of bodies
and entered into thousands of combinations, organic and inorganic; when
set free by their dissolution, it is ready to enter into a new series;
it is indestructible even by fire, the same now as when created. Nor has
the quantity of matter in our terrestrial abode ever been increased or
diminished; liable to perpetual change of place and combination, the
amount remains the same: the bed of the seas may be changed to dry land,
and the ocean may again cover the lofty mountains, but the absolute
quantity of matter changes not.
All substances, whether solid, liquid, or aëriform, are supposed to
consist of hard separate atoms or particles, and in conformity with that
supposition to be surrounded by the ethereal medium, otherwise they
could not transmit light and heat, which are merely vibrations of that
medium. Even the hardest and most compact substances are capable of
compression, and have been compressed to an enormous degree by the
hydraulic press; but it probably transcends mechanical force to bring
their atoms into contact: in fact, no known substance is impervious to
_both_ light and heat, however thin.
By far the greater number of terrestrial substances consist of
heterogeneous atoms chemically combined into atomic systems or
molecules; but there are sixty-four which have never yielded to chemical
analysis, and are therefore believed to be respectively formed of only
one kind of atoms. Thirty-five of these are metals found either pure or
as ores, and sixteen are metals existing naturally in chemical
combination with alkalies, alkaline earths, or earthy bases, that is as
salts, from which they have been obtained by the analytical power of
electricity or other means. The thirteen remaining simple substances are
non-metallic: some are aëriform, some solid, one liquid.
The alkaline metals are sodium, potassium, lithium, cæsium, rubidium,
and thallium. They are distinguished by their energetic affinities for,
and the simplicity of their compounds with, non-metallic elements. They
are never met with native, and are amongst the most difficult metals to
reduce from their ores, and their spectra are remarkable for simplicity.
Sodium and potassium—which have been such important agents in spectrum
science—were reduced from their alkalies of soda and potash by Sir
Humphry Davy by means of the voltaic battery, a discovery which led the
way to the reduction of many of the others. Lithium is a white metal
which burns brilliantly in air and oxygen; it swims in naphtha, and is
the lightest solid body known. Cæsium is the most energetic of all
metals in its chemical affinities.
The metals of the alkaline earths are barium, strontium, calcium, and
magnesium. They possess, like the preceding, energetic affinities for
the non-metallic elements, and are reduced with difficulty from their
ores. Barium is obtained from earth baryta: it is powerfully alkaline,
and its salts are colourless and poisonous. Calcium is obtained from
limestone, chalk, marble, and gypsum, which are amongst the most
abundant constituents in the crust of the earth; it is a bright ductile
metal of a bronze colour. Magnesium, which is a brilliant silver-white
hard brittle metal, is obtained from magnesium limestone or dolomite.
Although the ores of calcium and magnesium cover vast areas of the
globe, the metals form a very small comparative proportion of them.
The metals derived from non-alkaline earths are glucinum, yttrium,
thorinum, zirconium, and aluminium, which is the only one of any
interest: it is now becoming a very useful metal. It combines readily
with oxygen to form clay. The ruby, sapphire, and oriental topaz are
merely coloured varieties of corundum, which is nothing but crystallised
clay. Rubidium, cæsium, and thallium were discovered by spectrum
analysis.
The avidity of some of these metals for oxygen is quite remarkable:
potassium and rubidium inflame when they touch ice or cold water; they
decompose the water and combine with its oxygen. Calcium becomes
luminous in warm water, and burns with intense light when heated to
redness; but a magnesium wire burns with such intense brilliancy that it
has been employed for photography, and will probably become useful for
household purposes, as two ounces and a half of magnesium wire when
burnt give a light equal to that of twenty pounds’ weight of stearine
candles.
The metals whose oxides are not reducible by heat without the aid of
some form of carbon include nearly all the useful metals. They are all
polyatomic, that is, they combine with other elements in the number of
atoms varying from two to eight, and are divided into seven groups in
regard to this property. For instance, zinc, copper, and cadmium are
diatomic. Zinc is invaluable as a source of electric light and heat in
the voltaic battery, and its vapour burns brilliantly. Copper is one of
the most useful of metals, while cadmium is of no value at all. Nickel,
cobalt, and uranium form the triatomic group; they are remarkable for
their complex spectra. Nickel is usually an ingredient in meteorites;
cobalt is employed in pigments and in sympathetic inks; and the oxide of
uranium is used to stain glass, and gives it some very peculiar
properties, as will be shown. The precious metals have a feeble affinity
for oxygen at any temperature, and their oxides are decomposed by heat
alone, and sometimes even by the undulations of light.
Metals are excellent conductors of heat, but they vary exceedingly in
that respect; both theory and experiment prove that the best conductors
are invariably the worst radiators. In fact those atoms which transfer
the greatest amount of motion to the ethereal medium, that is, which
radiate most powerfully, are the least competent to communicate motion
to each other, that is, to conduct with facility. Silver and copper are
the best conductors of heat, but the worst radiators. These two metals
are the best conductors of electricity, but it is influenced by
temperature; for MM. Matthiessen and Von Bose’s experiments have proved
that all pure metals in a solid state vary in conducting power to the
same extent between zero and 100° Cent., and that the alkaline metals
conduct electricity better when heated than when cold.
All metals are capable of being vaporized, but at very different degrees
of temperature. Platinum requires the heat of the oxy-hydrogen blowpipe,
which by estimation amounts to 8801° Cent. This property makes it
valuable for terminal points to the conducting wires of the voltaic
battery and magneto-electric induction machine where great heat can be
employed without fusing the platinum terminals. Copper is always
employed for the conducting wire on account of its superior conductive
power. The coil of wire in the magneto-electric machine, which is often
miles long, is insulated by a coating generally of green silk thread.
But in experiments of extreme delicacy where magnetism might vitiate the
results, perfectly pure copper wire which is diamagnetic is used for the
conducting wires in the thermo-electric pile of the goniometer, and the
wires are coated with white silk thread, since it was discovered that
the green dye contains some magnetic metal.
The mass of the metals however constitutes comparatively but a small
part of the terrestrial globe, which is formed of chemical combinations
of only thirteen simple elementary substances,—a wonderful manifestation
of creative power that could form a world of such variety and beauty by
means of atoms so little diversified; still more wonderful is it that
four simple elements alone constitute the basis of nearly the whole
organic fabric. The air we breathe, water, the bodies of men and living
creatures, and the vegetation that adorns the earth, are chiefly
combinations of three invisible gases, oxygen, hydrogen and nitrogen,
with carbon, the purest amorphous form of coal.
Oxygen gas forms three-fourths of the superficial crust of the
terrestrial globe, its productions and its inhabitants. At least a third
part of the solid crust of the earth is oxygen in combination; it
constitutes eight parts out of nine in water, and water covers
three-fourths of the surface of the globe; it forms more than twenty
parts out of a hundred of atmospheric air, and in the organic kingdom it
is an essential constituent. Except in the atmosphere, oxygen is never
uncombined, but may be obtained by distilling chlorate of potash, by the
decomposition of water by voltaic electricity, and by other means. When
pure it is a colourless, tasteless, inodorous, invisible gas; it is
incombustible at ordinary temperatures, yet absolutely essential to
combustion; no animal can live long in it, and none can exist without
it. In the atmosphere oxygen is highly magnetic; its magnetism increases
with cold and decreases with heat; hence its intensity varies with night
and day, winter and summer, but its magnetic property vanishes when it
enters into composition.
Oxygen is perfectly quiescent and passive as a gas in the atmosphere,
and as a constituent of water and solid bodies, yet that inactivity
conceals the most intense energy, which only requires to be called into
action. Thus combustion of extreme intensity takes place when ignited
sulphur is put into a vessel containing oxygen gas; the metal potassium
is instantly inflamed by it on touching water; some of its combinations
with chlorine are highly explosive, and phosphorus burns in it with
dazzling splendour. Thus a stupendous amount of energy is latent in
oxygen under the most tranquil appearance.
M. Schönbein of Basle discovered that oxygen exists in another state,
which has neither the extreme quiescence on the one hand, nor the
intense violence on the other, of its ordinary form; and to express that
intermediate condition, in which its activity is less in amount and
different in quality, it has been called by another name, viz. ozone,
from the following peculiarity.
It had long been observed that there is a peculiar smell when an
electric machine is in activity, and when objects are struck by
lightning; that smell Professor Schönbein ascertained to arise from the
change of oxygen into ozone, and actually produced ozone by passing
electric sparks through that gas. Ozone differs from oxygen in having a
strong smell and powerful bleaching property; it purifies tainted air,
changes vegetable colours, and stains starch prepared by iodide of
potassium blue, which thus becomes a test of its presence; yet it
certainly is oxygen in an allotropic or changed state, for it readily
oxidizes or rusts silver and other metals, and when ozonized gas is sent
through a red-hot tube, it comes out pure oxygen. According to the
experiments of Messrs. Tait and Andrews, oxygen gas loses six eighths of
its volume, and becomes four times more dense by the change; it
contracts more readily with obscure electricity than with the spark. The
experiments of Professor Tyndall on the absorption of radiant heat by
gases give reason to believe that ozone is produced by the packing of
the atoms of elementary oxygen into oscillating groups, and that heating
dissolves the bond of union and restores the ozone to the form of
oxygen. Ozone chiefly exists in air that has passed over a great expanse
of sea, and the quantity is increased during the aurora, which alone
might lead to a surmise of that phenomenon being electric.
The change of oxygen into ozone is not the only instance of
Allotropism,—that is to say, the existence of the same substance in two
states differing from each other in every respect,—for ozone itself is
allotropic. Professor Schönbein has discovered that there are two kinds
of ozone standing to one another in the relation of positively and
negatively active oxygen; namely ozone and antozone, which neutralize
each other into common oxygen when brought into contact. In this respect
they are analogous to electricity, and, like electricity too, one kind
cannot be produced without a simultaneous development of the other.
When a metal, such as silver for example, is oxidized or rusts, it gives
polarity to the atoms of oxygen in the atmosphere and divides them into
the opposite states of ozone and antozone; the ozone combines with the
silver and rusts or oxidizes it, at the same time that the antozone is
dissolved in the moisture or aqueous vapour in the air and forms
peroxide of hydrogen. The oxidized or rusted silver, as well as every
other oxidized substance, is an ozonide, while the peroxide of hydrogen
is an antozonide.
Since both kinds of ozone are produced during the decomposition of water
by electricity, and as sea air is always found to contain more or less
free ozone, the ocean is probably an antozonide, for all the antozone
formed by electricity during thunderstorms must be either dissolved in
the sea-water, or carried into it in the form of peroxide of hydrogen by
the rain. Ozone must be exceedingly abundant in the zone of calms and
light breezes near the equator known as the variables, which is subject
to heavy rains and violent thunderstorms, and also in the regions of the
monsoons. On land one of the benefits arising from these formidable
phenomena is the production of ozone, which oxidizes decomposing organic
matter and hastens its decay, while the antozone, which is dissolved in
the atmospheric vapour, forms the peroxide of hydrogen and frees the air
from the antagonist principle.
The peroxide of hydrogen thus produced is a transparent colourless
inodorous liquid with a metallic taste, and contains one equivalent of
hydrogen and two of oxygen. It retains its liquid state under a great
degree of cold, and mixes with water in any proportion. It has a strong
bleaching property, instantly destroying vegetable colour. If exposed
suddenly to a temperature of boiling water it is decomposed with violent
explosion, and readily gives off oxygen at 59° Fahr. The mere touch of
an oxidized metal, as the oxide of silver, completely and
instantaneously decomposes it, and oxygen gas is evolved by the union of
the ozone and antozone so rapidly as to produce a kind of explosion
attended by an intense evolution of heat.
During the combustion of phosphorus in the atmosphere both kinds of
ozone appear, and Professor Schönbein considers the slow combustion of
that substance, which unites with the ozone and sets the antozone free,
as the type of all the slow oxidations which organic and inorganic
bodies undergo in moist atmospheric air; that true oxidation is always
preceded by the appearance of the peroxide of hydrogen, and that this
compound acts an important part in slow oxidations, and is deeply
concerned in animal respiration, and in many other chemical actions
going on in nature.
In confirmation of these views, it is certain that ozone is a powerful
minister in the work of decay. If wood be made explosive like gun-cotton
by a similar process, it becomes pulverulent after a time, and burns
without exploding, though it still retains its shape. In the natural
state of the wood the oxygen is passive and quiescent, for oxygen is a
constituent of wood; in its second state it is explosive, and after a
time that is succeeded by the semi-active state of ozone, which by a
slow imperceptible combustion causes the wood to decay. Mr. Faraday
observes that the force which would have been explosive had it been
concentrated into one effort, expends itself in a long continued
progressive change.
‘The majestic phenomena of combustion bespeak our admiration and rivet
our attention because of their imposing grandeur; yet these are but
spasmodic efforts in the grand economy of the material world,
occurrences of now and then. The slower but continuous progress of the
elements to their appointed resting-place, the silent, tranquil, ever
progressing metamorphic changes involved in the phenomena of
decomposition and decay, these we count for nothing and pass unheeded
by. Yet with all their majesty, with all their brilliancy, all their
development of tremendous energy, what are the phenomena of combustion
in the grand scheme of the universe compared with these? When the loud
crash of the thunder or the lightning’s flash awakens us from our
thoughtless abstractions or our reveries, our feelings become impressed
with the grandeur of Omnipotence and the might of the elements he
wields, yet the whole fury of the thunderstorm—what is that in
comparison with the electric energies which silently and continually
exert themselves in every chemical change? Why, the electric force in a
single drop of water, and disturbed when that water is decomposed, is of
itself greater than in the electricity of a whole thunderstorm. Those of
us who limit our appreciation of the powers of oxygen to the energies
displayed by this element in its feebly active state, form but a very
inadequate idea of the aggregate results accomplished by it in the
economy of the world.’ Oxygen is the only known gas that is allotropic,
and is the only known substance that is doubly allotropic, that is
existing in three different states similar to oxygen, ozone, and
antozone.
Hydrogen when pure is an invisible gas without smell or taste; it is a
constituent of various acids and alkalies, but is itself neither acid
nor alkaline. It is highly inflammable, burning with a pale light, and,
as already mentioned, a combined jet of oxygen and hydrogen produces
heat of 8801°, which is so intense that nothing can withstand it. It is
the lightest substance known. A balloon having the form of a globe ten
feet in diameter, would hold 32-1/2 pounds weight of common air, while
two pounds weight of hydrogen gas would fill it. Associated with this
small quantity of ponderable matter, hydrogen has an enormous power of
combination, but its activity is only called forth by some exterior and
exciting cause. A mixture of two measures of hydrogen and one of oxygen
gas would remain inert for ever, but the instant an electric spark is
sent through it, a bright flash and an explosion takes place, and the
result is water: thus a tremendous force lies quiescent in that bland
element.
Hydrogen gas is introduced into the atmosphere by imperfect combustion,
but it is instantly diffused and becomes harmless, for aëriform fluids
are capable of rapid and perfect diffusion through one another, each
having a capacity peculiar to itself, which under the same circumstances
is greater as its density is less; therefore hydrogen the lightest of
gases not only rises in the air on account of its levity, but is more
quickly and completely diffused than oxygen which is the support of
life. Though hydrogen is inferior in density to every other gas, it
surpasses them all in conducting electricity, just as silver and copper
conduct electricity better than platinum, though far less dense. The
great refrigerating power of hydrogen is owing to its extreme mobility
and consequent rapid convection of heat, in which it surpasses all other
gases. It is as permeable to radiant heat as atmospheric air, has a very
high refractive power, a specific heat of 3·2936, and may be substituted
in many chemical formulæ for a metal, without altering their character:
hence it is sometimes called a metalloid.
The quantity of nitrogen gas or azote that exists in nature is enormous.
It constitutes four-fifths of the atmosphere, whence it may be had in a
pure state, as well as by chemical means. Like oxygen, this gas is
permanently elastic, without smell, taste, or colour; it is neither acid
nor alkaline, it does not change vegetable colours, it neither burns nor
supports combustion, and is incapable when breathed of supporting animal
life. It abounds in organic bodies, in all parts of the animal texture,
in the blood, muscles, nerves, even in the brain; and is either a highly
nutritious or poisonous principle in the vegetable kingdom.
Nitrogen gas is altogether passive; it has no affinity for the metals,
and cannot be liberated from any of its compounds even by electricity.
Excepting boron and titanium, it will not combine directly or
spontaneously with any simple element, even under the highest
temperature, but its indirect combinations are numerous and violent:
those with hydrogen are either noxious or poisonous, those with oxygen
are all deadly poisonous. Had nitrogen combined spontaneously with
either of these gases, especially with oxygen, life would have been
impossible as the organized creation is constituted; its inertness
renders its mixture with oxygen in atmospheric air innocuous. However,
combinations of nitrogen and hydrogen, forming nitrate of ammonia, have
been discovered in the atmosphere by Professor Schönbein, the union of
evaporation, heat and air being the cause; and as evaporation is
continually going on, he concludes that nitrate of ammonia, nitrates and
other salts are generated in the moist air, and are speedily washed down
in our rainy climates into the springs and rivers. He considers the
formation of nitrates out of water as highly important for vegetation,
because each plant becomes a generator of a portion at least of its
azotized food, while the rain furnishes the ground on which it stands
with a supply of the same.
In the atmosphere, nitrogen has all the mechanical properties of common
air, but with a greater refractive power, and its specific gravity is
nearly the same with that of oxygen. Since the atmospheric gases are the
most permeable to radiant heat, the earth is in the most favourable
circumstances for being warmed by the solar rays, and thus the
properties of the elementary gases are admirably adapted for our
comfort, nourishment, safety, and pleasure.
Carbon, which combined with the three elementary gases forms the basis
of the organic creation, is widely distributed throughout the globe, in
enormous coal formations, the vegetation of former ages. Diamond is its
purest crystalline form; and charcoal, which is wood whence the volatile
matters have been driven off by heat, is its purest amorphous state. To
this simple substance and to hydrogen, we are indebted for terrestrial
light and heat, whether our fuel be coal or wood, our light a candle or
a lamp. The products of combustion are carbonic acid gas, whether pure
or mixed with smoke, for ashes are the incombustible earthy matter mixed
with coal or wood, and smoke is unconsumed carbon arising from the bad
construction of our chimneys; so that the waste is enormous in a great
city like London where coal is the only fuel. Light is given out by
incandescent solid particles, which become luminous sooner than gas, for
all gases have a feeble illuminating power, and heat results from the
chemical combination of the carbon with oxygen, a process in which the
chemical force merges into its correlative heat. Mr. Faraday observes,
that had the result of the combination of carbon and oxygen been a gas
only, we should have had very little light, and had it been a permanent
solid, the world would have been buried in its own ashes.
Diamond and pure carbon leave no residuum when consumed; they combine
with the oxygen of our atmosphere into carbonic acid gas, which is
invisible, poisonous, and so heavy, that it may be poured from one
vessel to another like water, thereby showing how much carbon it
contains in an invisible state. The quantity of carbonic acid gas thrown
into the atmosphere in this invisible yet ponderous state is immense,
since six tons weight of atmospheric air rushes hourly through an
average size blast furnace, carrying with it more than half a ton of
carbon in the form of that gas, whose constitution and properties are
always the same, whether it arises from combustion, fermentation, or
respiration, which latter may be regarded as a slow combustion,
consuming us to the bones if not supplied with carbon by means of food.
It has been computed that two thousand million pounds weight of oxygen
gas is daily converted into carbonic acid gas by these operations, and
given into the atmosphere, which would soon be contaminated by its
poison and suffocating quality, were it not for vegetables which
decompose it, assimilate the carbon and set the oxygen free to mingle
with the air and make it again fit for respiration. Carbon has a greater
power of combination than any other simple substance except hydrogen.
Mr. Faraday compressed carbonic acid gas into a liquid by the pressure
of its own elasticity when disengaged from combination in close vessels,
a force equal to the weight of thirty-five times that of our atmosphere;
and the liquid was reduced to a solid by M. Thilorier by rapid
evaporation, during which the heat was given out so quickly by one part
of the liquid, that the remainder was condensed into a substance like
snow, which could be touched with impunity, but when mixed with
sulphuric ether its temperature was reduced to 166° below zero of
Fahrenheit’s thermometer.
Carbon appears naturally under a great variety of forms, and exhibits
one of the most striking instances of allotropism, the same substance
showing the greatest contrast in appearance and physical properties. The
diamond, the most resplendent, transparent, and hardest of gems, is
identical with carbon, which is black, dull, opaque, and brittle. Both
are combustible; carbon is easily ignited, but it requires a heat of
1860° to consume the diamond.
However numerous the crystalline forms assumed by substances either
naturally or artificially may be, they are all capable of being grouped
into geometrical systems; each system possessing its own allied and
derivative forms capable of mutual variations among themselves, but the
forms of one system never assuming those of the other. With that law,
however, carbon and a few other substances are completely at variance.
The diamond crystallizes in octohedrons, while graphite, which is also
carbon, crystallizes in six-sided plates,—two forms that belong to
different systems quite irreconcilable with one another: and thus carbon
possesses the property of being dimorphous.
Sulphur is a simple inflammable mineral abounding in volcanic countries,
either in a crystalline or amorphous state, and forming a constituent in
organic substances, animal and vegetable. It is readily dissolved by
bisulphide of carbon, by benzine, and by a moderate heat; and copper
filings exposed to its vapour spontaneously take fire, the chemical
force of combination merging into light and heat. Sulphuretted hydrogen
gas, a combination of sulphur and hydrogen, forms naturally during the
putrefaction of organic matter, and Mr. Faraday observes with regard to
the affinities of sulphur, ‘so numerous are its relations, so extensive
its range of combinations, that we must consider it to be the very
foundation on which chemical manufacture is built up.’
Though a simple substance, sulphur exhibits the two remarkable phenomena
of dimorphism and the allotropic property. When reduced by heat to
vapour and cooled slowly, it crystallizes in rhombic octohedrons; when
merely melted and allowed to cool slowly, it takes the form of oblique
rhombic prisms. Here the same atoms when in vapour and in a liquid state
are acted upon by different forces; but however that may be, sulphur is
another singular exception to the law of the immutability of the
crystalline systems.
Sulphur becomes allotropic by the continued application of heat; that is
to say, it entirely changes its appearance and character, though it
remains chemically the same. Naturally it is yellow and brittle, but
when fused, it is a colourless pellucid fluid which by continued heat is
changed into a black tenacious substance that becomes like India rubber
or gutta percha when thrown into water. In this allotropic state it is
endowed with properties more powerful, energetic, and exalted; its
tendencies to act chemically being increased like those of ozone. That
this black tenacious substance is chemically the same with common
sulphur there can be no doubt, for when it is exposed to greater heat,
it again becomes a colourless pellucid fluid, which thrown into water
resumes the form of brittle yellow sulphur.
These new arrangements among atoms of the same kind show that the
immutability of matter is not without exceptions.
The animal kingdom is the great reservoir of phosphorus, a simple
substance that is never found uncombined. It is sparingly met with in
the vegetable kingdom, and still less in the mineral, but may be
procured abundantly from calcined bones. When pure it is colourless,
transparent, solid, extremely poisonous, and so inflammable that it must
be kept in water. In air it is in continual combustion with oxygen,
during which ozone is produced. When burnt in a current of air
phosphorus leaves a residuum consisting of two substances, of which one
is an acid, the other is red allotropic phosphorus, which has been
extensively used in the manufacture of lucifer matches, because its
fumes are not deleterious, and because it inflames less easily than
common phosphorus, to which it is reduced by heat or friction, which
generates heat.
Silicon is a simple substance, never found alone, but when forty-eight
parts of it are combined with fifty-two parts of oxygen gas it forms
rock crystal, the purest form of silica or quartz. Silica is so abundant
that it may be said to constitute the basis of the mineral world. The
sand on the sea-shore, which is the debris of quartz rocks, shows how
universally it prevails. It is even abundant in the vegetable kingdom,
giving strength to the stalks and leaves of the grasses, and may be felt
in the harshness of the beards of wheat and barley. Silicon exists in
three different states—the amorphous, which has no form; the graphic,
which takes the form of small hexagonal plates; and that of octohedral
silicon: hence this substance is dimorphous.
A singular analogy obtains between silicon and carbon: the amorphous
form of silicon corresponds to charcoal, the graphic form of silicon
corresponds to the graphic form of carbon, and the octohedral form of
silicon to the diamond; yet the chemical relations between the two
substances are very small.
Silica has hitherto been considered to be insoluble in pure water; at
least M. Bischoff states that only one part of silica dissolves in
769,230 parts of water; but by a method hereafter to be explained,
Professor Graham has actually obtained a limpid solution of silica in
pure water.
Boron is a constituent of boracic acid, a natural production in Thibet
and Monte Corbalo in Tuscany. It is a greenish-brown solid, insoluble in
water, but when heated to about 600° it burns in open air with a vivid
flame.
Fluorine is a constituent of a very beautiful mineral, well known as
fluor spar, which is found in cubic crystals of a green, yellow, or
purple colour. Hydrofluoric acid obtained chemically from the mineral is
highly volatile and extremely corrosive.
Three of the non-metallic simple substances, chlorine, bromine, and
iodine, are connected by the most remarkable analogies. They are marine
productions, for chlorine is obtained from common sea-salt and in
greater purity from rock-salt, both of which are compounds of chlorine
and the metal sodium. When sea-water is evaporated, salt and a substance
called bittern remain, which contains a salt whence bromine is
separated.
Again, when kelp, the ashes of burnt seaweeds, is purified from the
carbonate of soda and the chloride of potassium, a salt is left which is
the iodide of potassium, whence iodine is obtained. Iodine is also found
in sponges, oysters, and other low sea animals, as well as in certain
mineral springs, and sometimes in combination with silver. These three
elemental bodies have little affinity for one another, but they combine
powerfully with other substances.
Chlorine is a yellowish-green gas, twice as heavy as atmospheric air,
with a noxious suffocating smell and astringent taste. It has a powerful
bleaching property, and when combined with water, which absorbs twice
its volume of the gas, it is used for bleaching linen, in
calico-printing, and other arts. The clear solution of chloride of lime
is still more in use for the same purpose, as well as for an antidote
against contagion and unwholesome smells. Carbon does not burn in
chlorine gas, yet it is capable of supporting combustion, for oil of
turpentine, phosphorus, thin leaves of tin and copper, and powdered
antimony, take fire spontaneously in it. This gas shows its power by the
development of intense heat, but not by brilliant light, because the
results of its combustion are mostly vapours, or such gases as have a
feeble illuminating power; so chlorine differs materially from oxygen in
the phenomena of combustion. Mr. Faraday observes, however, that the
bleaching powder is analogous to ozone in being an intermediate state,
for chlorine is pernicious and violently destructive as a gas, perfectly
innocuous and quiescent in common salt and in its other natural
combinations, while in the bleaching substances its energy is subdued by
art, so as to make it an important agent in various manufactures.
Providentially, chlorine is never found free; but in a combined state it
exists in enormous quantities in the salt of the ocean, in salt lakes,
brine springs, and in extensive deposits of rock-salt, as well as in
organic liquids. It has a strong affinity for hydrogen, and forms
muriatic acid. A mixture of these two gases remains inactive in the
dark, but explodes in sunshine.
By chemical means chlorine is made to combine with oxygen so as to
produce four substances, two of which are gases of such unstable
equilibrium and weak affinity that the slightest cause makes them
detonate violently; the other two are more stable, though they contain a
greater quantity of oxygen. The only combination of chlorine with
nitrogen is the most powerful and dangerous explosive compound known.
Chlorine combines naturally with sulphur, and with the metals so as to
form ores.
Common salt affords a remarkable instance of change of volume by
chemical combination. Twenty-four parts in bulk of salt contain 20·7
parts of sodium and 23·3 parts of liquid chlorine; hence by chemical
combination a bulk of 44 is compressed into a bulk of 24, yet that great
compression is consistent with perfect transparency, crystallized salt
being perfectly transparent to light, and more so as regards radiant
heat than any other substance. Thus chemical affinity does what no
mechanical power could accomplish.
At an ordinary temperature and barometric pressure, bromine is an orange
red, extremely volatile fluid, which congeals and becomes brittle at a
temperature a little below the zero of Fahrenheit’s thermometer, and if
combined with water at that degree of cold it crystallizes in octohedral
crystals which are permanent even at 50° Fahr. Bromine is very
poisonous, corrodes the skin, has a disagreeable taste, and a smell
similar to that of chlorine, but more pungent and hurtful. It possesses
a powerful bleaching property, does not conduct electricity, and like
chlorine a taper will not burn in its gas, though it spontaneously sets
fire to phosphorus, and some of the metals. Reasoning from analogy
Professor Schönbein believes that chlorine and bromine are not simple
substances; he considers them to be ozonides analogous to the peroxides
of manganese, lead, &c. He believes chlorine to be the peroxide of
murium, and bromine to be the peroxide of bromium. Professor Tyndall’s
experiments on the absorption and radiation of gases show that the
action of these two substances is very different from that of the simple
gases.
Iodine is a dark purple solid, crystallized in scales or elongated
octohedral plates. It slowly evaporates at ordinary temperatures, and at
that of 350° Fahr. it is volatilized into a beautiful violet coloured
gas which changes starch into a bright blue, and for that reason a
little starch will detect the millionth of a grain of iodine in
composition. Iodine is slightly soluble in water, has a hot acrid taste,
and although used in medicine it is poisonous when taken in large doses.
Its bleaching properties are inferior to those of its congeners, but its
chemical combinations are the same. With hydrogen it forms a highly
explosive compound, which detonates with the slightest pressure.
These three simple substances are analogous in almost every respect.
They all possess a bleaching property, many of their compounds are
exceedingly explosive, combustible substances do not burn in their
gases, while their gases set fire spontaneously to substances generally
reckoned incombustible. Hence, though not combustible, they support
combustion, but in a very different manner from oxygen. Chlorine and the
gases of bromine and iodine diluted with common air, do not transmit
blue and violet light; that is to say, the spectrum of a sunbeam
transmitted through them is deprived of its most refrangible coloured
rays, and that which remains is crossed by more than a hundred
equidistant dark lines; their spectral properties however will be given
hereafter. They resemble oxygen in one respect—that when a current of
electricity is passed continuously through a glass tube filled with any
of these three gases, much attenuated, they slowly combine with the
platinum wire of the negative pole of the battery inserted in the tube.
The electricity by degrees passes in diminished quantity, and at last
ceases altogether, showing that matter, however attenuated, is requisite
to conduct it.
According to the experiments of M. Dumas, the volatility of a compound
is in the inverse ratio of the condensation of the substances composing
it, and simple bodies come under the same law. For example, chlorine is
more volatile than bromine, and bromine is more volatile than iodine;
hence according to that law, chlorine is the least dense of the three,
bromine is intermediate, and iodine is the most dense, which is actually
the case: for chlorine is a gas, bromine a liquid, and iodine a solid at
ordinary temperatures, which proves that there is a sequence in the
intensity of the cohesive forces in this triad.
SECTION II.
ON FORCE, AND THE RELATIONS BETWEEN FORCE AND MATTER.
FORCE is only known to us as a manifestation of divine power which can
neither be created nor destroyed. The store of force or energy in nature
is ever changing its form of action, its amount never. It may be
dispersed in various directions, and subdivided so as to become
evanescent to our perceptions; it may be balanced so as to be in
abeyance, or it may become potential as in static electricity; but the
instant the impediment is removed the power is manifested by motion.
Whatever form force may assume it has invariably a compensation or
equivalent, whether in the heavens or on the earth. The total sum of the
living forces, vis viva, or actual energy of the planets is the same
every time they return to the same relative positions with regard to one
another, to their orbits and to space, whatever may have been their
velocities or mutual disturbances. In the ocean, the energy by which
25,000 cubic miles of water flow over a quarter of the globe in six
hours, is exactly equal to the force or energy that makes it ebb during
the succeeding six hours. A body acquires heat in the exact proportion
that the adjacent substances become cold; and when heat is absorbed by a
body, it becomes an expansive energy at the expense of those around it,
which contract. Chemical action many miles distant from the
electro-magnet, as in telegraphs, is perfectly equivalent to the
dominant chemical action in the battery. The two electricities, positive
and negative, are developed in equal proportions, which may be combined
so as to produce many changes in their respective relations, yet the sum
of the energy of the one kind can never be made in the smallest degree
either to exceed or to come short of the sum of the other.
The mechanical energy of machinery or working power is exhausted by the
very act of working, and cannot be restored except by the action of
other forces. In clockwork, the weight must sink to move the wheel, and
when the weight is down, the store of energy is gone, and can only be
restored by raising the weight through the expenditure of energy in the
human arm, and the expenditure of human energy must be restored by food
and rest. The heat given off from the bodies of men and animals is
restored by the combustion of the oxygen inhaled during respiration and
the carbon of the food, and the light and heat given out by the
combustion of fuel, whether in the form of coal or wood, is compensated
by the light and heat of the sun stored up in living vegetables. It is
this equivalent for force or energy which prevails in every department
of nature that constitutes the universal and invariable law of the
Conservation of Energy, ‘a principle in physics as large and sure as
that of the indestructibility of matter or the invariability of gravity.
No hypothesis should be admitted nor any assertion of a fact credited,
that denies this principle. No view should be inconsistent or
incompatible with it. Many of our hypotheses in the present state of
science may not comprehend it, and may be unable to suggest its
consequences, but none should oppose or contradict it.’[1] Thus, ‘there
is a definite store of energy in the universe, and every natural change
or technical work is produced by a part only of this store, the store
itself being eternal and unchangeable.’[2]
Cohesion is a force which acting at inappreciably small distances unites
atoms and molecules of the same kind into solids, liquids, and aëriform
fluids, exactly according to the law of the conservation of energy; for
it requires the very same amount of force to dissolve their union as to
form it. Cohesion varies with temperature both in simple and compound
bodies, for metals can be fused and vaporized by artificial heat, and
ice becomes water and aqueous vapour as the seasons change from winter
to summer.
In solids the force of cohesion is so strong, that their atoms and
molecules always retain their respective places; that power is so weak
in liquids, that their atoms and molecules are capable of motion among
themselves, and in gases and the ethereal medium the atoms are free and
have no cohesion whatever. The resistance offered by substances to
compression is an equal and contrary force.
The reciprocal attraction between solids and liquids in capillary tubes
is a case of cohesion. If a glass tube of extremely fine bore be plunged
into a glass of water or alcohol, the liquid will immediately rise in
the tube above the level of that in the cup, and the surface of the
little suspended column will be a hollow hemisphere. If on the contrary
mercury be the liquid, it will not rise so high in the glass tube, and
the surface of the little column will be a convex hemisphere. There is a
reciprocal attraction between the glass tube and the liquid, and another
between the particles of the liquid itself; and the effect is produced
by the difference between the two. In the first case the attraction of
the glass is greater than that of the liquid, and in the second it is
less; hence the water rises higher in the tube than the mercury, and its
surface is concave, while that of the mercury is convex. The elevation
or depression of the same liquid in different tubes of the same matter
is in the inverse ratio of their internal diameters, and altogether
independent of their thickness; whence it follows that molecular action
is insensible at sensible distances, for when tubes of the same bore are
wetted throughout their whole extent with water, mercury will rise to
the same height in all of them whatever be their thickness or density,
the film of water being sufficient to intercept the molecular action,
and to supply the place of a tube by its own capillary attraction. The
action of this force is daily seen in the absorption of water by
sponges, sugar, salt and other porous bodies, and it is a most important
agent in the circulation of fluids in animals and vegetables.
Every atom of matter is subject to the force of gravitation, but each
substance has its own peculiar weight of specific gravity, that is to
say, the same bulk of different substances contains different quantities
of matter. Since nothing is known of absolute weight it is necessary to
have some standard of comparison, and for that purpose pure water at the
temperature 39° Fahr. (that of its maximum density) is chosen for solids
and liquids; while for gases and vapours atmospheric air at the
temperature of sixty degrees of Fahrenheit’s thermometer, and a
barometric pressure of thirty inches, is assumed as the unit of specific
gravity.
The foot-pound, which is the unit of mechanical force as established by
Mr. Joule, is the force that would raise one pound of matter to the
height of one foot; or it is the impetus or force generated by a body of
one pound weight falling by its gravitation through the height of one
foot. Now impetus or vis viva is equal to the mass of a body multiplied
by the square of the velocity with which it is moving: it is the true
measure of work or mechanical labour. For if a weight be raised ten
feet, it will require four times the labour to raise an equal weight to
forty feet. If both these weights be allowed to fall freely by their
gravitation, at the end of their descent, their velocities will be as
one to two, that is as the square roots of their heights, but the effect
produced will be as their masses multiplied by one and four; but these
are the squares of their velocities. Hence impetus or vis viva is equal
to the mass multiplied by the square of the velocity. Thus impetus is
the true measure of the labour employed to raise the weights, and of the
effect of their descent, and is entirely independent of time.
It is well known that iron becomes red-hot by percussion or impetus. The
atoms of the iron are thrown into vibration, and these minute motions
communicated to the nerves produce the sensation of heat. Now the
mechanical labour required to raise the hammer to any number of feet is
equal to the weight of the hammer multiplied by that number of feet; but
the impetus or mechanical effect of the fall of the hammer is equal to
its mass multiplied by the square of the velocity, that is to the vis
viva: hence the quantity of heat generated is proportional to the vis
viva. The circumstances being the same, if the mass be doubled the
amount of heat is doubled; and if the velocity be doubled the amount of
heat is quadrupled. If the weight and the perpendicular height through
which a body has fallen be known, the quantity of heat generated may be
determined. The same amount of heat is generated by the same amount of
force, whatever that force may be, whether impetus, friction, or any
other.
Dr. Thomson has put in a strong point of view the quantity of heat that
might be generated by percussion or impetus. He computed that if by any
sudden shock the earth were arrested in its orbit, the heat generated by
the impulse would be equal to 11,200 degrees of the centigrade
thermometer, even if the capacity of our planet for heat were as low as
that of water; it would therefore be mostly reduced to vapour, and
should the earth then fall to the sun as it certainly would do, the
quantity of heat developed by striking on the sun would be 400 times
greater. It is even supposed that the light and heat of the sun are
owing to showers of bodies falling on the surface with impetus
proportionate to his attraction, for had he been in combustion he would
have been burnt out ages ago. The masses of meteoric iron and stone that
occasionally fall on the earth show that matter may be wandering in
space; the vast zone of smaller bodies that in their annual revolutions
round the sun come within the earth’s attraction in August and November,
when thousands of them take fire and are consumed on entering our
atmosphere, show that a great amount of matter of small dimensions
exists within our own system. Much may be beyond it which drawn by the
sun’s attraction may fall on his surface.
When a body is heated, it absorbs one part of the heat; the other part
raises its temperature. The part absorbed increases the bulk or volume
of the body, the expansion being the exact measure, or mechanical
equivalent of the heat absorbed. In fact the coefficient of expansion is
the fractional part of the expansion in length, surface, or volume of
the body when its temperature is raised one degree. When the body is
cooled, its volume is diminished, and then the contraction is an exact
measure, or mechanical equivalent of the heat given out, and thus
expansion and contraction are correlatives with and represent heat and
cold.
Specific heat is the quantity of heat required to raise a given bulk or
a given weight of a body a given number of degrees. In the one case it
is distinguished as the specific heat for a constant volume, in the
other for a constant weight.
Although the specific heat of a substance remains the same, its sensible
and absorbed heat may vary reciprocally to a great extent.
As there can be no direct measurement of heat independent of matter, its
mutations and action on matter are the sole means we have of forming our
judgment concerning its agency in the material world.
Mr. Joule has proved that the quantity of heat requisite to raise the
temperature of a pound of water one degree of the centigrade thermometer
is equivalent to the mechanical work or force that would raise the same
mass of water to the height of 1,389 feet. This is the unit, or
mechanical equivalent of heat.
In fact, for every unit of force expended in percussion, friction, or
raising a weight, a definite quantity of heat is generated; and
conversely, when work is performed by the consumption of heat, for each
unit of force gained, a unit of heat disappears. For since heat is a
dynamical force of mechanical effect, there must be an equivalence
between mechanical work and heat as between cause and effect. That
equivalence is a law of nature. The mechanical force exerted by the
steam engine is exactly in proportion to the consumption of heat,
neither more nor less; for if we could produce a greater quantity than
its equivalent we should have perpetual motion, which is impossible.
When steam is employed to perform any work, the temperature of the steam
is lowered; the heat that disappears is transformed into the force that
performs the work, and is exactly proportional to the work done, and
vice versâ.
The heat which is the motive force in the steam engine is due to the
chemical combination of the carbon of the fuel with the oxygen of the
atmosphere. A pound weight of coal when consumed in one of our best
steam engines produces an effect equal to raising a weight of a million
of pounds a foot high, yet marvellous as that is, the investigations of
recent years have demonstrated the fact, that the mechanical energy
resident in a pound of coal and liberated by its combustion is capable
of raising to the same height ten times that weight.[3] The quantity of
coal existing in the whole globe is believed to be inexhaustible, hence
the energy in abeyance is incalculable. The chemical energy continually
and actually exerted in the great laboratory of nature is greater than
that which maintains the planets in their orbits.
The act of the combination of the atoms of carbon and oxygen in
combustion is ‘now regarded exactly as we regard the clashing of a
falling weight against the earth, and the heat produced in both cases is
referable to the same cause;’[4] so chemical combination in combustion
is only a particular case of falling bodies. Drummond’s light, the most
brilliant of artificial illuminations, is produced by a simultaneous
shower of the atoms of oxygen and hydrogen gas upon lime; and platinum,
the least fusible of metals, is vaporized by a similar shower from the
oxy-hydrogen blowpipe, and thus impetus generates both light and heat,
for although the atoms are too small to admit of an estimation of their
individual vis viva, there can be no doubt that like causes produce like
effects.
In what manner or under what form magnetism and electricity exist when
quiescent in matter we know not, but the compass needles show that
numerous lines of magnetic force, subject to periodic and secular
variations, perpetually traverse the earth and the ocean; and that waves
of magnetic force occasionally sweep rapidly over great tracts of the
globe. These phenomena would seem to stand in some periodic connection
with the solar spots. Professor Lamont of Munich has discovered that a
permanent and regular current of electricity is propagated parallel to
the equator all over the earth, and another similar to it in the
atmosphere. Besides these, there are currents of electricity in the
surface of the earth, sometimes in one direction and sometimes in
another, which decrease with the depth; and M. Lamont conceives that
this electric system is the cause of terrestrial magnetism. Electricity
of intense power and inappreciable quantity certainly exists in abeyance
in the atmosphere and in all terrestrial matter till the equilibrium
between the antagonist forces be disturbed, and then it bursts forth
with terrific violence in the lightning flash and stunning crash of
thunder. Since it requires electricity equivalent to that in activity
during a thunderstorm to form one drop of water, what must that power
have been which the Omnipotent wielded when he created that deep over
the face of which ‘darkness brooded.’
Electricity, though the most formidable power in nature, is made
available to man by the voltaic battery, and by the electro-magnetic
induction apparatus, in the battery of which it is generated by the
chemical action of dilute sulphuric acid on zinc. The positive and
negative electricities thus produced pass in opposite directions through
the two conducting wires of the machine by a continuous transmission of
force or vibration from atom to atom, a circulation that is accompanied
by a continual development of heat in overcoming the resistance it meets
with in the wires. The electricity decreases as the heat increases, and
vice versâ; the action is reciprocal. Thus electricity is merely a
transmission of force. Mr. Joule has proved that the quantity of heat
produced in a unit of time is proportional to the strength of the
current, whatever may be its direction, and that its power to overcome
resistance is as the square of the force of the current. The force is
exactly in proportion to the chemical action which produces it, and that
is measured by the quantity of zinc consumed in the battery. Thus
chemical action produces electricity, and conversely electricity is a
powerful agent in the chemical composition and decomposition of matter.
The light and heat of the electric spark are intense though
instantaneous; but a powerful induction apparatus like Ruhmkorff’s gives
so rapid a succession of sparks that the light and heat are sensibly
continuous and of great intensity. The light and heat, powerful as
lightning itself, are produced by the combined currents of two
batteries, each consisting of fifty Bunsen elements of moderate size.
This formidable united current passes through a circuit of thick copper
wire coated with silk thread, with an intensity of perpetually renewed
heat that no substance can resist. When the copper conducting wires are
fitted with charcoal terminals and brought near to one another, the
dazzling lights emanating from each pole combine in one blaze of
insupportable brilliancy. The most refractory substances, silica,
alumina, iron and platinum, when placed between the poles, immediately
melt like wax, and volatilize. Charcoal is so good a conductor of
electricity that when the terminals are in contact they complete the
circuit, and neither light nor heat appear. Air and glass are
non-conductors, yet the spark has passed through several inches of air
and perforated a mass of glass two inches thick. A long electric spark
combines or decomposes a greater quantity of gas or vapour than a short
one, and for a given induction apparatus and induction current, M.
Perrot has shown that there exists a length of spark corresponding to a
maximum chemical action.
Professor Seebeck of Berlin discovered that electric currents are
produced by the partial application of heat to a circuit formed of two
solid conducting substances as antimony and bismuth soldered
together,—another proof of the correlation of heat and electricity.
There cannot be a doubt that the atoms of a conducting wire are in
motion, and that they successively take definite and momentary positions
during the passage of an electric current, after which they return
successively to their normal state. When electricity is invariably sent
from the same pole of an inductive apparatus through the wire of a
telegraph, in a very short time the wire is torn or divided into small
sections, which destroy its continuity; but when the electricity is sent
from each pole alternately, the conducting wire is not injured. As each
atom of the wire has its own electricity, this seems to indicate that
during the successive transits of the same kind of electricity, the pole
of each individual atom is attracted more and more in the same
direction, till at last they no longer return to their normal state, the
cohesive force is overcome, and a rupture takes place, the more readily
if there be any imperfection in the wire. Since the electricity from the
other pole of the machine would have the same effect, but in the
contrary direction, an alternate motion in the atoms must maintain the
continuity of the wire.
A closed current of electricity or magnetism is accompanied by a
simultaneous current of the opposite force in the tangential direction
equal in quality and intensity. Thus the electric and magnetic currents,
which are merely transmissions of energy, differ by moving at right
angles to one another; their effects are alike, yet they are not
identical.
The amount of the chemical action of light has been determined by
Professor Roscoe to be directly proportional to the intensity of the
light; and when the light is constant the amount of action is exactly
proportional to the time of exposure. It appears that equal volumes of
chlorine and hydrogen explode in sunshine, but combine slowly in shade;
and as the combined gases are absorbed by water as soon as combined, the
gradual diminution of the volume of the mixed gases during the time of
absorption is a measure of the amount of action exerted by the light.
Professor Wm. Thomson has computed, by the aid of Poullet’s data of
solar radiations and Mr. Joule’s mechanical equivalent of heat, that the
mechanical value of the whole energy, active and potential, of the
disturbances kept up on the ethereal medium by the vibrations of the
solar light in a cubic mile of our atmosphere, is equal to 12,050 times
the unit of mechanical force: that is to say, twelve thousand and fifty
times the force that would raise a pound weight of matter to the height
of one foot. The sensible height of the atmosphere is about forty miles,
whence some idea may be formed of the vast amount of force exerted by
the sun’s light within the limits of the terrestrial atmosphere. The
green mantle which clothes the earth proves under a beautiful form the
influence of light on the organic world.
It has been proved that at _any given_ fixed temperature the amount of
light and heat absorbed and that which is emitted remains constant for
_all bodies_. The greater the amount absorbed, the greater the amount
radiated. The molecules or atoms of the bodies in consequence of the law
of resonance emit those ethereal undulatory motions which have been
previously impressed upon them, as a musical instrument resounds in
answer to the note impressed upon it. The whole is referable to
molecular or atomic motion, for in absorption the vibrations of the
ether are communicated to the atoms, and in radiation, the vibrations
are returned again to the ether. This principle is known as the law of
exchange.[5]
Matter has a decomposing and an elective power with regard to both
radiant light and heat; most coloured bodies, such as flowers, green
leaves, dyed cloth, &c., though seen by reflection, owe their colour to
absorption. The light by which they are seen is reflected, but it is not
in reflection that the selection of the rays is made which causes the
objects to appear coloured. When light falls upon red cloth, a small
portion is reflected at the outer surfaces of the fibres, and this
portion, if it could be observed alone, would be found to be colourless.
The greater portion of the light penetrates into the fibres, when it
immediately begins to suffer absorption on the part of the colouring
matter. On arriving at the second surface of the fibre, a portion is
reflected and a portion passes on, to be afterwards reflected from, or
absorbed by, fibres lying more deeply. At each reflection the various
kinds of light are reflected in as nearly as possible the same
proportion, but in passing across the fibres while going and returning
they suffer very unequal absorption on the part of the colouring matter;
so that in the aggregation of the light perceived the different
components of white light are present in proportions widely different
from those they bear to each other in white light itself, and the result
is a vivid colouring.
In certain substances however, as gold and copper, the different
components of white light are reflected with different degrees of
intensity, and the light becomes coloured by these reflections. Gold is
yellow by reflection; red cloth is red by absorption. In the same sense,
physically speaking, in which the red cloth is red, gold is not yellow
but blue or green; such is in fact the colour of gold by transmission
through gold leaf, and therefore gold is greenish blue by absorption. In
this case we see that while the substance copiously reflects and
intensely absorbs rays of all kinds, it more copiously reflects the less
refrangible rays with respect to which it is more intensely opaque. In
general absorption and radiation are independent of colour.
There is a vast diversity in the property which substances possess with
regard to the transmission of radiant light and heat; glass, for
instance, transmits light abundantly, but is impervious to heat from
non-luminous sources; while other substances, which are altogether
opaque to light, transmit heat copiously, as the bisulphide of carbon,
which of all liquids is the most diathermic, while water in all its
forms is almost impervious to heat.
Sir William Herschel discovered that invisible rays of high heating
power exist beyond the red end of the solar spectrum, and Mr. Tyndall
has shown that the reason of a substance being impervious to the light
of the most brilliant flame and at the same time pervious to these extra
red rays is, that the intercepted rays of light are those whose periods
of recurrence coincide with the periods of oscillation possible to the
atoms of the substance in question. The elastic forces which separate
these atoms are such as to compel them to vibrate in definite periods,
and when their periods synchronize with those of the ethereal waves, the
latter are absorbed. Thus transparency in liquids as well as in gases is
synonymous with _discord_, while opacity is synonymous with _accord_
between the periods of the waves of ether and those of the body on which
they impinge. All ordinary transparent and colourless substances owe
their transparency to the discord which exists between the oscillating
periods of their molecules and those of the waves of the whole visible
spectrum. The general discord of the vibrating periods of the molecules
of _compound bodies_ with the light-giving waves of the spectrum may be
inferred from the prevalence of the property of transparency in
compounds, while their greater harmony with the extra red periods is to
be inferred from their opacity to the extra red rays. Water illustrates
this transparency and opacity in the most striking manner. It is highly
transparent to the luminous rays, which demonstrates the incapacity of
its molecules to oscillate in periods which excite vision. It is as
highly opaque to the extra red oscillations, which proves the
synchronism of its periods with more of the longer waves. If, then, to
the radiation from any source water shows itself to be eminently or
perfectly opaque, it is a proof that the molecules whence the radiation
emanates must oscillate in extra red periods.
It has been already mentioned that many substances which transmit
radiant heat freely radiate badly, and vice versâ. Rock-salt is
extremely permeable to radiant heat but radiates feebly; the reason
according to Mr. Tyndall is, that the motion of the molecules of the
salt, instead of being expended on the ether between them and then
communicated to the ether external to the mass, is transmitted freely
from molecule to molecule.
Alum is exactly the reverse. Mr. Balfour Stewart proved that alum is an
excellent radiator, and Mr. Tyndall proved it to be a very bad
conductor, imparting freely and with ease the motion of its molecules to
the external ether, and ‘for that very reason it finds difficulty in
transferring the motion from molecule to molecule. The molecules are so
constituted that when one of them approaches its neighbour, a swell is
produced in the intervening ether; this motion is immediately
communicated to the ether outside, and is thus lost for the purposes of
conduction.’[6]
Melloni had investigated the laws of the radiation and absorption of
radiant heat in solid and liquid matter; but its radiation and
absorption by gases and vapours was unknown previous to the experiments
of Mr. Tyndall.
The apparatus employed was a horizontal brass tube four feet long,
between two and three inches in diameter, polished inside, and closed
air-tight at each end by a plate of rock-salt, which transmits more heat
than any other substance. The air could be pumped out of the tube by one
pipe, and the gas or vapour for the experiment introduced by another.
Close to one end of the brass tube there was a thermo-electric pile
connected with its goniometer. On each side of this arrangement there
was a vessel of water kept at the boiling point. These two vessels were
so placed that when the rays of heat from one of them passed through the
exhausted tube, and fell upon one face of the thermo-electric pile,
their effect was so neutralized or balanced by the rays of heat falling
on the opposite face of the pile from the other, thus the needle of the
goniometer was steadily maintained at zero, and its deflection instantly
showed the absorbent effect produced by any gas or vapour that was
admitted into the exhausted tube.
Since aqueous vapour has a very exalted absorbent power, a gas or vapour
was rendered perfectly dry before its absorbent capacity was determined.
For that purpose the pipe that introduced it into the brass experimental
tube was so constructed that the gas had first to pass over fragments of
pumice-stone wet with strong sulphuric acid, which absorbed its moisture
and dried it. Common atmospheric air, however, was not only dried in
this manner, but it was deprived of its carbonic acid by passing over
caustic potash, and many other precautions were taken to prevent the
possibility of error.
Under the ordinary pressure of the atmosphere, when the experimental
tube was exhausted, the needle of the goniometer stood at zero, but as
soon as pure dry atmospheric air was introduced into the tube its
absorption caused the needle to move from zero to 1°.
The tube was again exhausted; the needle stood at zero, but was
deflected from zero to 1° as soon as the tube was filled with oxygen. A
similar experiment was made with nitrogen and hydrogen with the same
result. Thus, dry air and the elementary gases, oxygen, nitrogen, and
hydrogen, have the same absorptive power, and consequently they all
deflected the needle of the goniometer one degree. The whole amount of
radiant heat that passed through the exhausted tube produced a
deflection of 71° 5ʹ; hence taking as unit of heat the amount that would
deflect the needle one degree, the number of units expressed by 71° 5ʹ
is 308, consequently the absorption of each of these four gases amounts
to 100/308 or 0·3 per cent. The most delicate tests could not show any
difference between the three first, but Professor Tyndall had reason to
believe that hydrogen has the lowest absorptive power of all gases and
vapours, though he was unable to express the amount. The absorptive
power of all four is very much less than that of every other gas or
vapour, and invariably deflects the needle to 1°, which thus becomes the
unit of comparison.
Olefiant gas, the most luminous of the constituents of coal gas,
possesses the highest absorptive power of the permanent gases. When sent
into the exhausted tube it deflected the needle of the goniometer from
0° to 70° 3ʹ, which is equivalent to 290 units. The whole heat that
passed through the exhausted tube before the gas was admitted produced a
deflection of 75° or 360 units, consequently more than 7/10ths or 81 per
cent. of the whole heat was cut off by the olefiant gas. Such opacity to
heat in so transparent a gas is quite marvellous. A current of it was
sent into the open air between the thermo-electric pile and one of the
sources of heat, and although it was perfectly invisible, it instantly
deflected the needle of the goniometer from 0° to 41°.
In order to ascertain the relation between the density of the gas and
the quantity of heat extinguished or absorbed, an ordinary mercurial
gauge was attached to the air-pump. The experimental tube was exhausted,
and the needle of the goniometer stood at zero. Then, from a graduated
glass vessel, measures of olefiant gas, each amounting to the 1/50th of
a cubic inch, were successively sent through the drying pipe into the
exhausted tube. The amount of the heat absorbed and the depression of
the mercurial column corresponding to each measure of gas as it was
introduced, was registered from one to fifteen measures. This experiment
showed that for very small quantities of gas, the absorption is exactly
proportional to the density or tension. One measure of the gas only
produced a depression of the mercurial column amounting to the 1/367th
part of an inch, or about the 1/15th of a millimetre.
In many of the vapours of volatile liquids, the preceding law only
prevails to a certain amount of pressure differing in each case, beyond
which increase of tension produces diminished effects. In sulphuric
ether the change begins at the eleventh term.
In bisulphide of carbon the law changes after the sixth measure, &c.
In order to adapt the apparatus for experiments on coloured gases, a
glass experimental tube 2 ft. 9 in. long, and 2 ft. 4 in. in diameter,
was substituted for the brass tube, and, instead of boiling water,
sources of radiant heat having a constant temperature of 270° Cent. were
adopted.
The following table shows the absorption of a number of gases at a
common pressure or tension of one atmosphere.
Dry air = 1
Oxygen 1
Nitrogen 1
Hydrogen 1
Chlorine 39
Hydrochloric acid 62
Carbonic oxide 90
Carbonic acid 90
Nitrous oxide 35·5
Sulphuretted hydrogen 390
Marsh gas 403
Sulphurous acid 710
Olefiant gas 970
Ammonia 1195
The absorptive power of ammonia is so great, that although as
transparent in the glass tube as if it had been a vacuum, a length of
three feet of it would be perfectly impervious or black to the heat here
employed, yet even this does not express the energy which it exhibits
under one inch of pressure.
When the relative absorptive actions of gases and vapours is compared,
it must be under the same amount of pressure. Hence, for one inch of
tension, the absorptive action of
Dry air = 1
Oxygen 1
Nitrogen 1
Hydrogen 1
Chlorine 60
Bromine 160
Hydrochloric acid 1005
Carbonic oxide 750
Nitric oxide 1590
Nitrous oxide 1860
Sulphide of hydrogen 2100
Ammonia 7260
Olefiant gas 7950
Sulphurous acid 8800
Thus, for a tension of an inch of mercury, the absorption of ammonia
exceeds that of air more than 7000 times; the action of olefiant gas is
7950 times, and that of sulphurous acid 8800 times, greater than the
absorption of air.
The effect produced by 1/30th of an inch of tension of air and the
elementary gases is equivalent to that produced by one inch in the
others, so the unit representing the absorption of these four gases is
only the 1/30th part of the unit in the preceding table.
It appears from the preceding tables of comparative absorption that
chlorine, a highly-coloured gas with a specific gravity of 2·45, has an
absorptive power expressed by 39° under the pressure of one atmosphere,
while, at the same tension, hydrochloric acid, a chemical compound of
chlorine and hydrogen which is perfectly transparent, with a specific
gravity of only 1·26, has an absorptive action amounting to 62, whence
it appears that the chemical change which renders chlorine more
transparent to light, makes it more opaque to obscure heat. Again,
bromine, which is far less permeable to light than chlorine, and has a
specific gravity of 5·54, has an absorptive power of 160 under a tension
of one inch; while hydrobromic acid, which is perfectly transparent to
light, has an absorptive action for obscure heat amounting to 1005. This
is a striking instance of transparency to light and opacity to heat
being produced by the very same chemical art.
The enormous difference between the absorptive power of compound and
simple gases and vapours is ascribed to their atomic structure; in fact
the radiant and absorptive powers augment as the number of atoms in the
compound molecule augments. The three elementary gases are formed of
simple atoms, the compound gases and vapours consist of different kinds
of atoms chemically united into groups. Both are free to receive the
vibratory motions of the ether which constitute heat; but single atoms
must produce a less effect than when a number of them are united into a
molecule. The atoms are loaded by their chemical union, which offers a
greater surface of resistance to the vibrations of heat, and renders the
motion of the molecule more sluggish and more fit to accept the slowly
recurrent waves of the obscure heat that strike upon it.
Thus when atoms of hydrogen and nitrogen are mixed in the proportion of
three to one, the absorption of the mixture is represented by unity; but
when they are chemically united in ammonia, the absorption is 1190 times
as great. Atoms of hydrogen and oxygen mixed in the proportion of two to
one absorb very feebly; when chemically united into a molecule of
aqueous vapour the absorptive power is enormous. The absorptive power of
nitrous oxide, a chemical compound of oxygen and nitrogen, exceeds that
of dry air 250 times; a convincing proof that the atmosphere is a
mixture and not a compound gas. Olefiant gas at five inches of tension
absorbs 1000 times that of its constituent hydrogen. In fact all the
compound gases and vapours far surpass the simple elementary gases and
dry atmospheric air in their capacity for absorption.
Chlorine and bromine, which have so many singular properties in common,
have this peculiarity also, that though simple substances respectively
formed of homogeneous atoms, their absorptive powers are similar to
those of compound substances, for the absorptive power of chlorine is 60
times that of the elementary gases, and that of bromine 160 times. This
high absorptive power is ascribed by Professor Tyndall to their atoms
being united into groups which act powerfully as oscillating systems,
instead of the feeble action of single atoms.
Ozone is an analogous instance of the presumed union of homologous atoms
into oscillating groups. By comparing the absorptive effect of ozonized
oxygen obtained from the electric decomposition of water with that of
the same oxygen deprived of its ozone by passing it over a very strong
solution of iodide of potassium, Professor Tyndall found that ozonized
oxygen possesses an absorption force 136 times greater than that of pure
oxygen. The quantity of ozone producing this astonishing effect was too
small even to admit of estimation, far less of measurement. This result
induced Professor Tyndall to believe that ozone is produced by the
packing of the atoms of elementary oxygen into oscillatory groups; and
that heating dissolves the bond of union and allows the atoms to swing
singly, thus disqualifying them from either intercepting or generating
the motion which as systems they were competent to intercept and
generate.
The indefinitely small and invisible constituents of perfumes of plants
and flowers are proved to be compound bodies by their absorptive and
radiating properties. The dried leaves of a flower or aromatic plant
such as thyme were stuffed into a glass tube 18 inches long and a
quarter of an inch in diameter. It was then inserted between the drying
pipe of the machine and the experimental glass tube, which was
exhausted, and the needle of the goniometer stood at zero. Then when the
air admitted into the drying pipe passed over the thyme and carried its
aroma into the experimental tube, the needle was deflected, and from
thence the absorption of the thyme was computed to be 33 times greater
than that of the air which carried it. By the same process it was found
that the absorption of peppermint was 34 times, spearmint 38 times,
lavender 32 times, and wormwood 41 times greater than that of the dry
air, which was unity as usual. When small equal squares of bibulous
paper rolled into cylinders and moistened with an aromatic oil, were
substituted for the dried herbs, the absorption corresponding to the
deflection of the needle was for dry air, equal to 1,—
Patchouli 30
Sandal wood 32
Geranium 33
Oil of cloves 33·5
Otto of roses 36·5
Bergamot 44
Lavender 60
Lemon 67
Orange 67
Thyme 68
Rosemary 74
Oil of laurel 80
Chamomile flowers 87
Spikenard 355
Anise seed 372
The absorption of thyme and lavender shows how much aroma is lost when
plants are dried. So great is the absorption of heat, that the perfume
of a flower-bed may be more efficacious than the entire oxygen and
nitrogen of the atmosphere above it.
The enormous absorption and consequently radiating power of the perfumes
of plants and flowers is a proof that their constituent parts are
molecules and not simple atoms, incredible as it may seem. The absolute
weight of the substances producing these wonderful effects is unknown,
but there must be great differences: some perfumes are carried to vast
distances, others are less volatile, and that of mignonette was remarked
by Dr. Wollaston to be absolutely so heavy that it was quite as powerful
below a balcony containing a box of that plant, as in the balcony
itself.
The perfumes during the experiments adhered to all parts of the
apparatus so pertinaciously, that after a continued stream of dry air
had been pumped through the tube till the exhaustion seemed to be
complete and the needle stood at zero, after a few minutes’ repose, the
residue of the perfume came out so powerfully from the crannies of the
apparatus as almost to restore the original deflection. ‘The quantities
of those residues must be left to the imagination to conceive. If they
were multiplied by billions they probably would not obtain the density
of the air.’
The absorptive power of the odour of musk was 72 or 74 times that of the
air that conveyed it into the experimental tube; the quantity that
produced it was quite inappreciable, yet the perfume was so persistent
that the pieces of the apparatus through which it had passed had to be
boiled in a solution of soda before they were fit for other experiments.
The absorption of many gases and vapours having been determined, their
radiation was measured by a very simple arrangement. The thermo-electric
pile was raised on a stand with a screen of polished tin in front of it.
A heated copper ball in a perforated ring on a low stand was placed
behind the screen; all direct radiation from the ball was thus cut off,
but the heated air rising in a column above the screen radiated its heat
on the pile and deflected the needle of the goniometer 60° when the ball
was red-hot; but the radiation of the hot air was neutralized by another
source of radiant heat on the opposite side of the pile which kept the
needle steadily at zero. Then a purified gas or vapour conveyed by a
pipe into the perforated ring which held the ball rose mixed with the
heated air above the screen, but the radiation of the gas or vapour
alone was shown by the deflection of the needle, because that of the air
was compensated. With this apparatus Professor Tyndall proved that the
amount of the absorption of each gas and vapour is exactly equal to the
amount of its radiation. He has shown that this result is a necessary
consequence of the dynamical nature of heat. For as no atom or molecule
is capable of existing in vibrating ether without accepting a portion of
the motion, the very same quality whatever it may be that enables it to
do so, must enable it to impart its motion to still ether when plunged
into it. ‘Hence from the existence of absorption we may on theoretic
grounds infallibly infer a capacity for radiation; from the existence of
radiation we may with equal certainty infer a capacity for absorption,
and each of them must be regarded as the measure of the other.’ This
reasoning, founded simply on the mechanical relations of the ether and
the atoms immersed in it, is completely verified by experiment.
Hitherto the absorption and radiation of heat by the same thickness of
different gases and vapours have been compared with each other, but in a
recent series of experiments Mr. Tyndall has compared the action of
different thicknesses of the same gas or vapour on radiant heat. The
experiments extend from a thickness of 0·01 of an inch to that of 49·4
inches. The instrument employed for ascertaining the action of the
smaller thickness was a horizontal hollow cylinder closed at one end by
a plate of rock-salt. A second cylinder was fitted into this with its
end also closed by a plate of rock-salt. This cylinder moved within the
other like a piston, so that the two plates of rock-salt could be
brought into flat contact with one another, or could be separated to any
required distance, and the distance between the plates was measured by a
vernier. At one end of the cylinder there was a source of constant heat,
and the differential goniometer already described at the other. With
this apparatus Mr. Tyndall found that olefiant gas maintains its great
superiority over the other gases in absorptive power at all thicknesses.
A layer of that gas not more than 0·01 of an inch thick intercepted
about one per cent. of the total radiation. This great absorption
corresponded to a deflection of 11° of the needle of the goniometer, and
such was the delicacy of the apparatus that it would be possible to
measure the action of a layer of this gas of less thickness than a sheet
of writing paper. A layer of olefiant gas two inches thick intercepts
nearly 30 per cent. of the entire radiation. A shell of olefiant gas two
inches thick surrounding our globe would offer no appreciable hindrance
to the solar rays in coming to the earth, but it would intercept, and in
great part return, 30 per cent. of the terrestrial radiation; under such
a canopy the surface of the earth would probably be raised to a stifling
temperature.
The apparatus for measuring the action of the greater thicknesses of gas
was a hollow brass cylinder 49·4 inches long, closed at both ends by
plates of rock-salt, and divided internally into two compartments or
chambers by a third plate of rock-salt movable in the interior; the
source of heat being at one end and the differential goniometer at the
other.
Carbonic oxide and carbonic acid are pervious to a vast majority of the
rays of radiant heat. When the cylinder was filled with carbonic oxide
gas and so divided, by moving the internal plate of rock-salt, that a
stratum of the gas 8 inches long was next to the source of heat, and
that 41·4 inches long farthest from it, the 8 inches of gas intercepted
6 per cent. of the whole radiation. But when the plate of rock-salt was
moved till the column 41·4 inches long was next to the source of heat,
and that of 8 inches farthest from that source, or behind the long one,
the absorption of the 8 inches was sensibly zero. In like manner eight
inches of carbonic acid gas when in front of a column of 41·4 inches of
the same gas absorbed 6-1/4 per cent. of the whole radiation, while
placed behind that column the effect was nearly zero. The reason is that
when the 8 inch stratum is in front, it stops the main portion of the
rays which give it its thermal colours,[7] while placed behind these
same rays have been almost wholly withdrawn, and to the remaining 94 per
cent. of the radiation the gases are sensibly permeable.
It is inferred from an extension of this reasoning that the sum of the
absorptions of the two chambers taken separately must always be greater
than the absorption effected by a single column of the gas of a length
equal to the sum of the two chambers; this conclusion is illustrated in
a striking manner by the experiments. It is also found that when the
mean of the sums of the absorptions is divided by the absorption of the
sum, the quotient is sensibly the same for all gases. It may farther be
inferred that the sum of the absorptions must diminish and approximate
to the absorption of the sum as the two chambers become more unequal in
length, and that the sum of the absorptions of the two chambers is a
maximum when the medial plate of rock-salt divides the long tube into
two equal parts.
When air enters an exhausted tube it is heated dynamically by the
collision of its particles on the sides of the tube as it rushes in to
fill the vacuum; and when the tube is exhausted again by the air pump,
chilling is produced by the application of a portion of the heat of the
air to generate vis viva. This dynamic principle occurred in some of the
experiments, and was dexterously adopted and applied to the solution of
a striking and unprecedented problem: ‘To determine the radiation and
absorption of gases and vapours _without any source of heat external to
the gaseous body itself_.’
The two external sources of heat being therefore dispensed with in the
absorptive apparatus, the thermo-electric pile was presented to the cold
glass tube which was exhausted, and the needle of the goniometer stood
at zero. Nitrous acid on entering the exhausted tube became heated and
radiated its heat upon the adjacent face of the pile which deflected the
needle of the goniometer through 28° in the direction that indicates
absorption. As the heat of the gas became gradually exhausted, the
needle returned slowly to zero. The pump was now worked, the rarefied
gas in the tube was chilled, and the adjacent face of the pile gradually
poured its heat on the chilled tube till the temperature of the pile was
so much lowered, that the needle was deflected 20° on the negative side
of zero, that is on the side denoting radiation.
When olefiant gas entered the exhausted tube, the needle showed an
absorption of 67°, and when the gas was pumped out again, the needle
showed a radiation amounting to 41°. When the gas was then pumped out,
very dry atmospheric air was introduced into the tube,—the needle
pointed to 59° indicating absorption; and when it was pumped out again
the needle swung to nearly 40° on the other side of zero, indicating
radiation. Remembering that the radiation and absorption of dry air only
produce a deflection of 1°, it is evident that the preceding great
deflection of the needle is entirely owing to the action of the small
residue of olefiant gas that remained in the exhausted tube. In order to
ascertain how much the quantity of a gas or vapour might be reduced
before its action became insensible, the vapour of boracic ether, which
has the greatest absorptive energy, was chosen.
The mercurial gauge for measuring the pressure or tension of the vapour
already mentioned remained attached to the apparatus. When one-tenth of
an inch of the vapour of boracic ether was admitted into the exhausted
tube, the barometer stood at 30 inches: hence the tension of the vapour
within the tube was the 1/300th part of an atmosphere. Dynamically
heated by dry air the radiation of the vapour produced a deflection of
56°. Again the tube was exhausted to 0·2 of an inch and the quantity of
vapour was thereby reduced to 1/150th of its first amount; the needle
was allowed to come to zero, and the residue of the vapour produced a
deflection of 42°. The pump was again worked till a vacuum of 0·2 of an
inch was obtained, this residue containing of course the 1/150th of the
quantity of ether present in the tube; and on dynamically heating the
residue, its radiation produced a deflection of 20°.
Thus it is evident that the tension of the ether in these experiments
was continually diminished by the 0·2 of an inch, consequently its
quantity was continually diminished by its 1/150th part, accompanied by
a corresponding decrease in the deflections of the needle. The final
result of this process showed that the radiation of an amount of vapour
in the tube possessing a tension of less than the thousand millionth of
an atmosphere is perfectly measurable. The temperature imparted to this
infinitesimal quantity of matter did not exceed 0·75 of a centesimal
degree. The molecules which constituted this intensely attenuated
vapour, though inconceivable, had as true an existence as the suns which
constitute the star-dust of the nebulæ. ‘A platinum wire raised to
whiteness in a vacuum by an electric current, becomes comparatively cold
in a second after the current has been interrupted; yet that wire, while
ignited, was the repository of an immense amount of mechanical force.
What has become of this? It has been conveyed away by a substance so
attenuated that its very existence must for ever remain an hypothesis.
But here is matter that we can weigh, measure, taste, and smell; that we
can reduce to a tenuity which, though expressible by numbers, defeats
the imagination to conceive of it. Still we see it competent to arrest
and originate quantities of force which on comparison with its own mass
are almost infinite, a small fraction of this force causing the double
needle of the galvanometer to swing through considerable arcs. When we
find ponderable matter producing these effects, we have less difficulty
in investing the luminiferous ether with those mechanical properties
which have long excited the interest and wonder of all who have
reflected upon the circumstances involved in the undulatory theory of
light.’
The dynamical principle was next applied to determine the radiation of a
gas through itself; or through any other gas having the same period of
vibration. For that purpose Mr. Tyndall made use of the hollow cylinder
49·4 inches long already mentioned, closed at both ends by plates of
rock-salt, and divided internally into two chambers by a movable plate
of the same substance. All sources of heat being dispensed with, the
chamber next the voltaic pile contained the gas which was to act as an
absorber, and the more remote as a radiator.
Heat is evolved in air when its motion is arrested; on entering an
exhausted tube, the more rapid the motion the greater the heat. Both
chambers of the cylinder were at first filled with the vapour to be
examined, the usual pressure being the 1/60 part of an atmosphere. But
the vapour entered so slowly, and the quantity was so small that the
radiation due to the warming of the vapour by its own collision was
insensible. The needle of the goniometer being at zero, dry air was
allowed to enter the chamber most distant from the pile; this air became
heated dynamically by the collision of its particles against the sides
of the tube, communicated its heat to the vapour, and the vapour
immediately discharged the heat thus communicated to it against the
pile. This case not only resembles, but is actually of the same
mechanical character as, that in which a vibrating tuning fork is
brought into contact with a surface of some extent. The fork, which
before was inaudible, becomes at once a copious source of sound. What
the sounding board is to the fork, the compound molecule is to the
elementary atom. The tuning fork vibrating alone is in the condition of
the atom radiating alone; the sound of the one and the heat of the other
being insensible. But in association with sulphuric or acetic ether
vapour the elementary atom is in the condition of the tuning fork
applied to its sounding board, communicating motion to the luminiferous
ether through the molecules, as the fork through the board communicates
its motion to the air.
Mr. Tyndall’s experiments show the great opacity of a gas to radiations
from the same gas, and may likewise show the remarkable influence of
attenuation in the case of vapour. The individual molecules of a vapour
may be powerful absorbers and radiators, but in their strata they
constitute an open sieve through which a great quantity of radiant heat
may pass. In such thin strata, therefore, the vapours as used in the
experiments were generally found far less energetic than the gases,
while in thick strata the same vapours showed an energy greatly superior
to the same gases, but the gases were always employed at a pressure of
one atmosphere.
Lastly Mr. Tyndall examined the diathermancy of the liquids from which
his vapours were derived, and the result leaves not a doubt that both
absorption and radiation are phenomena irrespective of aggregation. If
any vapour is a strong absorber and radiator, the liquid from whence it
comes is also a strong absorber and radiator.
Perfectly dry pure air is as pervious to light and heat as a vacuum
itself; consequently, if the atmosphere was quite pure and dry, the rays
of the sun would fall on the earth with unmitigated force during the
day, and would be radiated back again and dissipated in space during the
night to the destruction of vegetation. But the earth is protected from
these extremes by the absorptive power of aqueous vapour, which is
always present more or less in the atmosphere; even when the air is so
transparent that distant objects seem to be near, it is loaded with
vapour in an elastic invisible state, which a change of temperature may
condense into cloud or precipitate in rain.
The absorptive power of aqueous vapour was determined by placing tubes
containing fragments of glass moistened with water between the drying
apparatus and the experimental glass tube of the instrument, so that
perfectly pure dry air in passing over the wet fragments of glass
carried a portion of aqueous vapour with it into the exhausted
experimental tube, and the deflection of the needle of the goniometer
showed that the absorptive power of the aqueous vapour exceeded that of
the dry air 80 times. Now since in the atmosphere there is one molecule
of aqueous vapour with an absorptive power of 80 for every 200 atoms of
oxygen and nitrogen whose absorptive power is 1 like that of one of its
constituent atoms, it follows by comparison that the absorptive power of
the molecule is 16,000 times greater than that of an atom of either
oxygen or nitrogen. From this enormous opacity to obscure heat ‘it is
certain that more than ten per cent. of the terrestrial radiation from
the soil of England is stopped within ten feet of the surface of the
soil; remove for a single summer night the aqueous vapour from the air
which overspreads the country, and you would assuredly destroy every
plant capable of being destroyed by a freezing temperature.’
The quantity of vapour in each place varies with the latitude, the
season, and other circumstances; but whenever the amount of heat
radiated from the earth surpasses the absorption, the remainder passes
through the vapour into space, and for the same reason the residue of
that coming from the sun passes through the vapour and comes to the
earth, so that whatever may be the local differences it has been
decidedly proved with regard to the whole globe, that the quantity of
heat annually received from the sun is annually radiated into space; the
latter is a force lost to the earth, nevertheless it does not interfere
with the law of the conservation of force which extends to the universe.
By observations made during ten scientific ascents in a balloon to very
great altitudes, Mr. Glaisher has proved, that the theory of the uniform
decrease of temperature with increase of elevation is no longer tenable.
Since the absorptive force of aqueous vapour is 16,000 times that of dry
air, the whole of the heat radiated by the full moon is intercepted by
our atmosphere. It raises the temperature of the higher regions,
dissolves the vapour, dissipates the clouds, prevents the formation of
more, and allows the heat radiated from the earth to pass freely into
space: thus confirming the common, and almost universal, belief that the
full moon dispels the clouds. The absorptive power of aqueous vapour is
so enormous that even the planet Mercury may be habitable should his
atmosphere contain a sufficient quantity of it to mitigate the heat of
the sun.
No doubt all the heat from the stars must be absorbed by the atmosphere,
but their photographs show that it is pervious to the chemical rays.
Those from Sirius, the nearest and brightest of the stars, travelling
through 180 millions of millions of miles and decreasing in quantity
inversely as the square of the distance, still have sufficient energy to
give a perfect photographic impression of its spectrum; but Sirius is
sixty times larger than the sun, and is many times more luminous. A
photograph of the spectrum of Capella has been taken, though three times
more distant than Sirius. Photographs of double stars of the sixth and
seventh magnitude show that actinic rays from immeasurable distances in
space have power sufficient to decompose matter in unstable equilibrium
on the surface of the earth.
The chemical power of the moon’s light only surpasses that of Jupiter in
the ratio of 6 to 4 or 5, and Jupiter’s light has twelve times more
actinic energy than that of Saturn. For such comparisons a standard of
photographic intensity is requisite.
A paper coated with chloride of silver can be prepared which has a
constant degree of sensitiveness, and Dr. Roscoe has proved that a
constant dark tint is produced on this standard paper by a constant
quantity of light, the tint being the same, whether light of the
intensity represented by 1 acts for the time represented by 50, or light
represented by 50 acts for the time represented by 1; or in other words
the amount of the chemical action of light is directly proportional to
the intensity of the light, and when the light is constant, the amount
of action is exactly proportional to the time of exposure.
The ratio of the chemical action of the rays of light falling directly
from the sun to the chemical action of the light diffused over the whole
sky can be determined by means of an instrument, in which the shadow of
a little ball is made to fall on a sensitive paper so as to intercept
the direct rays of the sun, and allow it to be impressed by an action of
the light diffused over sky alone; this compared with a similar paper,
on which both the direct and indirect light has fallen, gives the ratio
required. From this it appears, that the relative amount of chemically
active light which comes directly from the sun, is very much less than
the amount of his direct visible light. For while Professor Roscoe was
making experiments at Manchester on the maximum effect of the chemical
action of light, he found when the sun had an altitude of 20°, that of
100 chemical rays which fell on a piece of standard paper, only about 8
came from the direct light of the sun; while on the contrary, of 100
rays of visible light, 66 came directly from the sun, and only 40 from
the light diffused over the whole sky, so that the diffused light is
richer in chemical rays than the direct solar beam, ‘a startling
result,’ but borne out by observations not only made at Manchester and
in its vicinity, but at Kew, Heidelberg, and at Pará on the Amazon
nearly under the equator.
On account of the increasing rarity of the atmosphere, the greater the
height above the level of the sea, the less the amount of diffused light
and consequently of actinic power. Hence photographers have to expose
their plates for a much longer time to the light on the snowy peaks of
the Alps and other great heights than in England or at the level of the
sea. During Mr. Glaisher’s tenth balloon ascent simultaneous
observations were made at Greenwich Observatory and in the balloon, when
at more than three miles above the surface of the earth, the standard
paper exposed to the full rays of the sun was not as much coloured in
half an hour as the corresponding paper at Greenwich in one minute.
By a series of observations at Heidelberg, Kew, and Manchester, it has
been proved that the very small relative chemical action of the sun’s
direct light decreases rapidly with his altitude, and at these three
places of observation, it has frequently happened when the sun’s
altitude was very low, as at 12°, that his direct light made no
impression on a sensitive paper. ‘The sun’s light had been robbed of its
chemical power in passing through the air.’ This singular result is
ascribed by Professor Roscoe to what he calls the opalescence of the
atmosphere.
Opalescent glass, slightly milky liquids, pure water with particles of
sulphur floating in it, are impervious to the chemical rays, whence
Professor Roscoe infers that the atmosphere, more especially its lower
regions, possesses that property in consequence of multitudes of solid
particles floating in it. What they are is unknown, but infinitesimal
particles of soda seem to be everywhere, and no doubt particles of other
substances mixed with them may be often seen as motes dancing in the
sunbeams. Besides, it is clearly proved that myriads of the eggs and
germs of organized beings, though invisible to the naked eye, are
continually floating in the air, and that they are more abundant in the
lower than in the higher strata of the atmosphere. Since opalescent
matter reflects the blue rays of light and transmits the red, Professor
Roscoe ascribes the blue colour of the sky and the bright tints at
sunrise and sunset to the opalescent property of the air.
The atmosphere is permeable to every kind of chemical rays, which is far
from being the case with bodies on earth, some of which though
transparent to all the visible rays, vary greatly in their transparency
to the chemical rays.
The atoms and molecules of matter not only have the power of turning the
rays of the solar beam out of their rectilinear path, but of changing
their refrangibility.
The myriads of ethereal waves or rays of light that constitute the seven
colours of the solar spectrum, decrease in refrangibility and increase
in rapidity of vibration and length of wave from the extreme violet to
the end of the red; each ray having its own rate of vibration, its own
length of wave, and its own colour. From the middle of the yellow, which
is the luminous part of the spectrum, the chemical spectrum extends
invisibly, but with increasing refrangibility and increasing velocity of
vibration, to a point far beyond the violet. On the contrary, the heat
spectrum, which may also be said to begin in the yellow light, extends
invisibly but with decreasing refrangibility, and decreasing velocity of
vibration to some distance beyond the visible red.
The rays of heat are absorbed by the humours of the eye, but were they
to reach the retina we should see that they differ from one another as
much as those of the luminous spectrum; the chemical spectrum from its
greater length is still more diversified.
The whole of the solar spectrum, visible and invisible, is crossed at
right angles to its length by innumerable dark rayless lines, differing
in breadth and intensity. Sir John Herschel discovered vacant spaces in
the extra-luminous part of the heat spectrum, and more recently M.
Edouard Becquerel, by throwing the solar spectrum upon a daguerreotype
plate, discovered that the chemical spectrum given by a glass prism,
from its beginning in the yellow to its extreme point beyond the violet,
is crossed by rayless lines, and that the lines in the part passing
through the visible spectrum coincide exactly with the rayless lines in
the luminous part. This coincidence was confirmed by the independent
researches of Dr. Draper at New York. By means of the rayless spaces or
black lines in the visible spectrum, M. Kirchhoff has proved that
thirteen terrestrial substances are constituents of the sun’s
atmosphere.
The length of the undulations of the ether which produce the impression
of the extreme violet rays of the solar spectrum on our eyes, is the
seventeen millionth part of an inch; the length of the ethereal
undulation that produces the sensation of the extreme red is the
twenty-six millionth part of an inch; the ethereal undulations beyond
these limits are invisible to human eyes. Nevertheless certain
substances have the power of increasing the length of the vibrations,
and reducing the rays of the spectrum to a lower grade in the scale of
refrangibility, so that the invisible rays of the chemical spectrum have
thus been brought within the limit of human vision.
For example, the chemical rays shine as visible light when they fall on
glass tinged with the oxide of uranium. When these dark rays fall upon
the glass, they put the whole of its molecules into vibrations, the same
with their own, while at the same time they give a more rapid vibration
to a certain number of the same molecules. The whole of the molecules
restore their vibrations to the surrounding ether. Those having the same
velocity with the chemical rays make no sensible impression on our eyes;
but the more rapid vibrations come within the limits of the visible
spectrum; they have consequently a lower refrangibility, and shine as
visible light. It is called degraded light on account of its lower
position in the prismatic scale, but more frequently fluorescent light,
because fluor spar was the first solid known to possess the property. A
number of substances are fluorescent, both solid and liquid, organic and
inorganic.
If in a dark room a non-fluorescent body be illuminated by a sunbeam
passing through glass stained deep blue by cobalt, it will reflect blue
light; but it will appear to be perfectly black if it be viewed through
glass tinged yellow by silver; while a piece of canary glass, which is
highly fluorescent, will shine with a vivid light under the same
circumstances. All the molecules of the canary glass give back to the
ether the undulations that have been impressed on them by the blue
light; while a certain number of them possess the power of receiving and
giving back more rapid vibrations to the ether. The yellow glass held
before the eye is impervious to the undulations of the blue rays, but
transmits those of the fluorescent light, which emanate from the smaller
number of molecules, and which thus become in reality new centres of
light, different from the sun’s light, though dependent upon it: the one
terrestrial, the other celestial. Since the vibrations of the
fluorescent light are more rapid than those of the blue light their
colour is lower in the prismatic scale. The vibrations of the molecules
in a fluorescent substance are analogous to those of a musical cord,
which give the fundamental note or pitch and its harmonics, for the
whole of the musical cord while vibrating the fundamental note divides
itself spontaneously into parts having more rapid vibrations, which give
the harmonics. Professor Stokes of Cambridge, who made this beautiful
experiment, computed that the vibrations which produced the fluorescent
light were a major or minor third below the pitch or vibrations of the
blue light.
One of the first discoveries of fluorescence was made by Sir John
Herschel—certainly the first who observed the property in a liquid. He
found that the blue light which emanates from all parts of a solution of
the sulphate of quinine, especially from its surface, is fluorescent,
and that the light transmitted through the liquid, though sensibly like
the incident white light, is no longer capable of producing
fluorescence; it has been deprived of its chemical rays by absorption.
The chemical rays having been rendered visible by an increase in the
length of the periods of vibration, unsuccessful attempts have been made
to change the periods of the rays of heat beyond the red end of the
spectrum so as to bring them within the limits of vision. The idea of
effecting such a change by employing a substance opaque to light, but
pervious to heat, is due to Dr. Akin; but it has since been accomplished
by Dr. Tyndall, who, in the course of his experiments on radiant heat,
found that a solution of iodine in the bisulphide of carbon excludes the
most dazzling light, but transmits the rays of heat freely. He employed
a mirror, lined in front with silver, to concentrate the rays emitted
from the charcoal points of the electric lamp, and interposed a vessel
containing the solution in question, so that the rays of heat alone were
brought to a focus almost undiminished. When the solar spectrum was
examined, the point of maximum heat was found to be as far beyond the
extreme red on one side as the green rays on the other. In the spectrum
of the electrical light the point of maximum heat was also found to lie
beyond the extreme red, but the augmentation of intensity was so sudden
and enormous as far to exceed the maximum heat of the sun previously
determined by Professor Müller. Aqueous vapour powerfully absorbs
radiant heat; so a solar spectrum beyond the earth’s atmosphere might
probably exhibit as great intensity as the electrical light. With the
apparatus described oxidizable substances burst into the flame of common
combustion when put into the focus; but when the chemical action of the
oxygen of the atmosphere was excluded by igniting substances in vacuo by
the invisible rays of heat, their periods of vibration were so changed
as to bring them within the limits of vision. When the electric light is
very powerful, a plate of platinized platinum in vacuo is raised to
white heat at the focus of invisible rays; and when the incandescent
platinum is looked at through a prism, its light yields a complete and
brilliant spectrum. ‘In all these cases we have a perfectly invisible
image of the charcoal points formed by the mirror; and no experiment
illustrates the change of heat into light’ more strongly than the
following:—When the plate of platinum or one of charcoal is placed in
the focus, the invisible image raises it to incandescence, and thus
prints itself visibly on the plate. On drawing the coal points of the
lamp apart, or causing them to approach each other, the thermograph
follows their motion. By cutting the plate of carbon along the boundary
of the thermograph, a second pair of coal points may be formed of the
same shape as the original ones, but turned upside down; and thus by the
rays of the one pair of coal points which are incompetent to excite
vision, we may cause a second pair to emit all the rays of the spectrum.
Fluorescence and calorescence act in contrary directions. Fluorescence
causes the molecules of a fluorescent substance to oscillate in slower
periods than the incident light, while calorescence causes the molecules
of a substance to oscillate in longer periods than the incident light.
The refrangibility of the rays is lowered in the first case, and raised
in the second.
Substances differ as much in their transmission of the chemical rays as
those of light and heat. Glass is impervious to the most highly
refrangible chemical rays, while rock crystal transmits them with the
greatest facility; and on that account the absolute length of the
spectrum was not known till the light was refracted by prisms of rock
crystal. Besides, the number, position, and intensity of the chemical
rays vary with the source of light. Some flames have scarcely any
chemical rays; that of the oxy-hydrogen blowpipe, though intensely hot,
has very few, and even the solar light is inferior in that respect to
electricity. The electric spark from the prime conductor of a common
electrifying machine, or the discharge of a Leyden jar, emits rays of
very high refrangibility, far surpassing those which emanate from the
sun. For, when the electric light from a highly charged Leyden jar was
refracted by two quartz prisms and thrown by Professor Stokes on a plate
of uranium glass, the chemical spectrum was highly luminous, and six or
eight times as long as the visible spectrum. An equally extensive
spectrum was obtained from the voltaic arc taken between copper points;
it consisted entirely of bright lines. The long spectrum also appeared
on the uranium glass when the spark refracted by quartz prisms was
obtained from the secondary terminals of an induction coil in connection
with the coatings of a Leyden jar. It consisted of bright lines, but was
not so luminous as that from a powerful voltaic battery. On changing the
metals of the points between which the sparks passed, the bright lines
were changed, which showed that they were due to the particular metals.
The heat of the electric spark volatilizes the metals which form the
points of the conducting wires; and all volatilized metals give
characteristic spectra, both visible and chemical. The visible part
differs from that of the solar spectrum in being crossed by bright lines
instead of dark ones; but the number, intensity, and position of both
the visible and invisible lines change with each metal. The changes in
the invisible part under consideration may be readily observed by
throwing the spectra either on a fluorescent or collodion plate. For
example: in the spectrum from the spark between thallium points thrown
on the latter, Dr. Miller found that there were two strong groups of
lines in the least refrangible part of the spectrum; at a little
distance from these there were three groups, the two first less intense
than the third; several rows of feeble dots followed, and the chemical
spectrum terminated rather abruptly with four nearly equidistant groups.
This spectrum bears a resemblance to those of zinc and cadmium, less
strongly to that of lead. Dr. Miller found that the photographic spectra
of iron, cobalt, and nickel, also have a strong analogy, but that the
metals arsenic, antimony, and tin showed as great a difference in the
invisible as in the visible part of their spectrum.
The fluorescent spectra of seventeen metals were examined by Professor
Stokes of Cambridge; several of them showed luminous lines of
extraordinary strength, especially zinc, cadmium, magnesium, aluminium,
and lead, which in a spectrum not generally remarkable contains one line
surpassing perhaps all other metals in brilliancy. Some other metals
exhibit in certain parts of their spectra lines that are both bright and
numerous; on the whole some parts of the spectra are strong and
tolerably continuous, while in others they are weak. This grouping of
the lines is most remarkable in copper, nickel, cobalt, iron, and tin.
Of all the metals examined, magnesium gave the shortest spectrum, ending
in a very bright line, beyond which however excessively faint light
extended to a distance equal to that of the long spectra. Aluminium, on
the other hand exceeded all the other metals in richness of the rays of
the very highest refrangibility. All the strong lines mentioned lie in
that part of the spectra.
In the course of these experiments Professor Stokes observed that even
quartz of a certain thickness is not transparent to invisible lines of
the highest refrangibility, for the highest aluminium line, which is
double, could only be seen by rays passing through the edge of the
prism. This leads to another branch of the subject, namely, the
absorption of the invisible rays by solids, liquids and gases. Mr. Wm.
Allen Miller has shown from his own experiments that bodies pervious to
the chemical rays in the solid form, are so also in the liquid and
gaseous form; that colourless transparent solids which absorb the
photographic rays, absorb them more or less also in their liquid and
gaseous states. He has moreover found that the following substances have
the same maximum transparency:—rock crystal, ice, and fluor spar among
solids, water among liquids, the three elementary gases and carbonic
acid among gaseous substances. The most opaque to the invisible rays
are, nitrate of potash, bisulphide of carbon, and sulphuretted hydrogen.
It appears that a thin plate of mica is intensely opaque to all the
invisible rays except a small portion of them of the lowest
refrangibility.
The absorptive property however is partial: an absorptive substance
either cuts off a portion of the light of a fluorescent spectrum or
stripes it with dark lines: each substance absorbs rays peculiar to
itself. Those employed by Professor Stokes were the alkaloids and
glucosides, and he assumed the spectrum of tin for their examination
because it has a long interval of continuity.
The fluorescent property of yellow uranite was discovered by Professor
Stokes some years ago, and now he has added another fluorescent mineral
in adularia or moonstone; from its natural faces and planes of cleavage
alike a beautiful blue fluorescence emanated under the induction spark.
As the same was observed in colourless felspars generally, Professor
Stokes concluded that fluorescence is an inherent property in the
silicate of alumina and potash constituting the crystal of moonstone.
The blue fluorescence extended to a very sensible though small depth
within the substance.
A particular variety of fluor spar found at Alston Moor in Cumberland,
which is very pale by transmitted light, shows a strong blue
fluorescence, and is eminently phosphorescent on exposure to the
electric spark. It is the same kind of crystal in which Sir David
Brewster originally discovered the property of fluorescence. On
presenting such a crystal to the spark passing between aluminium
terminals, besides the usual blue fluorescence, there was another of a
reddish colour extending not near so far into the crystal, produced by
the rays belonging to the strong lines of aluminium of extreme
refrangibility.
The cube of fluor spar which showed these effects was externally
colourless to the depth of 1/20 of an inch; then came one or two strata
parallel to the faces of the cube showing the ruddy fluorescence, while
the blue fluorescence extended to a much greater depth and had a
stratified appearance. This crystal was eminently phosphorescent, its
blue phosphorescence being arranged in strata parallel to the face of
the cube like the blue fluorescence, but it was not perceptible beyond a
very moderate distance below the surface at which the exciting cause
entered, that cause being the photographic rays of extremely high
refrangibility of the electric spark—taken, as in all these experiments,
between the secondary terminals of an induction coil in connection with
the coatings of a Leyden jar, and refracted by quartz prisms.
Mr. Stokes has employed fluorescence as a means of tracing substances in
impure chemical solutions. When a pure fluorescent substance is examined
in a pure spectrum it is found that on passing from the extreme red to
the violet and beyond, the fluorescence commences at a certain point of
the spectrum, varying from one substance to another, and continues from
thence onwards more or less strongly in one part or another according to
the particular substance. The colour of the fluorescent light is found
to be nearly constant throughout the spectrum. ‘Hence when in a solution
examined in a pure spectrum we notice the fluorescence taking as it were
a fresh start _with a different colour_, we may be pretty sure that we
have to deal with a mixture of two fluorescent substances.’
Experience as well as theory shows that rapid absorption is accompanied
by copious fluorescence. But experience has hitherto also shown what
could not have been predicted, and may not be universally true, that
conversely absorption is accompanied in the case of a fluorescent
substance by fluorescence.
The phosphorescent light of insects, fish, and plants is owing to
chemical action, which produces many luminous phenomena; but a great
number of inorganic and organic substances shine in the dark with a
phosphorescence which is nearly allied to fluorescence. It is produced
by exposure to the sun, by heat, electricity, insulation, cleavage,
friction, and motion. For if a bottle containing nitrate of uranium be
shaken, it shines spontaneously with a vivid light; even the hand shows
phosphorescence in the dark after being exposed to the sun.
The essential difference between fluorescence and phosphorescence
consists in the time during which the light lasts. Fluorescence ceases
almost immediately after the exciting cause is withdrawn, while a
phosphorescent body whether excited by heat, solar light, or
electricity, lasts a much longer time; besides, the fluorescent rays are
generally of lower refrangibility. Light and heat are temporarily
absorbed and given out again by every body on the surface of the earth,
more or less, that are exposed to the sun’s light. The nights would be
much darker even when illuminated by the stars were it not for earth
light, for the molecules restore to the ether, in the form of
phosphorescence, the undulations they have received from the sun’s light
during the day. The snow and ice blink of the sailors is a striking
instance; generally, however, it is of much shorter duration. The
phosphorescent property is nearly allied to electricity, for bodies that
are bad conductors are apt to become phosphorescent, while good
conductors of electricity rarely if ever show it. Ozone must be
phosphorescent, for oxygen exhibits persistent light when electric
discharges are sent through it, and Mr. Faraday saw a flash of lightning
leave a luminous trace on a cloud which lasted for a short time.
In the solar spectrum the chemical or actinic rays produce
phosphorescence, which the red rays have the power to extinguish. M.
Nièpce de St-Victor found that solar light impresses its vibrations so
strongly on substances exposed for a short time to its influence that
they not only shine in the dark, but that the phosphorescent light they
radiate has chemical energy enough to decompose substances in unstable
equilibrium, and leave daguerreotype impressions of great delicacy and
beauty.
The polarization of light and heat affords a remarkable instance of the
elective power of matter. Light and heat are said to be polarized,
which, having been once reflected, are rendered incapable of being again
reflected at certain angles. For example, a ray incident on a plate of
flint glass at an angle of 57° is rendered totally incapable of being
reflected at that same angle from another plate of flint glass in a
plane at right angles to the first. At the incidence of 57° the whole of
the ray is polarized: it is the maximum of polarization for flint glass,
but there is a partial polarization for every other angle; the portion
of the ray polarized increases gradually up to the maximum, as the
incidence approaches to 57°. All reflecting surfaces are capable of
polarizing light and heat, but the angle of incidence at which the ray
is totally polarized is different in each substance. Thus, the angle of
incidence for the maximum polarization of crown glass is 56° 55ʹ, and no
ray can be totally polarized by reflection from the surface of water
unless the angle of incidence is 53° 11ʹ. As each substance has its own
maximum polarizing angle, the effect is evidently owing to the action of
the molecules of matter, and not to any peculiarity in the light or
heat.[8]
Light and heat are also polarized by refraction, for certain substances,
especially irregularly crystallised minerals like Iceland spar, possess
the property of dividing a ray of light or heat passing through them in
certain directions into two pencils, namely, the ordinary and
extraordinary rays. The first of these is refracted according to the
same law as in glass or water, never quitting the plane perpendicular to
the refracting surface, while the second does quit that plane, being
refracted according to a different and more complicated law. Hence, if a
crystal of Iceland spar be held to the eye, two images of the same
object will generally be seen of equal brightness. But when they are
viewed through a plate of tourmaline it will be found that while the
spar remains in the same position the images vary in relative brightness
as the tourmaline is made to revolve in the same plane; one increases in
intensity till it arrives at a maximum, at the same time that the other
diminishes till it vanishes, and so on alternately at each quarter
revolution of the tourmaline, proving both rays to be polarized. For in
one position the tourmaline transmits the ordinary ray and reflects the
extraordinary, and after revolving 90°, the extraordinary ray is
transmitted and the ordinary ray is reflected.
The undulations of the ethereal medium which produce the sensation of
common light, are performed in every plane at right angles to the
direction in which the ray is moving, but the case is very different
after the ray has been polarized by passing through a substance like
Iceland spar, for the light then proceeds in two parallel pencils whose
undulations are still indeed transverse to the direction of the rays,
but they are accomplished in planes at right angles to one another. The
ray of common light is like a round rod, whereas the parallel polarized
rays resemble two long flat rulers, one of which lies on its broad
surface, and the other on its edge. By a simple mechanical law, each
vibratory motion of the common light is resolved into vibratory motions
at right angles to one another.
The polarization of light and heat by refraction is not owing to the
chemical composition, but to a want of homogeneity in the molecular
structure of the substances through which they pass; for regular
crystals and substances which are throughout of the same temperature,
density, and structure, are incapable of double refraction. The effect
of molecular structure is strikingly exhibited by the circular
polarisation in the dimorphic crystals of quartz. In one form the plane
of polarization revolves from right to left, and in the other that plane
revolves from left to right, although the crystals themselves differ
apparently by a very slight and often almost imperceptible variety of
forms.
Thus polarization forms the most admirable connection between light,
heat, and crystalline structure; showing peculiar arrangements of the
molecules in regions otherwise unapproachable, and too refined for our
perceptions. Besides, the gorgeously coloured images displayed by
depolarization are splendid examples of the power of matter in
decomposing light.
The perfect correspondence of the properties of the symmetrical,
elastic, and optical axes of crystals with light and heat is another
instance of the connection between the latter and crystalline form.
The axis of symmetry is that direction or imaginary line within a
crystal, round which all the parts or particles are symmetrically
arranged. A medium is said to be elastic which returns to its original
form with a resilient force after being relieved from compression, and
the axis of elasticity of a crystal is that direction in which it is
most elastic. The optic axis is that line or direction through which
light passes in one beam according to the law of ordinary refraction.
Crystals may have one, two or more optical axes according to their form.
Doubly refracting crystals such as Iceland spar have only one principal
optic axis in which the whole beam passes according to the ordinary law;
in every other direction the beam of light is divided into two polarized
rays, one of which called the ordinary ray passes according to the
ordinary law, while the other, known as the extraordinary ray, traverses
the crystal in a different direction, with more rapidity and according
to a different and more complicated law. The velocity of this
extraordinary ray is a maximum when at right angles to the principal
optical axis, and a minimum when parallel to it.
In perfectly regular crystals like the cube or die, the octohedron, &c.,
there are three axes of symmetry and of equal elasticity at right angles
to one another. In these regular crystals all the axes are optical, so
that they have no double refraction.
Right square prisms have two equal rectangular axes of symmetry, two
axes of equal elasticity, and one optical axis.
All crystals of the pyramidal and rhomboidal systems have one axis of
symmetry, two axes of elasticity, one optical axis; and form coloured
circular rings traversed by a black cross when viewed by depolarized
light.
Lastly oblique prismatic crystals which have three unequal axes of
symmetry have three axes of unequal elasticity, two optical axes; and by
depolarization give coloured lamnescata, that is coloured figures having
the form of the figure 8 which are traversed by a black cross in two
opposite quadrants, and when the crystal is made to revolve, the same
figure, but in the complementary colours and traversed by a white cross,
appears in the other two quadrants.
The right and left-handed circular polarization of quartz, according as
certain facettes of the crystal are turned to the right or left, and the
property of double refraction being exclusively possessed by crystals of
the rhomboidal form, are striking instances of the connection between
the geometrical arrangement of the molecules of matter and the optical
and thermal forces, for the polarization of heat and all its
consequences are in every respect analogous to those of light, and
similar phenomena would be seen were heat visible.
Heat changes the position of the optical axes of crystals. When applied
to a crystal of sulphate of lime, the two optical axes gradually
approach to each other and at last coincide; if the heat be continued
and increased, the axes open again, but in a direction at right angles
to their former position. Thus the force of heat throws every molecule
in the body into correlative motion. The angles of all crystals that are
not of the octohedral group are changed by heat and vary with the
intensity; the difference between the length of the greatest and least
optic axes in such crystals diminishes as the temperature is raised,
increases when it is lowered, and is constant at a given heat. In
Iceland spar heat indirectly affects the doubly refracting power, for
the expansion of the crystal in the direction of its axis is accompanied
by contraction at right angles to it, which brings the crystal nearer to
the cubical form, and consequently diminishes its doubly refracting
power.
According to the researches of M. Angström, in crystals with different
axes of elasticity the velocity of the molecular vibrations is different
in different directions when they are heated. In rock crystal and
tourmaline the heat radiates from a surface cut parallel to the axis of
the crystal; in felspar the radiating surface is at right angles to the
symmetrical axis.
The optical axes of crystals are also affected by pressure. Doubly
refracting crystals with one principal axis acquire two when the
pressure is perpendicular to it. The new principal axis coincides with
the line of pressure or is at right angles to it according as the
crystal is positive or negative, that is, according as the extraordinary
ray is refracted to or from the optic axis of the crystal. The colours
produced by polarization are affected by compression and dilatation
according as the crystal is positive or negative.
Sir David Brewster is of opinion that all the properties of double
refraction and the gorgeous phenomena of polarization, whether by
crystals or produced in various substances permanently or transiently by
heat, cold, rapid cooling, compression, dilatation, and induration, are
wholly the result of the forces by which the atoms are held together;
but these phenomena may rather be said to depend upon a reciprocal
action between an irregular molecular structure and the agency of light
and heat: which indeed seems to be confirmed by the transit of these two
forces through right and left-handed quartz, for there is no reason to
believe that there is any difference in the form of the particles in
these two crystalline substances.
The experiments of M. Becquerel show that electricity is a power which
makes the atoms of matter aggregate in crystalline forms; for he has
succeeded in forming crystals of gold, silver, cobalt, nickel, platinum,
and a variety of the gems undistinguishable from those in nature, by
exposing saturated solutions of these substances for a very long time to
feeble voltaic electricity; and crystals of earthy matter have been
obtained in the same manner. The electric and magnetic state of mineral
veins in mines which contain a vast variety of crystals, metallic and
non-metallic, strongly favours this view of the origin of crystalline
form.
M. Regnault has proved that the ratio between the specific heat and the
weight of the atoms of matter is intimately connected with the mode of
their aggregation; and indeed if it be considered that the atoms have
not only specific heat and weight, but specific affinity, electricity,
magnetism, consequently polarity, and probably specific forms, these
peculiar forces must necessarily influence the structure of crystals
according as they combine with or oppose the natural or artificial
forces acting upon them, or upon their dissimilar faces, and this may be
the cause of the great variety of forms that matter appears under.
Carbonate of lime alone assumes more than 1,200 different modifications
of its primitive type, but whatever be the variety of forms which any
one substance may take, they are found to be all compatible with and
derivative from a common type. The circumstances which have caused
dimorphous crystals to deviate from the general law have not yet been
explained.
It is very singular that when chlorate of soda is dissolved in water the
solution does not possess the property of circular polarization, but
when evaporated and allowed to crystallise, some of the crystals turn
light to the right, and others to the left. Now if all the crystals that
have the same property be picked out and dissolved in water a second
time, the liquid will still have no circular polarization, but when
allowed to crystallise, some of the crystals make light revolve through
them to the right and others to the left as before. From this it is
supposed that the atoms of liquids, which are free to move in every
direction, already possess part of the characters which the change to
solidity renders evident and permanent.
Although the relations between the force of magnetism and the atoms of
matter do not exhibit such brilliant phenomena as light does, they are
nevertheless most interesting and wonderful. Mr. Faraday discovered that
all substances, whether solid, liquid, or aëriform, are either magnetic
like iron, or diamagnetic like bismuth, the latter being by far the most
numerous. Thus if a bar of iron be freely suspended between the poles of
an extremely powerful magnet or electro-magnet, it will be attracted by
both poles and will rest or sit axially, that is, with its length
between the poles or in the line of magnetic force; whereas an equal and
similar bar of bismuth so suspended will be repelled by both poles and
will rest or sit equatorially, that is with its length perpendicular to
the line of magnetic force. Magnetism and diamagnetism are both dual
forces, but they are in complete antithesis to one another, which is
strikingly illustrated by their action on crystalline matter.
A sphere of amorphous substance freely suspended under magnetic
influence is indifferent, that is to say it has no tendency to set one
way more than another; but a sphere cut out of a crystal whether
magnetic or diamagnetic, is more powerfully attracted or repelled in one
direction than in any other, which shows a connection between the
magnetic forces and crystalline structure.
Crystals of carbonate of iron and carbonate of lime are isomorphous,
that is, they have exactly the same crystalline form, but the carbonate
of iron being highly magnetic is most powerfully attracted in the
direction of its greatest optical axis which therefore sets axially,
that is, in the line of magnetic force; while the principal optic axis
of the carbonate of lime, which is diamagnetic, is most powerfully
repelled and therefore sets equatorially. In both cases the antithetic
forces follow the same law of decrease in intensity from the greatest
optical axis to the least.
A bar of soft iron sets with its longest dimensions axially, but a bar
of highly compressed iron-dust, whose shortest dimensions coincide with
the line of pressure, sets equatorially, because it is most powerfully
attracted in the line of greatest density. A bar of bismuth sets
equatorially, but a bar of highly compressed bismuth dust, whose
shortest dimensions coincide with the line of pressure, sets with its
length axially, because it is most strongly repelled in the direction of
its greatest density. Hence the action of magnets upon matter is most
powerful in the line of maximum density, the force being attractive or
repulsive according to the kind of magnetism possessed by the atoms. It
follows therefore that the density is greatest in the line of the
principal optical axis, and gradually decreases to the least optical
axis, where it is a minimum.
The position which crystals take with regard to the magnetic force
depends also upon their natural joints of cleavages, and upon their
power of transmitting electricity. The diamagnetic force is inversely as
the conducting power of bodies, and the conducting power of crystals is
a maximum in the planes of their principal natural joints. Hence the
action of the diamagnetic power is least in the natural joints, and
conversely the magnetic force is greatest. In fact, the magnetic
phenomena of crystals depends upon unequal conductibility in different
directions, and their set is determined by the difference between the
forces of attraction and repulsion of the poles, for one pole of the
magne-crystallic axis is attracted and the other repelled. It is
unnecessary to give more examples to show the action of the magnetic
forces upon the atomic structure of crystals.[9]
Magnetism changes the relations and distances between the ultimate atoms
of matter, a circumstance which probably depends upon their polarity. It
changes steel permanently, iron temporarily, and it elongates a bar of
iron, which loses in breadth what it gains in length; and as heat is
developed in one direction and absorbed in the other, the temperature of
the bar remains the same. Heat being an expansive force, diminishes the
magnetism of iron and nickel in proportion as it increases the distance
between their atoms, till at length they lose their cohesive force
altogether. But there seems to be a temperature at which the magnetic
force is a maximum, above and below which temperature it diminishes.
Thus the magnetism of cobalt increases with the temperature up to a
certain point; it then decreases as the temperature increases, and it
loses its magnetism altogether when the heat amounts to 1996°.
Sir Humphry Davy and M. Arago noticed that the voltaic arc takes a
rotatory motion on the approach of a magnet; and the effect of magnetism
on the stratified appearance of the electric light in highly rarefied
air shows how powerful its action is. In the year 1858, Mr. Gassiot
published a series of observations on stratified light; subsequently
various publications appeared on the subject both by Mr. Gassiot and by
Professor Plücker, who made a series of very interesting observations on
the nature of the stratifications, but more especially on the effects
produced when they are under the influences of magnetism. Since that
time, Mr. Gassiot has published several papers on the subject, and still
continues his experiments on the stratifications of electric light,
which give a visible proof of the connection between electricity and
magnetism. He first showed that the stratified character of the electric
discharge through highly attenuated media is remarkably developed in the
Torricellian vacuum; latterly he has made his experiments by passing
electricity through closed glass tubes of various lengths and internal
diameters, filled with highly attenuated gases and vapours.[10] Two
among the many brilliant experiments of this gentleman may be selected
as illustrations of the property of electric light.
One of these closed glass tubes containing a highly attenuated gas was
38 inches long with an internal diameter of about an inch, and had the
extremities of two platinum wires fused into the same side 32 inches
apart. When these wires were put in connection with the wires of an
induction battery and brought into contact, and the electricity passed
through the tube, the luminous appearances at the extremities or poles
of the platinum wires were very different, but simultaneous. A glow
surrounded the negative pole, and in close approximation to the glow, a
well defined black space appeared, while from the positive pole there
issued in rapid succession a series of alternate dark and brilliantly
luminous curved strata, which formed a column of stratified light, the
concavities of the strata being turned to the positive pole. The
stratifications do not extend to the black band round the negative wire
or ball, which is quite different to the dark intervening space between
the stratified discharge and the luminous negative glow. On making and
breaking the electric circuit, the stratified discharge emanates from
each pole alternately, the concavities of the strata turning alternately
in different directions; in fact the whole phenomena are reversed, but
not changed. ‘The stratified discharge arises from the impulses of a
force acting on highly attenuated but resisting media,’ a new proof of
the wonderful power inherent in highly attenuated gases; the number of
stratifications given out at each discharge, depending upon the
intensity of the electricity and rarity of the gas.
[Illustration: Fig. 1.]
Fig. 1 represents the form which the stratified discharge assumes in a
vacuum tube one inch diameter and 38 inches in length, + and -
representing platinum wires attached to the terminals of a Ruhmkorff’s
induction coil.
When the tube, with its stratifications just described, was laid
horizontally on the pole of a magnet, the stratified column showed a
tendency to rotate as a whole round it. According to the theory of
Ampère, the polarity of a magnet is owing to a superficial current of
electricity perpetually circulating in a direction perpendicular to its
axis; and he also showed that currents of electricity flowing in the
same direction attract one another, while currents flowing in opposite
directions repel each other. Hence, since the currents of electricity in
the magnet and tube were flowing in the same direction on one side of
the magnet, and in opposite directions on the other side, the stratified
column was attracted at one end and repelled at the other, so as to take
the form [sideways S], in consequence of its tendency to rotate as a
whole round the pole of the magnet.
When narrow bands of tin foil wrapped round the glass tube near the
platinum wires were put in communication with the poles of the induction
battery, brilliant stratifications filled the whole tube between the tin
coatings every time the electric circuit was broken or renewed; and when
the tube was placed horizontally on the pole of a magnet, the
stratifications no longer showed a tendency to rotate as a whole, they
were divided into two parts tending to rotate in opposite directions;
when the tube was placed between the poles of a powerful electro-magnet,
one half of the stratifications were repelled and the other half
attracted. When the tube was placed on the north pole, the divided
stratifications arranged themselves on each side of the tube, changing
their respective positions when placed on the south pole, but in every
case each half was concave in opposite directions.
Fig. 2 (p. 81) represents the form which the induced stratified
discharges assume when the vacuum tube is placed on or between the poles
of a powerful electro-magnet—the tin foil coatings C+ C- being attached
by wires to the terminals of an induction coil.
[Illustration: Fig. 2.]
If a vacuum tube with or without wires or tin coatings be laid upon the
induction coil of a battery, or upon the prime conductor of an
electrifying machine, stratifications are produced by induction which
are divided by a magnet. Thus there are two distinct forms of the
stratified discharge, one direct, the other induced.
When Professor Plücker of Bonn sent an induced current of electricity
from Ruhmkorff’s coil through a vacuum tube having a platinum wire fused
into each extremity, and extending a little way into the interior of the
tube, electric light radiated from every point of the negative wire, and
when exposed to the action of an electro-magnet the whole tube was
filled with a luminous atmosphere. But when all the negative platinum
wire except its extreme point was insulated by a coating of glass, the
rays of electric light which radiated from the point were united into
one single and perfectly regular magnetic curve, upon the approach of an
electro-magnet; when the negative platinum wire was partially insulated
by glass coating, electric light emanated from every exposed part, and
assumed the form of magnetic curves under electro-magnetic action.
Whence Professor Plücker concluded that the luminous atmosphere in the
first experiment was the locus of an infinite number of magnetic curves,
and consequently that magnetic light emanates from the negative or
warmth pole, and electric light from the positive or light pole. These
magnetic curves of light are precisely similar to those assumed by iron
filings from magnetic action.
The most remarkable of these experiments is the absolute extinction of a
powerful electric discharge by magnetic action. Mr. Gassiot sent a
discharge from a voltaic water battery, containing 3,520 insulated
cells, into a tube filled with attenuated carbonic acid gas. The
discharge was so strong that it was capable of passing through more than
six inches of the gas, yet, on the approach of a very powerful
electro-magnet, the stratifications were arrested as soon as they
appeared, as if blown out, and finally extinguished. A stratified
discharge, in vacuo, from 400 insulated cells of a nitric acid battery,
was extinguished by the large electro-magnet of the Royal Institution;
the luminous strata rushed from the positive pole of the battery, but
under the magnetic force they retreated; cloud followed after cloud with
deliberate motion, appearing as if swallowed up by the positive
terminal. The amount of electricity that passed through the tube
appeared to be materially increased by exciting the electro-magnet; the
discharge was so intense on one occasion as to fuse half an inch of the
positive terminal. A very powerful magnet is also capable of
extinguishing a stratified discharge. In fact, according to the law of
the reciprocal action of magnetism, the forces are equal in intensity
and opposite in direction.
The electric discharge from an induction coil is discontinuous, or
eruptive sparks of high tension are given out producing stratified
discharges.
The discharge of the voltaic battery had hitherto been considered
absolutely continuous; and so it is for chemical action, whether of
analysis or combination; nevertheless certain phenomena gave reason to
doubt its continuity. Mr. Gassiot has proved that the tension of a
single cell of a galvanic battery increases in force according to the
chemical energy of the exciting liquid, and in all his experiments he
found that ‘the higher the chemical affinities of the elements used, the
greater was the development of evidence of tension.’ These observations
induced him to institute a series of experiments with galvanic batteries
of different chemical affinities, and to compare the resulting phenomena
with those produced by the induction coil, whose sparks are of high
tension. The same carbonic acid vacuum tubes were made use of in all the
experiments; a copper wire formed the positive terminal, and a copper
plate was fixed at the extremity of the negative terminal. In other
tubes platinum terminals extended into the interior, coated with glass,
except the points, to which charcoal balls were fixed. One end of the
tubes was of small diameter and contained caustic potash.
When a discharge from an induction coil was sent through these tubes,
there were either minute luminous spots, narrow stratifications, or a
well defined cloud-like discharge at the positive pole, according to the
size and structure of the terminal, but the characteristic phenomenon in
all the tubes was a large cloud-like luminosity or circular glow on the
brass plate or charcoal ball at the negative terminal.
With 512 insulated cells of copper and zinc of Daniell’s constant
battery, the exciting liquid being dilute sulphuric acid, a brilliant
glow appeared round the charcoal ball of the negative terminal on the
passage of the electric discharge through the tube, with very trifling
luminosity of the positive pole.
Two copper plates that could be separated or closed by a screw, were
placed between the poles of a nitric acid battery, so that the circuit
could be made or broken gradually, and spark discharges were obtained
between them. The vacuum tubes were placed between one of these plates
and a pole of the battery; one of these tubes was 24 inches long, 18 in
circumference, and had a circular copper disc 4 inches in diameter on
its negative terminal. On completing the circuit, the discharge of the
battery passed with a display of magnificent strata of dazzling
brightness; on separating the plates by the screw, the luminous
discharges presented the same appearance as when taken from an induction
coil, but brighter. On the copper disc within the vacuum tube, there was
a white layer, then a dark space about an inch broad, and then a bluish
atmosphere curved like the disc, evidently three negative envelopes on a
great scale. When the disc was made the positive pole, the effect was
feeble.
In vacuum tubes 6 inches long and 1 inch diameter, with carbon balls on
the terminals, the discharge of the nitric acid battery elicits extreme
heat. In one of these the discharge presented a stream of light of
intolerable brightness, but when viewed through a plate of green glass
strata could be seen. This soon changed to a sphere of light on the
positive ball, which became red hot, the negative being surrounded by
magnificent envelopes; with a horse-shoe magnet the positive light was
drawn out into strata. The needle of a galvanometer in circuit was
violently deflected and the polarity reversed. When the caustic potash
was heated, the discharge burst into a sunlike flame, subsequently
subsiding into three or four large strata of a cloud-like shape, but
intensely bright. This is called the arc discharge: it occurs in vacuum
tubes with charcoal balls; when the potash is heated intensely, dazzling
stratifications suddenly emanate from the positive ball, and powerful
chemical action takes place in the battery, after which the discharge
ceases.
This process facilitates the discharge and assists the disintegration of
the carbon particles, and these in a minute state of division are
subsequently found attached to the sides of the glass. It is these
particles which produce the arc discharge with its intense vivid light
so suddenly observed with far more brilliant effects than the usual
stratified discharge. During its passage the conducting power of the
vacuum tube is greatly enhanced.
It was already mentioned that a stratified discharge was obtained from
3,520 insulated cells of a water battery, which differs but little in
intensity from 400 cells of the nitric acid battery. On one occasion the
electricity seemed to pass through the vacuum tubes in a continuous
stream, but when examined with Mr. Wheatstone’s revolving mirror it was
decidedly stratified. Mr. Gassiot never could obtain a continuous
discharge in air, whether between the points or metallic plates of the
water battery. The discharge was invariably in the form of minute
clearly defined and separate sparks.
Thus it was proved by the preceding experiments that a spark discharge
could be obtained in air from both the nitric acid and water battery;
and that when these discharges were passed through the highly attenuated
matter contained in carbonic acid vacua, the same luminous and
stratified appearance was produced as by an induction coil; a proof that
whatever may be the cause of the phenomena it could not arise from any
peculiar action of that apparatus.
Mr. Gassiot finally concludes that the cause of the stratified discharge
arising from the impulses of a force acting upon highly attenuated but
resisting media is also applicable to the discharge of the voltaic
battery in vacuo; while the fact of this discharge even in its full
intensity having been now ascertained to be also stratified leads to the
conclusion that the ordinary discharge of the voltaic battery, under
every condition, is not continuous but intermittent, that it consists of
a series of pulsations or vibrations of greater or less velocity,
according to the resistance in the chemical or metallic elements of the
battery or the conducting media through which the discharge passes.
Caustic potash absorbs the carbonic acid gas by degrees, and at last so
completely exhausts a vacuum tube that electricity cannot be conducted.
Air is a non-conductor, and an electric discharge that will not pass
through an inch of air, will pass through more than 30 or 40 inches of
attenuated gas.
It has already been mentioned that the stratified discharge can be
obtained by a single discharge of the primary current of an inductive
coil, however long may be the vacuum tube through which the discharge is
passed. If no addition be made to the battery and no alteration be made
in the arrangement of the coil so as to increase or diminish the
intensity of the discharge, the stratifications will always present the
same appearance and form, occupying the same spaces and positions in the
vacuum tube; but if any change be made so as to alter the intensity,
then a corresponding alteration will appear in the discharge, the striæ
assuming a different shape, and the bright and dark divisions occupying
different positions.
In order to try what effect a change of intensity would produce, three
separate insulated voltaic batteries, in which the exciting liquid was
brine, formed an electric circuit which was completed by two long wires.
It was so arranged that the discharge of one, two, or all the three
batteries could be separately employed. In order to vary the resistance
at pleasure, two tubes 18 inches long containing distilled water and
connected at their base were introduced into the circuit. By varying the
depth to which the terminal wires of the circuit were plunged into the
water, the resistance could be regulated at pleasure, and it was
immaterial in what part of the circuit the vacuum tube was introduced
provided the circuit was completed.
The first experiments were made with a carbonic acid vacuum tube 20
inches long and 4 inches in diameter. The negative terminal at one
extremity of the tube was of aluminium, cup-shaped, about 3 inches in
diameter; the positive terminal was a wire of the same metal fused into
the other extremity of the tube; the point of the wire and cup were
about four inches and a half apart. With this tube and 2,240 cells of
the battery the discharge when the resistance was introduced had the
appearance of a positive and negative discharge, impinging on and
intermingling with each other, without any dark space intervening.
Around the negative terminal the luminosity extends to the sides of the
tube and tapers to the point of the positive wire. The light round the
negative terminal becomes brighter, a dark space appears next to it when
the resistance is diminished, and increases as the resistance decreases,
by the rolling back of the light in bright clouds to the point of the
positive terminal. These changes can be perfectly regulated by the
resistance, and various luminous phenomena occur at each stage.
With 2,240 cells distinct sounds were heard in the tube; with the whole
battery of 3,360 series the sounds were not heard till a magnet was
applied to the striæ, when they again became audible and the striæ were
spread over the surface of the tube.
A carbonic acid vacuum tube with platinum terminals fused into the same
side far apart was now put into the circuit, the part of the wires that
penetrated within the tube being coated with glass up to the carbon
balls in which they terminated. When a discharge from all the three
batteries passed through the tube, changes occurred in the form and
number of the striæ corresponding to the greater or less amount of the
resistance offered in the circuit.
At the commencement of the experiments there were 18 inches of water in
each of the tubes, which formed the maximum resistance. The wires
attached to the terminal wires of the battery were placed inside of
these tubes, and as soon as they touched the surface of the water a
faint luminous discharge was seen at each ball in the vacuum tube. As
the wire attached to the negative end of the battery was slowly
depressed, the two luminous discharges appeared to travel towards or
attract each other, and at times a portion of the positive luminosity
passed over and mingled with the negative; in this state the discharge
was extinguished by a magnet.
When the wire was pressed farther into the water a dark space about an
inch in length divided the light into two parts, the positive glow being
sharply defined, the negative glow having an irregular edge. When the
wire had been about three inches deep in the water, the positive and
negative glows became more brilliant, and a single clearly defined
luminous disc burst from the positive side and occupied the middle of
the dark space. When the wire was pressed down till 13 inches of it were
in the water, a second luminous disc travelled from the positive side,
and then the two luminous discs or striæ occupied the dark space at a
little distance from one another. As the wire was pressed more into the
water, three parallel luminous striæ appeared, then four, then five, and
so on till as many as thirteen or fourteen striped the dark central
space. With the full power of the battery, the adjacent disc impinged on
the glow that surrounded the negative ball. This disc was of a pale
green, those adjacent were reddish, while the negative glow was of a
bluish white; minute bright scintillations emanated from the negative
ball, while distinct luminous flash discharges took place through the
striæ. Thus by the amount of resistance introduced into the circuit, the
number of striæ can be regulated, their position fixed, separating or
closing up the dark space between the luminous glows round the balls.
In these experiments there is indication of a force emanating from the
negative wire. The actual disruption of the particles from the negative
terminal also indicates a force, and the disruption is as freely
obtained by the continuous discharge of the battery as it is by the
intermittent discharge of the induction coil. Besides, when Mr. Gassiot
sent discharges from the induction coil through Torricellian vacua, he
several times observed that while a cloud-like discharge issued from the
positive terminal, a long tongue of the most brilliant blue
phosphorescent light emanated from the upper part of the negative
terminal, and a brilliant white tongue of light was also seen close to
the negative wire: so there is reason to believe that force emanates
from both terminals.
Some of the preceding striated discharges ‘present an appearance
somewhat analogous with the stationary undulations (or nodes) which
exist in a column of air when isochronous progressive undulations meet
one another from opposite directions, and on the surface of water by
mechanical impulses similarly interfering with each other.’[11]
‘May not the dark bands be the nodes of undulations arising from similar
impulses proceeding from positive and negative discharges? or can the
luminous stratifications which we obtain in a close circuit of the
secondary coil of an induction apparatus, and in the circuit of a
voltaic battery, be the representations of pulsations which pass along
the wire of the former, and through the battery of the latter, impulses
probably generated by the action of the discharge along the wires?’
The action of magnetism and electricity on light is similarly
illustrated by the rotation of the plane of polarization. Sir John
Herschel was the first who tried to rotate the plane of polarization of
a ray of light by surrounding it with a spiral wire electrized by the
great battery of two enormous plates of copper and zinc at the London
Institution, but he obtained no evidence of any such action. Long
afterward Professor Faraday succeeded by sending a ray of light through
a piece of silico-borate of lead, which formed the core of a magnetic
helix. The silico-borate took on a quasi-crystallised state during the
passage of the electric current round it, giving it for the moment the
property of circular polarization, analogous to that of glass in a state
of tension or compression.
Substances vary exceedingly in the facility with which they transmit
electricity; even the same substance under another form differs
remarkably in that property: charcoal, which next to the metals is the
best conductor known, when under the form of diamond is quite impervious
to electricity. In general, substances that are the best conductors of
heat are also the best conductors of electricity, as for example the
metals, which however, possess the transmissive property in very
different degrees. Silver and copper are the best conductors, lead one
of the worst; its resistance to the passage of electricity is twelve
times greater than that of silver and copper, consequently it becomes
twelve times as hot, for when a current of electricity is impeded it is
changed into heat. So great is the resistance offered by a fine platinum
wire, that the heat amounts to 3280° and the wire is melted, a striking
instance of the correlation of electricity and heat, and of the power of
the cohesive force.
When electricity is passing through conducting substances or when it is
static, it induces an electric state in bodies at a distance by
transmission through non-conducting substances or air, for it gives
polarity and tension to the adjacent atoms, and these to the next, and
the next in succession, throughout the whole intervening mass,—a strong
proof of the individuality and polarity of the atoms of matter.
Motion, which is the result of all the physical powers, has itself a
strong action upon the ultimate elements of matter; in cases of unstable
equilibrium it accelerates and even determines their chemical union.
Some substances will remain merely mixed as long as they are at rest,
but no sooner is their inertia disturbed by a slight motion than they
rush into permanent combination. In newly sublimed iodide of mercury the
vibration impressed by the scratch of a pin is so rapidly transmitted
through the mass that its colour is immediately changed from yellow to
bright red. By a new arrangement of the molecules their action on light
is altered.
Catalysis or the chemical decomposition and composition of substances by
the contact of a foreign body, is well illustrated by the chloride of
nitrogen, that explodes when touched by substances which at ordinary
temperatures would neither combine with the chlorine nor with the
nitrogen. The iodide of nitrogen explodes if touched by a feather, and
M. Becquerel decomposed the iodide of nitrogen by the vibrations of
sound. When substances only exist in consequence of the inertia of their
atoms, the instability of their chemical attractions and repulsions is
only increased by an external agent, so that a great effect is produced
by a slight cause, as in an avalanche, the snowy mass is on the point of
falling, and the smallest motion, a breath of wind, hurls it down. In
such cases the potential energy of the unstable mass is in a moment
changed into vis viva or impetus. Daguerreotype impression shows the
power of the chemical rays on substances in unsteady equilibrium, and
the length of time required to make the impression under the same
circumstances is a measure of the instability.
Most of the fulminates are compounds of nitrogen; of that the fulminate
of aniline is a recent instance, since it is formed by the slow action
of nitrous acid on aniline. Explosion takes place on the sudden
evolution of gas, or the sudden change of a solid into vapour. In these
cases fire or percussion are the foreign causes of change. They are all
particular instances of the general principle of catalysis, which is the
chemical combination of heterogeneous atoms by the action of a substance
that does not participate in the change. Thus it has long been known
that when platinum is plunged into a mixture of oxygen and hydrogen it
combines these gases into water. Acids in some cases seem to have the
same effect; for when rags or starch are dissolved in an acid the starch
is changed to dextrine and the liquid has acquired the power of turning
the plane of polarised light to the right. The acid has undergone no
alteration, but it has changed the properties of the starch though not
its chemical composition. After a time, a second transformation takes
place, the liquid ceases by degrees to turn the plane of polarisation to
the right, and ends by turning it to the left. The acid is still
unchanged, but the dextrine has now disappeared: it has combined with
the water and is transformed into glucose or sugar of grapes.
The quantity of the physical powers, active and latent, is inappreciably
great. The quantity of heat or potential energy generated by chemical
combination alone is enormous.
SECTION III.
ATOMIC THEORY, ANALYSIS AND SYNTHESIS OF MATTER, UTILITY OF WASTE
SUBSTANCES—COAL-TAR COLOURS, ETC.
THE chemical combination which forms the infinite variety of substances
in the organic and inorganic creation consists in an intimate union of
their ultimate atoms which produces substances differing from their
constituent parts in every respect except gravitation, the sum of the
weights of their constituent parts being invariably equal to the weight
of the resulting substance. Thus the chemical union of oxygen and
hydrogen forms water, and the weight of the water so formed is exactly
equal to the sum of the weights of the two gases.
All chemical changes whether of analysis or composition are subject to
definite unalterable laws of weight, measure and number; nothing is by
chance or casual, the relative weights of the invisible atoms of matter,
and their combination in definite proportions reveal the laws which
prevailed in the primeval structure of created things. By the wonderful
discovery of these laws Dr. Dalton has placed chemistry on a strictly
numerical basis.
The chemical union of different kinds of atoms and volumes of matter in
the definite proportions of whole numbers entirely changes their
character and properties, as for example the chemical combination of one
atom of hydrogen and one atom of oxygen into water. The condensation is
often unexpected and wonderful; two different liquids are often
condensed into a solid, and the result of the chemical combination of
two different gases or vapours in quantitative proportions may be solid,
liquid or aëriform, a fact which could only have been discovered by
experiment. The powers of the atoms are changed and often highly exalted
by chemical union as in ammonia, a chemical compound of three atoms of
hydrogen and one of nitrogen, which absorbs 1,195 times more radiant
heat than its constituents whether simple or mixed. During chemical
combination light and electricity are often evolved, heat always. The
quantity given out is exactly proportional to the energy of the chemical
action, and is often so great and so rapidly evolved as to produce an
explosion by the sudden expansion of the air around. Whatever the
temperature may be, which is given out during the union of the atoms,
the very same quantity of heat is requisite to dissolve their union, and
the atoms are separated in the same definite proportions in which they
were combined.
Voltaic electricity both combines and resolves substances into their
component parts, strictly according to the law of definite proportions.
It combines eight parts by weight of oxygen and one part by weight of
hydrogen into water; and again when it decomposes water, one part by
weight of hydrogen is given out at the negative pole of the battery, and
eight parts by weight at the positive or zinc pole. For an electric
current weakens or neutralizes the force of affinity in one direction
and strengthens it in the other, so that the heterogeneous atoms of the
substance under its influence have a tendency to go in different
directions and appear at opposite poles. Mr. Faraday has established as
a general law, that the quantity of electricity requisite to unite the
atoms of matter, is precisely equal to the quantity requisite to
separate the same atoms again. Electro-chemical action, or the power of
electricity to combine and separate the heterogeneous atoms of matter,
is in direct proportion to the absolute quantity of electricity that
passes in the current. Hence the superior analytical power of voltaic
over static electricity, which has enormous intensity, but is very small
in quantity. The electric current separates molecular combinations which
yield to no other means: it is the most powerful instrument of analysis;
light is the most delicate.
Two simple substances are only capable of a certain number of chemical
combinations, which form a regular series of new substances; as for
example oxygen and nitrogen. Two measures of nitrogen gas will unite
with one measure of oxygen to form the protoxide of nitrogen; with two
measures of oxygen it unites to form the binoxide of nitrogen; with
three measures of oxygen it forms the hyponitrous acid; with four it
forms nitrous oxide; and with five measures of oxygen it forms nitric
acid. Thus there are five compounds of nitrogen and oxygen, no more.
Affinity of _kind_ is merely the attraction of one element or atom of
matter for another; affinity of _degree_ consists in the grades and
limits of combination; the preceding series is of the fifth degree; the
limit is the last term, for no further combination of these two gases
can take place, and these are accomplished by art. All the five
substances are deleterious, most of them deadly poisons, for the
protoxide of nitrogen, which is the laughing gas, could not be long
inhaled with impunity. For a long time the middle term of the preceding
series was wanting, but Gay-Lussac formed it by attending to the laws of
definite proportion and sequence.
The atoms of different kinds of matter possess an affinity, or
attractive force, which binds them together chemically in different and
very unequal degrees. Two substances may unite and form a third
differing from both, as water does from oxygen and hydrogen; but if a
new substance be added which has a greater attraction for one of the
substances than for the other, it will dissolve their union, combine
with that for which it has the strongest attraction, and set the other
free. Thus the metal potassium, which has a greater attraction for
oxygen than it has for hydrogen, decomposes water, combines with the
oxygen, and sets the hydrogen free. Both chlorine and ozone have the
property of liberating the iodine in a weak solution of the iodide of
potassium; the liquid stains starch blue, a proof of the free iodine.
The facility with which acids and alkalies combine affords the means of
eliminating either the one or the other from a compound so as to
liberate what remains.
The constituents of compound substances may be separated from one
another by a variety of means depending upon their greater or less
fusibility, volatility, and other properties. Water, acids, alcohols and
other liquids hot or cold, different degrees of temperature,
sublimation, solution, distillation, evaporation, together with static
and voltaic electricity, are the most powerful means of analysis.
But the animal and vegetable creation rear their fabrics by a synthetic
process. A plant after having absorbed carbonic acid and water,
decomposes the carbonic acid, returns the oxygen to the atmosphere, and
combines the carbon and water into wood, leaves, and a variety of
organic substances. Now MM. Berthelot, Wöhler, and other distinguished
chemists, by following this example of nature, have established a system
of synthetic chemistry, by which they have produced from the chemical
combination of the three elementary gases and carbon alone more than
1,000 complete organic substances, precisely the same with those formed
within the living plants and animals. Yet we are as far as ever from any
explanation of the mystery of life, whether animal or vegetable.
Carbon and hydrogen will not combine at any artificial heat however
great; but when the electric arc between highly purified charcoal
terminals passes through hydrogen gas, acetylene, a new carburet of
hydrogen, is formed, consisting of four equivalents of carbon and two of
hydrogen. This substance, which no organized being is capable to form,
was discovered by M. Berthelot, and being assumed as a base, yielded an
extensive series of organic substances. Thus when two atoms of carbon
are added to acetylene it becomes olefiant gas; when two equivalents of
oxygen are added to olefiant gas, the result is alcohol, which is
transformed into acetic acid by the addition of two atoms of oxygen, and
from this by a similar process have been obtained the malic, tartaric,
succinic, and the other acids; glycerine also, which is the sweet
principle of the oils, wax, essential oils, the perfumes of fruit and
flowers, the principle of the balms, the essential oil of mustard, and
numerous other organic substances, simply from carbon, oxygen and
hydrogen; but nitrogen was introduced by combining alcohol with ammonia,
an inorganic substance consisting of three equivalents of hydrogen and
one of nitrogen, from whence a vast number of nitrogenized substances
were derived, both animal and vegetable.
Chemical combination, which has from the beginning of created things,
and still is, building up organic and inorganic matter in the earth, in
the air, and the ocean, exerts forces of transcendent power, though
silent, unperceived, and for the most part unknown. Professor Tyndall
has given a striking instance of this in water, the most simple compound
of oxygen and hydrogen, a constituent alike of organic and inorganic
nature. ‘In the combustion of the two gases to form a gallon of water
weighing ten pounds, an energy is expended, the atoms clash together
with a force, equal to that of a ton weight let fall from a height of
23,757 feet; and in the change from the state of vapour to water, an
energy is exerted equal to that of a ton weight falling from a height of
3,700 feet, or of a hundredweight falling from a height of 74,000 feet.
The moving force of the stone avalanches of the Alps is but as that of
snowflakes compared with the energy involved in the formation of a
cloud. In passing finally from the liquid to the solid state,’ that is
from water to ice, ‘the atoms of ten pounds exercise an energy equal to
that of a ton weight falling down a precipice of 550 feet of
perpendicular height.’
From Mr. Joule’s investigation of the relation existing between chemical
affinity and mechanical force, it appears that when affinity is feeble
it can be overcome mechanically. He formed amalgams of different metals,
that is he combined them with mercury, by electricity. The affinity of
iron for mercury is so feeble that the amalgam is speedily decomposed
when left undisturbed by the pressure of the atmosphere, and if a
greater pressure be added, almost all the mercury is driven out. The
efficacy of mechanical force to overcome feeble chemical affinities is
strikingly illustrated by the amalgam of tin, out of which nearly the
whole of the mercury is driven by long continued pressure. In these
cases the force of affinity did not amount to chemical equivalency,
otherwise the mercury could not have been driven out by so small a
force. Instances from the weakest to the strongest affinity show that it
is only when the power reaches a definite point that the law of chemical
equivalents comes in. The intense energy which then begins to be exerted
has just been shown.
It is vain to hope for a knowledge of the _absolute_ weight of the
ultimate atoms of matter, and nothing seems to be more beyond the power
of man than to determine even their relative weights; yet the definite
proportions in which they combine have enabled him to do so. Thus, an
atom of oxygen unites with an atom of hydrogen to form water; but as
every drop of water, however small, contains eight parts by weight of
oxygen, and one part by weight of hydrogen, it follows that an atom of
oxygen is eight times heavier than an atom of hydrogen. Now, since
hydrogen gas is the lightest body known, its atom has been assumed as
the unit of comparison. Hence, if the unit of hydrogen be represented by
1, that of oxygen may be represented by 8. Again, carbonic acid gas
contains six parts by weight of carbon, and eight parts by weight of
oxygen, and as an atom of oxygen is eight times heavier than an atom of
hydrogen, therefore an atom of carbon is six times heavier than an atom
of hydrogen, and consequently may be represented by 6. In this manner
the relative weights of many substances have been determined. But the
property of isomorphism also affords the means of ascertaining the
atomic weights of certain substances with unerring certainty. It is
exactly the contrary of dimorphism, for in the latter substances are
chemically the same under different forms; whereas isomorphic bodies are
chemically different under the same form. Now the peroxide of manganese
contains one atom of oxygen for one atom of metal; but in 100 parts of
the protoxide there are 21·94 parts of oxygen and 78·06 of manganese.
Comparing these numbers with 8 the atomic weight of oxygen, the result
is 28 the weight of an atom of manganese. The same number is obtained
from two other isomorphic compounds of oxygen and manganese, which
proves the accuracy of this result. The atomic weights of many bodies
have been determined, of which the following are the most important.
_Atomic Weights, an Atom of Hydrogen being the Unit._
Hydrogen 1
Carbon 6
Oxygen 8
Nitrogen 14
Sulphur } 16
Phosphorus }
Sodium 23
Iron }
Nickel } 28
Manganese }
Bromine 80
Copper 32
Zinc 32·5
Chlorine 35·5
Potassium 39
Rubidium 86
Cæsium 133
Iodine 127
Thallium 204
In the determination of atomic weights a few cases have occurred of
fractional numbers, and although it cannot yet be affirmed that no such
cases exist, yet it seems to be established by the new and more perfect
analyses of MM. Dumas, Isidore, Williamson, and others, that the atomic
weights of substances compared with an atom of hydrogen are in whole
numbers.
This law leads to very important results. For example, the equivalent
weights of the chemical elements of bodies derived from their specific
gravities are either identical with, or simple multiples or
sub-multiples of, their relative weights. Thus the specific gravity of
hydrogen is 0·0693, and that of oxygen is 1·111; hence taking hydrogen
as the unit of comparison, it is easy to see that 0·0693 : 1·111 :: 1 :
16, the simple multiple of 8, the relative atomic weight of oxygen. In
fact since each substance has its own specific gravity or weight, that
weight must depend upon the weight of its atoms, so that the weights of
equal bulks of different substances are proportional to the weights of
their atoms, and thus a relation is established between the atomic
weights and specific gravities of bodies, so that one being given the
other may be found.
Atoms like their substances have many different capacities for heat and
electricity. It was proved by MM. Petit and Dulong, that specific heat,
or the quantity of heat required to raise a simple substance to a given
temperature, is inversely as the weight of its atoms, so that the
specific heat or repulsive force of simple substances multiplied by
their atomic weights is a constant quantity. Such is the condition
requisite for the equilibrium or equality of force; or the law may be
thus expressed: A given quantity of heat will raise to the same number
of degrees a portion of every simple substance represented by its atomic
weight. For instance, the atomic weight of sulphur is 16, that of zinc
32·5; hence it requires twice as much heat to raise a pound of sulphur
ten degrees as it does a pound of zinc. It has also been proved that the
atoms of compound bodies of analogous composition are endowed with the
same capacity for heat, so that there is a perfect correspondence
between the weight of atoms and their specific heat. The numbers
representing the atomic weights derived from the specific heat of bodies
are connected with their equivalent atomic weights by the simple ratios
of equality, multiples or sub-multiples.
Mr. J. Croll has made experiments showing that the specific heat of
compound gases and liquids is generally less, and those of solids more,
than that of their component elements, which is contrary to the hitherto
received opinion. Moreover it appears that the changes in the specific
heat of bodies which occur during combination are not only due to
chemical action, but also to molecular changes; the real specific heat
of a simple atom probably remaining the same under all conditions.
Mr. Faraday has proved that the specific electricity of different
substances is also in proportion to their atomic weights, that is to
say, a given quantity of electricity will separate combined substances
into parts represented by their atomic weights. For example, 32·5 parts
of zinc will generate voltaic electricity enough to separate nine parts
of water into eight parts of oxygen and one part of hydrogen gas. The
weights thus derived from decomposition are exactly the same with those
determined by composition, and thus the atomic weights derived from
electro-decomposition accord exactly with those obtained from chemical
composition. Moreover, Mr. Faraday, as already mentioned, proved that
the very same quantity of electricity necessary to decompose a body into
its elementary atoms, is requisite to unite them again. The analysis and
synthesis of compound matter, solid or fluid, show a constant and
definite proportion of the component elements expressed by number, and
by an equivalent or multiple ratio of parts in every chemical change.
The atomic theory unites, by a common bond, specific gravity, chemical
affinity, heat, and electricity. Taking atmospheric air at the
temperature of 60° Fahr. and a barometric pressure at 30 inches as the
standard unit of specific gravity; the quantity of heat required to
raise a volume of water 1° Fahr. as the unit of specific heat; hydrogen
gas as the unit of atomic weight; and atomic electro-chemical
electricity as the unit of specific electricity, the following numbers
have been established:
+----------+----------+------------+----------+-------------+
| | Specific | Equivalent | Specific | Specific |
| | gravity | atomic | heat | electricity |
| | | weight | | |
+----------+----------+------------+----------+-------------+
| Hydrogen | 0·0693 | 1 | 0·2936 | 1000 |
| Oxygen | 1·111 | 8 | 0·2361 | 125 |
| Carbon | 13·2 | 6 | 0·2631 | |
| Nitrogen | 0·978 | 14 | 0·2750 | |
+----------+----------+------------+----------+-------------+
The distances between the atoms of the gases are equal, hence the atomic
weights of simple gases are proportional to their densities; and for the
same reason, equal volumes of the same fluid contain an equal number of
atoms, and the number of atoms in the same volume of different fluids is
in the simple ratio of one to one, one to two, one to three, &c.
It follows from the atomic theory that the number of atoms in equal
weights of any two solid substances, is in the inverse ratio to the
weights of these atoms. Now since the bodies that have the greatest
specific gravities are the heaviest, if the specific gravities and
atomic weights of equal bulks of two simple substances be known, the
relative number of atoms they contain may be found. For the density
divided by the atomic weight of the one, is to the density divided by
the atomic weight of the other, as the number of atoms in the first to
the number of atoms in the second. By the preceding law it is found that
in equal bulks of the three metals, sodium, platinum, and potassium,
platinum contains five times as many atoms as sodium, and ten times as
many as potassium. When substances which have strong analogous qualities
are compared in this manner, the results are either equality, or a
simple ratio.
It has already been mentioned that the protoxides of iron, copper, zinc,
nickel and manganese, have the same form, and contain the same quantity
of oxygen, but differ in the respective metals that are combined with
it; and by the preceding law it appears that equal bulks of these
isomorphous bodies contain also the same number of atoms.
Mr. Hermann Kopp has proved that the atomic weight of a substance
divided by the specific gravity, that is to say, its atomic volume, is
the same for all isomorphic bodies simple and compound, and as a general
law that the atoms of isomorphous substances are not only the same in
form, but equal in dimensions. It follows, therefore, that any one of
the preceding metals might be substituted for any other in the
respective protoxides, and on that account, according to the modern
theory, they are the chemical equivalents of each other, for that
expression is used now in a different sense from what it formerly had.
Chemical equivalency between two or more substances consists in their
capacity for being exchanged one for the other. Direct or indirect
substitution forms the basis of the modern doctrine of chemical
equivalents.
Substances which are capable of replacing one another in compounds, and
which are endowed with qualities mutually analogous, are said to be
isomeric. Many isomeric compounds are formed of the same materials, in
the same proportions, and yet differ essentially both in their physical
and chemical properties; whence M. Daniel observes, that a specific and
definite arrangement of the constituent molecules in space appears to be
no less essential to the individual constitution of bodies than a
certain proportion between their heterogeneous ingredients.
Successive substitution in isomeric bodies does not alter the character
of the chemical formulæ of these bodies; thus chlorine, bromine and
iodine, are chemically equivalent with an atom of hydrogen, for they may
be put for one or more atoms of hydrogen in various compounds without
changing the character of the chemical formulæ of these compounds. The
peroxide of hydrogen consists of one atom of hydrogen and two of oxygen;
hence if 32·5 parts of zinc, 28 of manganese, and 32 of copper be
successively put for the atom of hydrogen, the result will be the
peroxides of zinc, manganese and copper respectively. Here the character
of the chemical formula of the original compound remains the same, and
the three metals are chemically equivalent to one another, and to the
atom of hydrogen. In many compounds organic and inorganic, one or more
atoms of hydrogen may be replaced by an equal number of atoms of sodium,
potassium, zinc, &c., without altering the character of the chemical
formula of the compound.
Olefiant gas, olefiant oil and paraffin, form an isomeric series of a
gas, a liquid and a solid, consisting of carbon and hydrogen. The gas
contains 86 parts in 100 of carbon, and forms the most luminous part of
coal gas.
M. Dumas has proved it to be a general law, that when three isomeric
bodies are arranged in the sequence of their chemical properties, there
will also be a sequence in their respective atomic numbers, and that
whenever this symmetry of chemical properties and atomic weights
obtains, any one of these substances may be substituted for the other
without changing the chemical character of the formula.
Sulphur, selenium, and tellurium, form an isomeric group; that is, they
form a sequence, with analogous qualities, for sulphur is the most
volatile; selenium, a simple substance found in iron pyrites in Sweden,
is less volatile; and tellurium is the least volatile and with regard to
their atomic sequence, the atomic weight of sulphur is 16, that of
tellurium is 64, and half the sum of these numbers is 40, the atomic
weight of selenium, the mean term. Hence selenium might be put in any
compound for the sulphur, and the tellurium for the selenium, without
changing the chemical character of its formula.
The metallic group of calcium, strontium, and barium, are endowed with
analogous properties, perfect harmony in their chemical qualities, and
in the numbers expressing their atomic weights. That of calcium is 20,
that of barium is 68, and the half sum is 44, the atomic weight of
strontium. So calcium might be put for strontium, and strontium for
barium, in any compound without altering the character of its formula.
Professors Johnson and Allen have shown that the new metalloids cæsium
and rubidium form an isomeric triad with potassium, for the atomic
weight of cæsium is 133, that of rubidium 86, and that of potassium 39.
Transmutations of one isomeric substance for another may also be made in
organic bodies, but chlorine, bromine, and iodine form an exception to
M. Dumas’s law, because the arithmetical relation is wanting.
There are certain groups of substances, especially among the metals,
whose atomic weights are in regular arithmetical series, as those of
titanium, tin, and tantalum, which are 25, 59, and 92, the common
difference being 34.
Certain groups of combined atoms called compound radicles are much more
important than the preceding. They unite chemically with one another,
and with other substances in definite proportions, precisely as if they
were ultimate atoms. They are even capable of being substituted one for
the other, forming groups of infinitely varied properties, and thus
chemical equivalency extends to them.
Cyanogen, amidogen, and the peroxide of hydrogen are compound radicles
which combine with other substances and with simple atoms as if they
themselves were simple elements; though the first is a chemical compound
of two atoms of carbon and one of nitrogen, the second a chemical
compound of one atom of nitrogen and two of hydrogen, and the peroxide
contains as before mentioned two atoms of oxygen and one of hydrogen.
All three are capable of replacing hydrogen, chlorine, and metals by
equivalent substitutions. For example, the chlorate of potash consists
of one atom of potash, an atom of chlorine, and five atoms of oxygen; if
then an atom of cyanogen whose weight is 26, be put for the atom of
chlorine, the result would be the cyanate of potash.
Cyanogen, formed by passing nitrogen over red-hot carbon, consists of
two equivalents of carbon and one of nitrogen. It is a frequent
constituent of organic and inorganic compounds, and travels in the
voltaic circuit as if it were a simple substance.
Ammonia consists of three equivalents of hydrogen and one of nitrogen;
now, when the radical phenyle, which consists of twelve equivalents of
carbon and five of hydrogen, is put in the ammonia for one equivalent of
hydrogen, the result is aniline, whence most of the coal tar colours are
obtained. In like manner carbazotic acid, a beautiful yellow dye from
coal tar, is carbolic acid, three of whose equivalents of hydrogen have
been replaced by three equivalents of an oxide of nitrogen.
Compound radicles, consisting of carbon and the three elementary gases,
have been discovered which enter into combination in definite
proportions as simple atoms, and all compound radicles travel in the
galvanic circuit as equivalents to the elementary substances. Hitherto
they have been regarded as representatives or equivalents of one atom of
hydrogen. Now it is generally admitted that each has the property of
replacing two, three, or more atoms of hydrogen by equivalent
substitution. This multiple equivalency among compound radicals forms
the basis of what is called the polyatomic theory, now so much employed
by MM. Hofmann, Berthelot, and other great modern chemists.
Water is the most common radicle both in the inorganic and organic
world. Though a compound of oxygen and hydrogen, it enters, according to
the law of definite proportion, into the composition of various
amorphous bodies in a dry state, that is in the form and proportion of
its gases. It is an essential element in the greater number of crystals,
and abounds in organic matter. In certain cases the same substance
crystallizes at different temperatures, unites with different quantities
of water under the form of oxygen and hydrogen, and assumes
corresponding forms. For example, the seleniate of zinc unites with
three different portions of water and takes three different forms,
according as its temperature is hot, lukewarm, or cold. Thus each
particle of water, containing one atom of oxygen and one of hydrogen,
combines with one atom of zinc in three different proportions as if it
were a simple atom.
The water of crystallization may be driven off from many substances by
heat, as from the hydrates of lime, iron, copper, &c., but when combined
with the oxides of certain metals, potassium for instance, it cannot be
driven off by any means whatever. In general a heat of 212° Fahr. is
sufficient, but some crystals lose their water of crystallization at the
ordinary atmospheric temperature.
Crystals whose atoms are in unstable equilibrium, are readily altered
both externally and internally by a very moderate degree of heat.
Arragonite and calcareous spar are isomeric, that is, they are
chemically the same but differ in form and hardness, which shows that
their molecules are grouped differently. When the arragonite is heated,
the inertia of its atoms is overcome, the crystal explodes with force,
and becomes a mass of crystals of calcareous spar. The expansive force
of the heat suddenly overcoming the force of cohesion causes the
explosion, and at the same time disturbs the unstable repose of the
atoms, which immediately obey their natural attractions and assume the
stable form of calcareous spar.
Dialysis is a method of separating and analysing substances by means of
their diffusion in alcohol or water. If a wide-mouthed vial nearly full
of a solution of common salt be placed in a jar of water, after a few
days it will be found that the particles of salt have come out of the
vial and have diffused themselves through the superincumbent water, even
to its surface. Now Professor Graham, Master of the Mint, with whom this
subject originated, made three arrangements precisely like that
described; the three vials were exactly similar and equal, the three
jars exactly the same in size and form, and contained the same quantity
of water; but the first vial contained a solution of gum arabic, the
second a solution of Epsom salt, and the third a solution of common
salt. After fourteen days the diffusion of the gum had risen through one
half of the superincumbent water, while the particles of both the salts
had risen to the surface. However the common salt would have risen much
higher, for when the strata of water at the two surfaces were drawn off
by a siphon and evaporated to dryness, there was fifteen times as much
common salt as Epsom salt. The three solutions are heavier than water,
yet they rise notwithstanding their gravitation, whence Mr. Graham
thinks that there is probably an attraction between the particles of the
dissolved substances and those of the water. The force of molecular
attraction is more powerful than gravitation, hence the particles must
rise by the difference of the two forces.
After many comparative experiments the professor concluded that most
substances differ in diffusibility, and that crystalloids or crystalline
substances such as salts, sugars, &c., are much more diffusible than
colloids or amorphous sticky bodies, such as gum, caramel, jellies, and
substances that combine with the hydrogen of the water to form
gelatinous hydrates.
The partial decomposition of definite chemical compounds may be effected
by diffusion. Alum, which is a double sulphate of the two metals
potassium and aluminium, furnishes an example; when allowed to diffuse
itself from its aqueous solution, the diffusive tendency of potassium
compounds is so much greater than the diffusive property of aluminium
compounds, that a portion of the sulphate of potassium actually breaks
away from the sulphate of aluminium with which it was combined, in order
to diffuse itself in the superincumbent external water more freely than
the sulphate of alumina can do.
Common salt diffuses itself in a solid mass of jelly almost as easily
and extensively as in the same bulk of free water. Thus colloid bodies
do not interfere with the diffusion of crystalloids such as salts, but
they almost entirely arrest the diffusion of one another. Solutions of
salts, sugars, and other crystalloids pass freely through colloid
substances, such as parchment-paper, vellum, and membrane into water,
although they have no pores, because the particles of the crystals unite
diffusively with the water combined in these substances, which solutions
of gum, caramel, and other colloids cannot do. These colloid substances
are permeable to solutions of crystalloids, impermeable to solutions of
colloids. This constitutes Dialysis.
The instrument used by Mr. Graham was a little tray formed of vellum or
membrane stretched tightly over a hoop of gutta-percha and capable of
holding a liquid and floating on water. When a mixed solution of equal
parts of salt and gum is put into the tray, after a time all the salt
will have passed into the water below, leaving nothing in the tray but
an aqueous solution of gum.
The following is one of the most extraordinary results of dialysis. Mr.
Graham took a silicate of soda, a soluble crystalline salt formed by
fusing quartz with carbonate of soda at a red heat, which diffuses
readily. He acidulated the aqueous solution of the salt with
hydrochloric acid, which changes the constituent silica from being a
crystalloid substance into a colloid form. When the liquid was poured
into the tray floating on water, after four days, the whole of the acid
and the chloride of sodium had been diffused in the water and nothing
remained in the tray but an aqueous solution of quartz. There remained
in fact, a solution of sand in water, a substance so hard that no pure
aqueous solution of it had ever been obtained. Many other crystalline
substances besides quartz can exist both in the colloid and crystalloid
states.
All colloid substances are characterized by non-crystalline habits, low
diffusibility, chemical inertness, high atomic weight, and above all by
their mutability. The aqueous solution of quartz is limpid and liquid,
even if it contains 14 per cent. of silica, but after a time it becomes
opalescent, viscous, and ultimately sets into a firm insoluble jelly,
capable however of solution by chemical means. This jelly gradually
shrinks, exudes pure water, and when perfectly dry it forms a glassy,
transparent, but not anhydrous substance, and the residue left by
ignition has a specific gravity of 2·2, that of crystallized silica
being 2·6.
Mr. Graham has obtained many pure aqueous solutions of organic and
inorganic matter, most of them being unstable. Ice near or at its
melting point is believed to be a colloid body, consequently it is
unstable and resembles a firm jelly, having a tendency to rend and
recombine. ‘The constant intervention of colloid septa in so many of the
phenomena of animal and vegetable life gives to the subject of dialysis
a high physiological interest, and it will doubtless exercise an
important influence on the progress of physiological research.’[12]
Subsequently to these researches Mr. Graham published a memoir on a new
method of analysing gases which he had called atmolysis. The memoir may
be regarded as consisting of four parts, the first of which is
preliminary, being on the reciprocal diffusion of gases through porous
plates. The next three parts relate to effusion, or the passage of gases
under constant pressure through a minute opening in a very thin plate
into a vacuum; transpiration, or the passage of gases through capillary
tubes into vacuo; and lastly atmolysis, which is the partial separation
of a mixture of gases and vapours of different degrees of diffusibility
by permitting them to diffuse themselves through a porous plate into a
vacuum: a new kind of analysis, which possesses a practical character of
extensive application.
The diffusing instrument employed by Mr. Graham was a cylindrical glass
tube about an inch in diameter, ten inches long, with one end closed by
a very thin porous disc of compressed artificial graphite fixed by a
resinous cement. While the tube was being filled with hydrogen gas over
a trough of mercury, the escape of the gas was prevented by covering the
graphite very carefully with a thin sheet of gutta percha. As soon as
the gutta percha was removed, the reciprocal diffusion of the gases
began, and in from forty to sixty minutes the whole of the hydrogen had
escaped from the tube, and a quantity of atmospheric air amounting to
about one fourth of the volume of hydrogen had entered the tube and
taken its place, according to the ordinary law of the diffusion of
gases. During this time the mercury rises in the tube so as to form a
column several inches high, a fact which is a striking demonstration of
the intensity of the force with which the reciprocal penetration of
different gases effected.
Natural plumbago or graphite has little or no porosity and cannot be
used in these experiments, but the pores of artificial graphite of which
pencils are made, appear to be so minute that only isolated molecules of
gas are able to pass, without however being at all impeded by friction;
for the smallest pores that we can suppose to exist in the graphite must
be real tunnels compared with the minuteness of the ultimate atoms or
molecules of a gaseous body. The cause of motion appears to reside
solely in that internal movement of molecules which is now generally
admitted as an essential condition of matter in a gaseous state. The
molecules and atoms are assumed to be perfectly elastic and to move in
all directions with different velocities according to the nature of the
gas. Enclosed in a porous vessel the moving atoms constantly strike
against its walls and against one another, but in consequence of their
perfect elasticity, no loss of movement results from the collision. When
the gases inside and outside of the tube are of the same density and
molecular movement, an exchange takes place without any perceptible
change of volume; but when the two gases are of different densities and
molecular velocities, then the reciprocal penetration ceases to be equal
on the two sides. Reciprocal diffusion of gases is accelerated by heat
and retarded by cold; the tension of the gases is increased in the first
case, and diminished in the second.
In Mr. Graham’s experiments relating to effusion, a gas under a constant
pressure was on one side of a minute opening in a very thin plate, and a
vacuum on the other. The rapidity with which air or gases enter the
vacuum depends upon their specific gravity. A gas rushes into a vacuum
with the speed acquired by a heavy body in falling from the height of an
atmosphere of the gas in question supposed to be everywhere of the same
density. The height of this uniform atmosphere will be in an inverse
ratio to the density of the gas. An atmosphere of hydrogen, for example,
will be 16 times higher than one of oxygen. But the velocity acquired by
a heavy body not being in direct proportion to the height, but to the
square root of the height, it follows that the rate of flow of different
gases into a vacuum will be in an inverse ratio to the square root of
their respective densities. The rate of flow of oxygen being represented
by 1, that of hydrogen will be represented by 4 the square root of 16.
This law has been verified by experiment, and is quite analogous to that
which regulates molecular diffusion, but the phenomena are essentially
different. It is the gas _en masse_ which partakes of the movements of
effusion, whilst only the molecules or atoms of a gas are affected by
the movements of diffusion. For that reason the swiftness of the
effusion of a gas is many thousand times greater than that of diffusion.
The swiftness of the efflux of atmospheric air is as rapid as the
velocity of sound.
The rate of the flow of different gases under constant pressure through
capillary tubes into a vacuum, constitutes the capillary transpiration
of gases. These rates bear a constant proportion to one another, but
they are singularly unlike the rates of effusion. They are independent
of the material of the tube; they are not governed by specific gravity;
and ‘they appear to be in constant relation with no other known property
of the same gases; and they form a class of phenomena remarkably
isolated from all else at present known of gases.’
The pores of graphite are so fine that it is incapable either of
effusion or transpiration, but it is readily penetrated by means of the
molecular or diffusive movements of gases, as appears on comparing the
time requisite for the passage of equal volumes of different gases under
constant pressure into a vacuum. For oxygen, hydrogen and carbonic acid
gas, the times are nearly as the square roots of their densities.
The atmolysis or partial separation of mixed gases and vapours of
unequal diffusibility, can be effected by allowing the mixture to
penetrate through a graphite plate into a vacuum. The amount of
separation is in proportion to the pressure, and attains its maximum
when the gases pass into a perfect vacuum. One of the results of
atmolysis was the concentration of oxygen in atmospheric air. When a
portion of air confined in a vessel was allowed to penetrate into a
vacuum through graphite or unglazed earthenware, the nitrogen passed
more rapidly than the oxygen in the ratio of 1·0668 to 1, and the
portion of oxygen is proportionally increased in the air left behind in
the vessel. The increase of oxygen actually observed when the air in the
vessel was reduced from 1 volume to 0·5 was 0·48 per cent. The diffusion
was continued till the air in the vessel was reduced to 0·0625 and the
concentration of the oxygen in it amounted to 2·02 per cent. The
molecular or diffusive mobility exercises a certain influence on the
heating of gases by contact with heated liquid or solid substances. The
more rapid the molecular movement of a gas is, the more frequent will be
the contact of the molecules and the quicker will be the communication
of heat. The greater cooling power of hydrogen compared with that of
oxygen or air is probably owing to that cause. ‘Oxygen and hydrogen gas
have the same specific heat for equal volumes; but a hot object placed
in hydrogen is really touched 3·8 times more frequently than it would be
if placed in oxygen gas. Dalton had already ascribed this peculiarity of
hydrogen to the high mobility of the gas.’[13]
It appears that isomorphic substances such as chloride, bromide, and
iodide of sodium, have a similar diffusibility, another of the many
analogies between these singular marine substances.
Modem chemistry is essentially experimental; the unprecedented magnitude
to which British manufactures have risen is chiefly owing to experiments
conducted with consummate skill and dexterity. In these investigations,
accidental circumstances have sometimes occurred which led to other
researches quite different from that originally in view, which have had
unexpected and invaluable results. Although the simple elements are few,
they are capable of an infinite variety of combinations, so that by
analysis and new combinations, the most useful and valuable materials
are now obtained from obnoxious or useless substances, formerly thrown
away. The instances are numerous; but sawdust may be mentioned as one of
the most remarkable. It was not even fit for fuel, but now oxalic acid,
a bleaching principle most extensively used in the various processes of
calico printing, is procured from it; the quantity required may be
imagined, since the cotton cloth annually printed in Great Britain
previous to the American war, would surround the earth’s equator
nineteen times. Oxalic acid, which is a vegetable substance, found
combined with potash in wood sorrel or Oxalis acetosella, used to be
made from sugar or starch, by the action of nitric acid. Now starch,
sugar, and woody fibre or fibrine, all contain twelve parts of carbon
and different portions of oxygen and hydrogen, always in the proportions
that form water; hence the name of carbohydrates. Their composition is
so similar that the one may be changed into the other by the addition or
subtraction of one or two atoms of water under its atomic form; thus
when fruits ripen, the starch they contain is changed into sugar by the
addition of one atom of water under its dry form.
Now sawdust is woody fibre, and might be changed by nitric acid into
oxalic acid like the others. But a less expensive method is actually
employed.
When sawdust, mixed with two equivalents of the hydrate of soda and one
equivalent of the hydrate of potash, is exposed to a heat of 400° for a
few hours, the substances are fused, and when raised to a still higher
temperature the hydrates are decomposed: hydrogen is evolved, and the
carbon combines with the oxygen to form the oxalate of soda and the
oxalate of potash. In order to separate these oxalates they are put into
a filter, a solution of carbonate of soda is passed through it; the
oxalate of soda remains in the filter, the carbonate of potash passes
through it; and when lime is added to the oxalate of soda, the soda is
liberated, passes through the filter, and the oxalate of lime remains.
Sulphuric acid is then added to the oxalate of lime, sulphate of lime is
formed, and oxalic acid mixed with water remains, and by evaporation
forms into beautiful crystals of oxalic acid. This is an instance of a
complicated chemical process; nevertheless it is carried on to a vast
extent in Manchester, nine tons a week being furnished by one
manufactory alone. Two pounds of sawdust yield one pound of oxalic acid.
In ordinary distillation a volatile substance such as water, by
absorbing the heat applied to it, becomes converted into vapour; by
abstracting the absorbed heat from the vapour, it is reconverted into
the original substance. Destructive distillation, on the contrary,
consists of an entire destruction of the original substance and a
simultaneous production of new substances. Of this the destructive
distillation of coal furnishes the most interesting illustration, and
shows at the same time the success of modern chemistry in utilizing
waste substances.
Coal had been distilled for years to furnish gas for the illumination of
our cities before it was discovered that the refuse contained principles
of the greatest value. The products of the distillation are threefold:
gas, coal water, and coal tar.
Coal gas is a combination of various gases, whose illuminating
properties depend upon, and are exactly in proportion to, the quantity
of carbon they contain. The particles of carbon raised to a white heat
give the light, for the gaseous part has a feeble flame, and requires a
higher temperature than solid matter, which becomes luminous at about
700° in the dark, and at from 1000° to 2000° in bright daylight. Coal
gas consists of a combination of illuminants: olefiant gas, which
contains 86 per cent. of carbon, carburetted hydrogen or marsh gas,
which contains 75 per cent., carbonic oxide, carbonic acid gas,
hydrogen, sulphuretted hydrogen, and a very small quantity of nitrogen,
besides the bisulphide of carbon, and benzol, a pure hydro-carbon,
consisting of 12 equivalents of carbon and 6 of hydrogen.
The poisonous quality of coal gas is owing to the carbonic oxide, which
is fatal to life, and its explosive quality to carburetted hydrogen,
which also is generated by decomposition of vegetable matter in stagnant
pools and marshes; and in the firedamp of mines it still bears testimony
to the vegetable origin of coal. That fatal gas increases in explosive
force as it mixes with atmospheric air, and is at a maximum when it
amounts to 12 per cent. Hydrogen, carburetted hydrogen, and carbonic
oxide do not add much to the light, on account of the feeble flame of
hydrogen and the small quantity of carbon they contain, but they force
the chief illuminating gases out of the iron retorts in which the coal
is distilled before the heat has had time to decompose them, and they
also enable them to burn without smell or smoke.
Carbonic acid, bisulphide of carbon, and sulphuretted hydrogen are
impurities from which coal gas is freed before it is fit for use. By
passing the gas over lime, the lime absorbs both the carbonic acid and
the sulphuretted hydrogen; one per cent. of carbonic acid diminishes the
illuminating power six per cent., and the sulphuretted hydrogen has an
abominable smell.
The bisulphide of carbon, consisting of one equivalent of carbon and two
of sulphur, is got rid of by passing the gas over hot lime. The water of
the lime is decomposed, and carbonic oxide and sulphuretted hydrogen are
produced; but the latter may be absorbed by passing the gas again over
lime, or through a mixture of sawdust and the oxide of iron. The oxide
of iron decomposes the sulphuretted hydrogen, forms water and sulphide
of iron, then the air restores the sulphide to oxide, and the sulphur is
deposited in the mixture. After passing the gas through it till none of
that impurity remains, the gas is fit for use. The test is the
nitro-prusside of sodium, which the gas stains purple if any of the
impurity remains.
Paraffin, already mentioned as isomeric, is a pure hydrocarbon,
colourless, transparent, and of crystalline texture. It melts at a heat
of 120° or 130°, burns like wax without smell or smoke, and makes
beautiful candles, which give a brilliant light on account of the 86 per
cent. of carbon they contain. Paraffin oil is much used for lamps; the
manufacture of these two substances at Bathgate is one of the largest
chemical establishments in the world.
The black fœtid gas water resulting from the distillation of coals,
formerly thrown away, is so rich in the salts of ammonia, that it has
become the chief source from which these materials so important in the
arts are obtained.
Ammonia is well known to be a colourless gas, with an acrid pungent
smell, consisting of one equivalent of nitrogen and three of hydrogen.
It has an alkaline character, combining with acids, and is extremely
soluble in water.
Now the gas water contains carbonate of ammonia and sulphide of
ammonium, and when any acid strong enough to decompose these substances
is put into the liquid, the carbonic acid and sulphuretted hydrogen
being volatile are driven off, and the acid combines with the ammonia to
form a salt. For example, when muriatic acid is put into the liquid, it
drives off the volatile gases and combines with the ammonia in solution
to form muriate of ammonia, which is dissolved in water and evaporated
till it crystallises; then it is vaporized and sublimed to free it from
impurities.
When ammonia and muriatic acid are separately vaporized, the two
colourless transparent vapours, when mixed, combine into solid muriate
of ammonia, a result so unexpected that as Mr. Playfair justly observes,
it could only have been taught by experiment. About 4,000 tons of
muriate of ammonia are annually made from gas water in England for
soldering, and for making alum.
Sulphate of ammonia to the extent of 5,000 tons is annually made by
adding oil of vitriol to the liquid. It is also used for making alum, as
well as for manure; it supplies our grain with nitrogen, an important
article of vegetable food. To these may be added 2,000 tons of carbonate
of ammonia, so that a substance that was considered to be good for
nothing yields 11,000 tons of valuable materials, but even this quantity
forms only part of the enormous amount annually consumed in the
manufactures of Great Britain.
Coal tar is of complicated nature, containing a variety of substances,
many of which are more or less volatile. When it is distilled by sending
a current of steam through it, the steam collects the volatile parts,
condenses them into naphtha; the first product is condensed steam or
water with naphtha swimming on its surface, the next product is dead
oil, and the remainder is pitch.
By the aid of the crude naphtha thus produced, Indian rubber is
dissolved and waterproof clothes are made. When purified by sulphuric
acid, it forms a substance like tar which is thrown away, and the
remaining products when clarified are acid oils and neutral
hydro-carbons. The carbolic and cressylic acids are the most important
of these acid oils. The carbolic acid, which has the property of
arresting the putrefaction and decay of organic matter, consists of 12
equivalents of carbon, 6 of hydrogen, and 2 of oxygen. The cressylic
acid only differs from the preceding by having two more equivalents of
hydrogen and two of oxygen in its chemical composition.
Creosote is a mixture of these two acids. Those vast beams of wood that
are driven as piles into the sand or mud at the bottom of the sea, as
well as the timbers that form marine superstructures, are saturated with
it to a certain depth to preserve them from the attacks of marine
insects, especially Limnoria terebrans, an isopod crustacean, which is
so destructive in some of our harbours. The wood is deprived of its air
by heat and the creosote easily enters.
Carbolic acid is liquid, but becomes solid when purified and dried; and
as already mentioned the brilliant yellow dye, carbazotic acid, one of
the coal tar colours, is a compound radical, in which the peroxide of
nitrogen has replaced three equivalents of hydrogen. The other coal tar
colours are obtained from the neutral hydro-carbons, that is to say,
compounds of hydrogen and carbon, such as benzol, toluol, and other
analogous substances.
Benzol, which consists of 12 equivalents of carbon and 6 of hydrogen, is
very volatile, boiling at 117° Fahr., and when acted upon by nitric
acid, it forms a compound radicle in which one equivalent of oxide of
nitrogen takes the place of one of hydrogen. It smells strongly of
bitter almonds, and may be used with safety instead of them. When water
and iron are mixed with nitro-benzol, the iron combines with the oxygen
and forms oxide of iron, and the result is rusted iron and aniline,
which is the origin and foundation of the coal tar colours. Now aniline
consists of 12 equivalents of carbon, 7 of hydrogen, and 1 of nitrogen.
It is a compound radical: it is ammonia in which one equivalent of
hydrogen has been replaced by the radical phenyle, consisting of 12
equivalents of carbon and 5 of hydrogen. It may be remarked that in all
these chemical operations the quantity of carbon has remained the same.
Aniline is a colourless liquid, and, being an analogue of ammonia, it
readily combines with the different acids to form the beautiful coal tar
dyes, for which the world is indebted to the brilliant researches of Dr.
Hofmann, professor of chemistry.
By combining a solution of the chloride of lime with the colourless
liquid aniline, he obtained the beautiful colour mauve, but it could not
be used as a dye till it was rendered permanent by his pupil, Mr.
Perkins. His next discovery was the rich crimson crystalline dye
magenta, which M. Verguin first introduced into trade at Lyons as a
dyeing agent. It may be produced by mixing the anhydrous bichloride of
tin with aniline and then driving off the excess of aniline by heat.
Other metallic chlorides, nitrates, and many oxidizing agents, have the
power of converting aniline into magenta; as for example when the two
colourless liquids acetic acid and aniline are mixed and heated, a
chemical combination takes place in which three atoms of ammonia have
coalesced into one, a salt is formed which is the acetate of aniline or
magenta. Here two liquids unite to form a solid and as in many other
instances the resulting substance has the power of decomposing light
which neither of its constituents can do. Magenta has a redder tint than
mauve, and on that account it is sometimes called aniline red. Professor
Hofmann has discovered quite recently that pure aniline has not the
property of producing these colours, but that they originate in an
impurity of the aniline called toluidine.
Rosaniline or roseine, a white substance, is the base of aniline. It is
a powerful alkali, readily combining with acids to form highly coloured
salts, many of which have a tendency to crystallize, like magenta. This
base is most easily extracted from the acetate of aniline. The boiling
solution of that salt decomposed by a large excess of ammonia, yields a
crystalline precipitate of a reddish colour, and when the colourless
liquid is separated by filtration from the precipitate, it deposits on
cooling perfectly white needles and tablets of pure rosaniline. This
substance unites to acids in three different proportions forming three
kinds of salts. The salts that contain one equivalent of acid are
extremely stable compounds; for the most part they have a green metallic
reflection like some insects’ wings; by transmitted light they are red,
and their solutions in alcohol have the magnificent crimson colour of
magenta.
A bright purple dye is furnished by mixing equal weights of magenta and
aniline. When this mixture is kept at the temperature of 329° for some
hours and then mixed with water and hydrochloric acid to remove any
excess of magenta or aniline, the result is an insoluble purple residuum
or precipitate, but which when well washed with water becomes soluble in
alcohol and boiling water slightly acidulated with acetic acid. When the
insoluble purple residue is boiled several times with dilute
hydrochloric acid, a fine blue dye is formed; azuline, the most
beautiful of the blue dyes, which resists the action of the strongest
acids, and which is produced by oxidizing aniline under high pressure.
It was first prepared at Lyons from phenic acid, a product of the
distillation of coal; when pure it appears under the form of copper
bronze-coloured crystals soluble in alcohol, to which they communicate a
magnificent blue colour tinged with red; but most of the blue dyes are
derived from carbolic acid and from creosote. A blood-red colour is the
direct result of mixing the muriatic and phenic acids. Aniline, the
great source of the coal tar colours, yields also a fine yellow. A vast
deal of talent has been employed in the research of colouring dyes both
at home and abroad, in which the manufacturers themselves have shown
great scientific knowledge.
Attempts have been unsuccessfully made to obtain a green dye from
chlorophyll, the green colouring matter of plants. The want was for a
short time supplied by Lo-hao, a Chinese dye, but being unstable it was
given up. However the very same substance has been procured from the
Rhamnus cathartica (Buckthorn), one of the commonest European trees. M.
Charwin of Lyons, who made the discovery, has utilized a waste
substance, and rendered it permanent as a dye. It is the only known
substance which with proper reagents is capable of producing all the
seven colours of the spectrum.[14]
The coal tar colours have nearly superseded those from lichens which
incrust rocks, walls and stems of aged trees with brilliant colours,
which do not however furnish dyes directly; they yield a colourless
crystalline substance which combines with alkalies to furnish very
beautiful dyes; it is exactly the opposite of rosaniline, which is a
base. The Variolaria dealbata yields litmus or orchil, from which the
beautiful French purple is made. The Rocella tinctoria and fusiformis
give blue and purple, and the pale yellow lichen, Parmelia parcolerina
furnishes a bright yellow dye, which a little ammonia changes to a rich
red, inclining to purple. Mauve was first made from orchil, but was not
permanent. The fine dyes, alizarine blue, Turkey red and garancine, are
still much in use. They are derived from madder, the dried roots of the
Rubia tinctorum; the madder dyes most extensively employed are alizarine
and flower of madder. Mauve and other dyes are derived from guano, the
offal of seabirds, which is imported in large quantities for manure.
The coal tar colours are manufactured on a highly scientific plan and
most extensive scale in Great Britain, to supply the enormous quantity
annually consumed in dyeing silk and printing cotton. In general, animal
substances such as silk and wool can be permanently dyed at once,
because they have a strong affinity or attraction for coloured dyes. If
silk is destined to be a moiré, the silk before it is woven undergoes a
chemical process in order to introduce fatty matter into it which gives
a softness to the silk when woven and renders it fit to receive the
moiré by intense pressure.
Cotton cloth has no affinity for dyes, which are washed out at once if
not fixed by art, because cotton fibre consists of minute tubes
generally open at the extremity, which imbibe the dye by capillary
attraction, but cannot retain it unless fixed by a mordant, such as the
white of a raw egg, which readily absorbs any dye that is mixed with it,
and being then laid on the cloth in any pattern it is absorbed by the
tubular fibres, and when coagulated by steam or any other application of
heat it is immovably fixed. Both animal and vegetable substances afford
a variety of mordants. Caseine or cheese, the curd of milk, which may
also be obtained from pease and beans, is the mordant most used by
calico printers; for if caseine be dissolved in twice the quantity of
alkali necessary for its solution, it coagulates like white of egg and
may be used in the same manner. Skimmed milk cheese from Scotland and
Holland when purified is extensively used in calico printing. The
quantity of mordants required is very great, for of all the cotton that
was imported into Britain before the late American civil war, one
seventh only was manufactured into muslin and printed calico, yet as
already mentioned that was sufficient to envelope the earth’s equator
nineteen times, and twenty-seven millions of pieces were exported
annually. Atmospheric electricity and ozone affect the process of
dyeing, and east wind has a retarding and injurious effect. The Lyons
manufacturers, not less celebrated for their scientific skill and taste
than for the brilliancy of the colours, have an advantage in their fine
climate and bright sun.
It is a singular circumstance that petroleum has existed in enormous
quantities throughout the North American States and a great part of
Canada, unnoticed and neglected till the year 1859, when its value was
discovered, and it almost immediately formed a new and extensive branch
of commerce, for during the succeeding year at least 1,000 wells were
dug, some of which enriched the proprietors; others were a failure.
Petroleum from the fountains of Is, on the banks of the Euphrates about
120 miles from Babylon, furnished the asphaltic mortar for building
Nineveh 2,000 years before the Christian era. There are many sources of
naphtha, petroleum, and asphalt in Europe and Asia, which like those in
Trinidad and Venezuela occur for the most part in rocks of the newer,
secondary and tertiary formations, though sometimes in the lower. But in
the northern part of the United States and Canada these substances occur
in rocks of all ages from the lower silurian to the tertiary period
inclusive; they are usually found in the limestones and more rarely in
the sandstones and shales. Petroleum collects in the fissures of the
rocks, chiefly in those that have a tendency downwards; in wells dug for
it near one another, an abundant supply is furnished at all depths from
70 to 300 feet. In some parts of Ohio and Canada the ground is saturated
with petroleum, so that it is believed there is enough in North America
to supply the world for ages. In 1861 no less than 42,000,000 gallons of
petroleum were sent to England. The wells are not without danger, for
when they pass through the coal strata, the petroleum is accompanied by
a highly inflammable gas which on one occasion was accidentally set on
fire; it ignited the petroleum, which was forced out as from the mouth
of a volcano, and covered the ground with liquid fire far around; at the
same time the burning gas formed an incandescent atmosphere which
extended to a still greater distance.
The distillation of petroleum yields substances for the most part
identical with those arising from the distillation of coal. The crude
petroleum is put into an iron retort connected with a coil of iron pipes
surrounded by cold water, called the condenser. Heat is applied to the
retort, and from the open extremity of the condenser, a pale coloured
liquid with a strong smell flows, which is very volatile and explosive
naphtha. After the naphtha has passed over, an oil of excellent
illuminating quality is distilled over. Steam is then forced into the
retort, and a heavy oil is driven over, and there remains a black, oily,
tarry matter, and a black cake used for fuel. After the naphtha has been
repeatedly distilled, benzol is formed, and when the heavy oil is cooled
to 30° Fahr., crystals of paraffin appear, which are separated from the
oil by pressure, and when they are purified by alternate pressure and
agitation in a melted state, they are moulded into candles. This
paraffin is identical with that from coal. Among the products of the
distillation of petroleum are naphthalin whence aniline is obtained,
which yields mauve, magenta, and the other coal tar colours, also
solferino which yields dianthine and other dyes and has been proposed as
a substitute for chloroform and ether. Many other substances have been
separated from petroleum which like some from coal have not yet been
chemically examined. Most of the substances obtained from petroleum and
the distillation of coal are common also to distilled peat, and now it
is proposed to utilize sea weeds, in which the northern coasts of
Scotland and Ireland are so rich. They were burnt for many years chiefly
to furnish soda, but as that substance is obtained at a cheaper rate
from salt, kelp or sea weed ashes has only been made lately to obtain
iodine for medical purposes, and more than one half is wasted in the
process. Besides iodine and six other substances generally procured from
kelp, Mr. Stanford has discovered that it contains naphtha, paraffin oil
and volatile oil rich in benzol, which yields aniline and magenta dyes
and shows that marine vegetation as well as terrestrial abounds in
colouring matter.
Every substance is now of use, no substance is without its value, but it
would be a vain attempt to mention the innumerable discoveries made by
experimental chemistry, which is daily extending its empire over the
three kingdoms of organic and inorganic nature.
_Composition of some of the preceding Substances._
Acetylene C_{2}H_{2}
Olefiant gas C_{2}H_{4}
Ammonia H_{3}N
Benzol C_{12}H_{6}
Phenyle C_{12}H_{5}N
Aniline C_{12}H_{7}N_{3}
Rosaniline C_{40}H_{9}N_{3}
Carbolic acid C_{12}H_{6}O_{2}
Cressylic acid C_{14}HO_{2}
SECTION IV.
THE SOLAR SPECTRUM, SPECTRUM ANALYSIS, SPECTRA OF GASES AND VOLATILIZED
MATTER, INVERSION OF COLOURED LINES, CONSTITUTION OF SUN AND STARS.
TO the unrivalled genius of Sir Isaac Newton we owe the solar spectrum,
and the laws of coloured rings, by aid of which, Dr. Thomas Young proved
and established the undulatory theory which forms the basis of the whole
science of light. The visible part of the solar spectrum forming a band
of seven colours was supposed to be continuous till the year 1802, when
Dr. Wollaston looking with a prism whose axis was parallel to a narrow
slit in a window shutter, at a sunbeam passing through it, discovered
seven dark lines crossing the coloured band, at right angles to its
length.
Twelve years afterwards, Fraunhofer of Munich, a celebrated optician,
magnified the spectrum of a vertical line of light passing through an
upright prism by receiving it upon the object glass of a telescope, and
discovered 600 dark lines. Having ascertained that the position of the
lines in the spectrum, and their distances from one another, are
invariable under every circumstance, he determined their places
accurately and drew the diagram known as Fraunhofer’s lines, which is
universally referred to as a standard of comparison. For that purpose,
the principal lines are designated by letters; thus the dark line A is
in the red near the least refrangible end of the spectrum, B and C are
in the orange, the very remarkable double line D is in the yellow, _b_
and E are in the green, F is at the limit between the green and the
blue, G is in the blue, and the double line H is in the violet.
The instrument used by MM. Bunsen and Kirchhoff, though more
complicated, is constructed on the same principle as the preceding. A
sunbeam transmitted by a very narrow vertical slit passes through four
prisms, which disperse it so much, that if drawn on the scale seen with
the magnifying telescope which receives it, the spectrum would extend
over twenty feet. By means of a micrometer screw, the telescope can be
turned round a vertical axis, and as the dark lines come successively
under the cross wires in its eye-glass they are seen to pass over a
graduated scale, so that the distances between two thousand of them have
been measured in millimetres with unerring accuracy, but that is only a
small part of the whole. When viewed through the telescope, the retina
of the eye is the screen on which this wonderful spectrum falls, crossed
by innumerable dark rayless lines of various breadths and intensities.
Black bands given by the inferior refraction of one prism are here
resolved into numerous dark lines as fine as a spider’s thread.
Mr. Glaisher during his tenth scientific balloon ascent devoted his
attention for a time almost entirely to the dark lines on the solar
spectrum. At a height of about four miles and a half, they were almost
innumerable; all he had seen on the earth were there, and many more. The
nebulous lines H were both seen, the spectrum was a good deal lengthened
at the violet end, and at the red end the line A was visible. The light
from the sky near the sun gave a shorter spectrum; the lines were only
visible from B to G.
Besides these cosmical or permanent lines, Sir David Brewster observed
that certain dark bands and lines in the red and green parts of the
spectrum are only visible when the sun is near the horizon, whence he
concluded that they are occasioned by the absorption of the solar light
while traversing a thicker stratum of air than when the sun is in the
zenith. Various groups of these absorption bands are to be seen at times
on the solar spectrum, especially a remarkable one near Fraunhofer’s
line D, and Dr. Miller observed that temporary dark lines appeared
during a heavy shower, which vanished when the rain ceased.
When the sun was high, M. Kirchhoff mentions that he had noticed traces
of lines and nebulous bands in different parts of the spectrum, which he
thinks might be resolved by a greater number of prisms than those in his
apparatus.
Sir David Brewster was led to his discovery of atmospheric bands by
observing that the brownish red vapour of nitrous oxide has the property
of absorbing solar light, resolving the spectrum into a series of bright
and dark bands, alternating. Professors Daniel and Miller found that
bromine, iodine, and chlorous acid do the same, and Sir John Herschel
observed a multitude of similar bands in the flame of cyanogen; but Dr.
W. A. Miller, who has particularly studied the phenomena of absorption
bands, has proved that the colour of a vapour does not necessarily
determine the position or even the existence of dark bands. He has shown
that some simple substances which do not occasion dark bands produce
them abundantly by the absorptive power they acquire when in
composition, while lines that are produced by a simple vapour, vanish
when it is in combination. Dr. W. A. Miller has proved also that none of
the preceding vapours exist in the atmosphere. He computed that if free
bromine constituted only one in a thousand million parts of atmospheric
air, it would betray its presence by absorptive bands; nevertheless he
suspects that there may be some substance in the air that occasions
certain unaccountable changes. Possibly ozone, so intimately connected
with atmospheric electricity, may produce some unknown effect.
The spectra from glowing solids and liquids, such as Drummond’s light,
which is incandescent lime, the still more brilliant flame of the
electric arc between charcoal points, glowing solid and fused metals,
and coal-gas flame, are continuous; the spectra exhibit the seven
colours, but they are not crossed by dark rayless lines, because such
incandescent substances give off light of all refrangibility. But solids
and liquids reduced to glowing vapours, and incandescent gases, only
give out rays of certain refrangibilities, which cross their spectra at
right angles, as bright lines of various colours and intensities. Each
glowing vapour and gas has bright lines on its spectrum peculiar to
itself.
In order to compare these bright lines with Fraunhofer’s dark lines,
solar light is transmitted through one half of the vertical slit in
Kirchhoff’s apparatus, and the light of the luminous vapour or gas
through the other half. Then by prismatic refraction two spectra are
seen in looking through the telescope, the gaseous one immediately below
the solar one, and only divided from it by an almost imperceptible dark
line. So that the bright lines appear to be continuations of the dark
lines if they occupy the same position in the two spectra; if not, the
deviation is at once visible. The coincidence or deviation of the bright
lines on the spectra of two volatilized substances may be determined by
the same method.
The coloured light that has so beautiful an effect in fire-works is
owing to the combustion of the salts of different metals: as soda, or
common salt, which gives a perfectly pure homogeneous yellow; potash
gives a violet light, strontia red, baryta green. The colour is given
out by the glowing atoms of the vaporized metals sodium, potassium,
strontium, and barium in a state of violent ignition; for as the salt
and its metal give the same colour and the same spectrum when ignited,
it is evident that the colour is independent of the oxygen of the
alkali.
Sir David Brewster appears to have been the first who analysed coloured
light with a prism; and in 1822 Sir John Herschel, besides having made a
series of observations on coloured flames, had determined the spectra of
the muriates of strontia and lime, the chlorides and nitrate of copper
and boracic acid; and observes that ‘the colours thus communicated by
different bases to flame afford, in many cases, a ready and neat way of
detecting extremely minute quantities of them.’[15]
The same opinion was afterwards formed by Mr. Fox Talbot, who after many
experiments on metallic salts, says in his paper,[16] that a glance at
the prismatic spectrum of a flame may show it to contain substances
which it would otherwise require a laborious chemical analysis to
effect. In that paper this gentleman noticed that the glowing salts of
lithium and strontium give a crimson or red colour to flame so exactly
of the same tint that if these metals were in combination it would be
impossible to decide to which metal the colour is due. But when he
passed their respective lights through a prism, he found that the bright
lines on their spectra are entirely different. ‘The strontia flame,’ he
observes, ‘exhibits a great number of red rays well separated from each
other by dark intervals, not to mention an orange, and a very definite
bright blue ray. The lithia exhibits one single red ray,’ Whence Mr. Fox
Talbot observes, ‘I hesitate not to say that optical analysis can
distinguish the minutest portions of these two substances from each
other with as much certainty, if not more, than any other known method.’
Thus Sir John Herschel and Mr. Fox Talbot laid the foundation of a
spectrum analysis of unrivalled delicacy and beauty, since carried to
perfection by Messrs. Bunsen, Kirchhoff and other experimenters,
presently to be mentioned.
M. Bunsen detected the characteristic crimson lithium line in the
spectra of numerous substances; in granite, in the earliest geological
strata, in meteoric stones, in the ashes of most land plants, in blood
and other animal matter; so that instead of being one of the rarest
metals, it exists in all the three kingdoms of nature. In the year 1857
Mr. Swan gave an instance of the extreme minuteness of spectrum
analysis, by detecting the 1/2,300,000th part of a grain of salt by its
yellow light; but by the same reaction M. Bunsen not only recognised the
180 millionth part of a grain of sodium, but found that there is hardly
any substance that does not contain it. It exists in the dust on our
clothes and furniture, particles of it float in the air we breathe, so
that while examining the spectra of other incandescent substances,
flashes of yellow light appear as these atoms are volatilized and
instantly burnt up, which shows that common salt is perhaps more
universally diffused than any other kind of matter.
By spectrum analysis, M. Bunsen has discovered the two new metals,
rubidium and cæsium. While examining with a prism the spectrum of the
hundredth part of a grain of an alkaline substance separated from the
residuum of the Durckheim mineral water, he saw coloured lines, which he
had never seen before on the spectrum of any other alkali, and at once
concluded that they belonged to a new metal; and having obtained about
200 grains of the substance by the evaporation of forty tons of the
water, he found that they contained the chlorides of the two new metals
in question. Moreover he perceived that these metallic chlorides
resemble the chloride of potassium so nearly in spectrum and chemical
character, that a refined prismatic analysis could alone determine the
difference. He thus ascertained that the spectra of all the three have
two red lines in the red part of their spectrum, and two violet lines in
the indigo, while the middle part is occupied by a continuous diffused
light. The only difference is that the two red lines in the rubidium
spectrum are less refrangible than the red lines in the potassium
spectrum, and that the cæsium spectrum is distinguished by two bright
blue lines in the diffuse middle part. Rubidium received its name from
rubidus, on account of the dark red of its lines, and cæsium from its
sky-coloured blue lines.
M. Bunsen thinks that there can hardly be a doubt of rubidium having
been mistaken for potassium, but he has shown that they may be
distinguished by the difference in the solubility of the double salts
which the chlorides of these two metals form with the chloride of
platinum. An aqueous solution of the bichloride of platinum and
potassium gives an insoluble yellow precipitate, consisting of the
bichlorides of platinum and potassium. An aqueous solution of the
bichlorides of platinum and rubidium gives an insoluble yellow
precipitate of the bichlorides of platinum and rubidium. These two
precipitates are undistinguishable to the eye. Now if a solution of
platinum be added to the first, no further precipitate can take place,
but if a solution of rubidium be added to it, a yellow precipitate is
formed consisting of the bichloride of rubidium and potassium, because
the chloride of rubidium resolves the precipitate, combines with the
chloride of potassium, and sets the chloride of platinum free. Thus the
precipitate of the bichloride of rubidium and potassium is the least
soluble of the two. The yellow colour is evidently due to the potassium.
Cæsium may be distinguished from potassium by the same process. The
carbonates, hydrates, and other salts of the two metals were determined;
their carbonates were shown to be readily separated, because the
carbonate of cæsium is soluble in alcohol, which the carbonate of
rubidium is not, and finally the metal rubidium was separated. It has an
extreme avidity for oxygen, and burns in water like potassium, and
possesses many other analogous qualities. It melts at the temperature of
38·5° Cent., and has a specific gravity of 1·516. Rubidium is abundant
in the mineral lepidolite in many parts of Europe and North America, and
M. Grandeau has detected it in the ashes of beetroot, tobacco, coffee,
tea, and grapes by spectrum analysis. It exists in various mineral
waters, and in fact is very general. Traces of various metals are met
with in the same vegetable; thus the spectrum of tobacco gives lines
characteristic of lithium, potassium, rubidium, and lime.
Mr. W. Crookes discovered the new metal thallium by means of its
spectrum, which differs from every other in having one bright green line
upon a dark ground. He obtained its various salts, and the metal itself,
which he describes as being heavy, dense, and very like lead, but of
greater specific gravity. Its fresh surface has a bright metallic
lustre, not so blue as that of lead, but it tarnishes more easily. It is
so soft that it can be indented by the nail, yet it can be drawn into
wire, and in chemical properties it resembles mercury, lead, and
bismuth. Altogether it is more like a metal than a metalloid, perhaps
something between the two. Thallium is completely volatilized at a
temperature below red heat, whether single or in composition. If the
quantity be small, the green line appears in a sudden flash, lasting but
the fraction of a second. If a larger quantity of the metal be gradually
put into the flame, it lasts a little longer, appearing as a single
green line of extraordinary purity and intensity, sharply defined on a
black ground. With respect to volatility, thallium is analogous to the
non-metallic element selenium, which is so volatile that its beautiful
blue light only lasts a few seconds. The green light of thallium comes
out more rapidly, and with less of the substance, than the blue light of
selenium, a quantitative distinction which accords with Dr. Miller’s
observation that the rapidity with which a result is obtained, and the
minuteness of the quantity required for the examination, gives this
method a superiority over every other for the qualitative analysis of
the alkalies and alkaline earths. Thallium has been detected in mineral
waters, wine, treacle, tobacco, and chicory.
Drs. Reich and Richter discovered a fourth new metal in the zinc-blende
at Freiberg in Saxony, which has been called indium, from two beautiful
indigo-blue lines in its spectrum, which have a greater refrangibility
than the blue lines in strontium. The chemical relations of indium
resemble those of zinc, with which it is associated in nature. The metal
can be reduced before the blowpipe into a bead, which marks paper and
has the colour of tin.
The practical importance of spectrum science has been beautifully
illustrated by Professor Roscoe by its application to overcome a
difficulty in Bessemer’s process for the manufacture of steel. According
to that process, steel is made by sending a blast of air through a
quantity of melted iron; the difficulty was when to stop the blast, for
if stopped too soon, the metal retains so much carbon that it crumbles
under the hammer; if continued a few minutes too long, the molten metal
is so viscid that it cannot be poured into the moulds. Experience had
hitherto enabled the manufacturer to judge of the right time from the
appearance of the flame which issued from the mouth of the converting
vessel, but now Professor Roscoe has determined the exact moment for
cutting off the blast by a spectral examination of the flame, the light
of which is most intense. The flame spectrum in its various phases
revealed complicated masses of dark absorption bands and bright lines,
showing that a variety of substances were present in the flame in a
state of incandescent gas; and by a simultaneous comparison of these
with well-known spectra of certain elementary bodies, Mr. Roscoe
ascertained the presence of sodium, potassium, lithium, iron, carbon,
phosphorus, hydrogen, and nitrogen in the flame.
Both Dr. Wollaston and Fraunhofer noticed that the spectrum of the
electric spark was crossed by bright-coloured lines; and in the year
1835, Professor Wheatstone determined the spectra of the electric spark
taken from fused zinc, cadmium, tin, bismuth, lead, and from mercury,
and found that each is crossed by bright lines differing in number,
position, and colour, but which are the same whether the electric spark
be from a static, voltaic, or magneto-electric machine. Having given a
plate showing the colours of these bright lines on the respective
spectra, he proved that they are not owing to the electricity, but to
the incandescent atoms of the metals, for by using different metals as
terminals to the conducting wires, he determined the spectra of these
metals in vacuo, which proved that they were due alone to the
volatilization of the metallic terminals, and concluded that any one
metal may be distinguished from another by the appearance of the spark.
Wheatstone discontinued his spectrum researches, for he had invented the
electric telegraph, and was busy in extending the first telegraphic wire
that ever carried the thought of man to man between London and
Manchester. Soon after he laid the first aquatic line across the Thames,
and he has lived to see his telegraphic lines spread over the surface of
the earth and the bottom of the ocean.
Mr. Wheatstone had perceived that the bright lines on the spectra of the
metals are different and more complicated when taken in air than in
vacuo, and Professor Angström made the important remark that the
electric spark gives two superposed spectra, one due to any metal that
may be under examination, the other to the incandescence of the air
through which the spark passes. Hence the importance of the spectrum
analysis of gaseous substances, especially of those which constitute our
atmosphere, a subject that has been ably and successfully investigated
by Professor Plücker. For that purpose he made use of the Geissler or
vacuum tubes, similar to those he used in his experiments on the
stratification of electric light. When electricity was sent through a
tube containing oxygen gas, the gas combined so rapidly with the
platinum of the negative terminal of the battery that there was little
time to examine the spectrum. The electral light in the tube was too red
at first, but as the attenuated gas gradually disappeared it changed
through flesh-colour to green, then through blue to reddish-violet, and
at length there was too little gas to convey the electricity. However,
the oxygen spectrum has a remarkably bright red band at its red
extremity, two bright orange lines divided by a black one in the orange,
and some bright bands in the green.
The electric light of attenuated hydrogen is red, and almost the whole
light in its spectrum is concentrated into six bright bands of nearly
equal breadths. There is a dazzling red band near the red end of the
spectrum, which, however, does not coincide with the oxygen band; then
comes a very beautiful yellow band, in which the whole of the yellow
rays seem to be concentrated, followed by a grey interval which
separates the yellow from three bright lines in the green, the first of
which is yellowish green, the last a beautiful greenish blue; a black
and a dark space separates the latter from the violet in which there is
a bright line. The electric light in a tube containing highly rarefied
aqueous vapour is red, the vapour is resolved into its simple elements
by the electricity, the oxygen combines with the platinum of the
negative or heat pole, and the spectrum is that of pure hydrogen with
the three most prominent bands only.
The nitrogen spectrum is brilliant with all the seven colours; there are
no broad dark spaces like those which divide the bright bands in the
hydrogen spectrum, but it is crossed by numerous very fine black and
grey lines. Fifteen of the latter stripe the red and orange; the green
is separated from the yellow by a black narrow band; it is terminated by
two bright blue lines, and very fine dark lines cross it and the rest of
the spectrum. The tube light is yellowish red.
The spectrum of highly rarefied atmospheric air is chiefly that of
nitrogen, for the oxygen combines with the platinum of the negative
terminal, and is in too small a quantity to transmit the electricity
through the tube.
The rarefied vapours of chlorine, bromine, and iodine are so rapidly
combined with the platinum of the negative terminal, that it is
difficult to determine their spectra; but they have peculiarities in
common, which distinguish them from all other spectra. The bright lines
that cross them are first at rest, but soon become flickering. In the
iodine spectrum, five of those lines of flickering light of great beauty
are in the green, two of them close together. The bromine spectrum shows
a greater number, which extend across the colours of its middle part,
accompanied by dark lines; and in the chlorine spectrum there are many
lines, both of flickering light and darkness. New lines are brought out
in the iodine spectrum by increase of temperature. At a low heat it is
crossed by a number of dark lines, but with a higher temperature the
vapour has a greenish hue, which is resolved by the prism into green
lines at some distance from one another, and fainter blue light, crossed
by groups of luminous bands.
Rarefied compound gases are resolved by the electricity into their
component parts, and the result is superposed spectra, one belonging to
each element. M. Seguin considers the aspect of the electric spark to be
a sure indication of chemical action, for while the decomposition is in
progress, the electric spark is encompassed by a halo, and the bright
lines of the double spectrum are less distinct; but when the reaction is
finished, the spark becomes slender, and the spectrum bands distinct. In
the decomposition of highly carburetted and attenuated hydrogen gas, the
spark resembles a flame, and the spectrum is like that of white light.
When the gas is decomposed, the hydrogen is disengaged, and the carbon
deposited on the extremities of the conducting wires; the spark becomes
slender, and then the lines of the hydrogen, the lines belonging to the
hydro-carbon and to carbon itself may be seen on the spectrum.[17]
The bright and coloured lines on the spectra of the gases, and the
vapours of a great number of the metals and metallic salts, were known
before MM. Bunsen and Kirchhoff began their systematic researches,
during which they added many more, some so difficult and analogous, that
it required all their skill and experience to make them out.
Of all the spectra that have been determined, those of sodium and iron
are the most important and interesting. In that of sodium, the only
light is of the purest yellow condensed into a double line of intense
brilliancy on a dark ground. The iron spectrum on the contrary is
crossed by bright lines of all intensities and colours in such
multitudes, that their number has not been ascertained. The calcium
spectrum has one very bright green band in the orange, a red line in the
yellow, and a well-defined yellow line in the indigo. As already
mentioned, the red and orange parts of the strontium are crossed by many
red lines separated by dark intervals; there is a bright blue line
between the orange and yellow, and an orange line in the blue. One
intense crimson band in the orange characterises the lithium spectrum.
Seven broad green bands stripe the yellow and a part of the green, in
the barium spectrum, and that of magnesium has many green bands and
lines.
All of these were determined by the heat of white coal gas flame, which
amounts to 2350° Cent., and at the time MM. Bunsen and Kirchhoff were
not aware that by an increase of temperature new bright lines were added
to some of the spectra. That discovery was made by Professor Tyndall,
while examining the spectrum of chloride of lithium, which with the low
temperature has only one crimson band in the orange, but with the hotter
flame of hydrogen gas, amounting to 3259° Cent., an orange line appeared
in the yellow, and when Mr. Tyndall employed the electric lamp,[18] the
spectrum acquired a broad brilliant blue band between the orange and
yellow, while the crimson band remained unchanged. Professors Roscoe and
Clifton confirmed Tyndall’s discovery, and upon comparing the spectra of
strontium and lithium, they found where only one prism was employed that
the blue line of lithium appeared to coincide with the blue line, delta,
of strontium; but with an apparatus having several prisms like that of
Kirchhoff, they saw that the two blue lines differed by one division of
the measuring scale, the lithium line being the most refrangible. A
great change was produced on the strontium spectrum by increased
electric temperature: three of the red bands vanished, and new bright
lines appeared, that were not coincident with those they replaced; the
blue line was not affected, but four new violet lines were added. With
the intense heat of the electric spark, the broad green band of the
calcium spectrum is replaced by five green lines of less refrangibility,
the well-defined yellow line vanishes, and instead of the red band three
red or orange lines appear, of greater refrangibility than those that
have vanished. Six of the bright green bands in the spectrum of barium
entirely vanish, and bright new non-coincident lines appear. Thus, not
only new lines appear at very high temperature, but the broad bands,
characteristic of the metal or metallic compound at a low temperature of
the flame or a weak spark, totally disappear at the higher temperature.
The new bright lines, which supply the part of the broad bands, are
generally not coincident with any part of the band, sometimes being less
and sometimes being more refrangible. The gentlemen who made these
experiments add, that possibly the cause of the disappearance of the
broad bands and the production of the bright lines may be, that at the
lower temperature of the flame or weak spark, the spectrum observed is
produced by the glowing vapour of some compound, probably the oxide of
the difficultly reducible metal, whereas, at the enormously high
temperature of the intense electric spark, these compounds are split up,
and the true spectrum is obtained, namely, the narrow bright lines. No
such changes take place in the easily reducible metals, potassium,
sodium, or lithium, which remain unaltered by change of temperature. In
these experiments, a bead of the metallic salt on a platinum wire was
placed between the platinum terminals, from which the spark of a
powerful inductive coil could be passed, but in order to have a more
intensely hot spark the coating of a Leyden jar was placed in
communication with the terminals of the secondary current respectively.
By this addition of static electricity, the intensity of the current was
increased four-fold, and must have been beyond estimation.
By high temperature the cæsium spectrum has been so changed, that for
number, colour and distinctness of its lines, it is the most beautiful
of those of the alkaline and earthy metals, for besides its
characteristic blue lines, it has six red and an orange-red line in the
red part of its spectrum, a fine yellow line, and nine green lines, the
last coinciding with Fraunhofer’s E. The thallium spectrum also acquires
more lines when evaporated by electricity, for besides the remarkable
green line in the green, it acquires a faint one in the orange, two of
nearly equal intensity in the green, a third fainter, and a fifth in the
blue.
MM. Plücker and Hittorf, in recent experiments, proved that many
non-metallic bodies, such as nitrogen and sulphur, give two distinctly
different spectra on change of temperature, and that the transition from
one spectrum to the other is sudden. The change is particularly striking
in sulphur, for at the moment the first spectrum attains its maximum
brightness, it disappears, and gives place to the second or high
temperature spectrum, which is one of the richest in brilliant rays
known. When the temperature is lowered the first spectrum reappears.
These changes M. Plücker ascribes to the existence of the elements in
two allotropic conditions. M. Plücker has also found that each metalloid
possesses a peculiar and characteristic spectrum: as hydrogen, which has
three bright lines, all of which are coincident with dark solar lines,
and nitrogen, which exhibits a complicated series of bands.
The experiments of the Rev. Dr. Robinson on a variety of gases and
vapours, inclosed in glass tubes, show that a greater change is produced
by pressure than by heat. At the ordinary atmospheric pressure, the
spectra show a number of bright lines on a coloured ground, the light of
which is, in general, stronger towards the red than the violet end, and
strongest in the green. In some the ground is so bright as to efface all
but the most luminous lines. This is especially the case with hydrogen.
On gradually exhausting the tube in which the vapour is contained, the
spectra rather suddenly fade away, leaving only a suspicion of one or
two lines, but upon exhausting the tube still more, these transition
spectra become bright again, fresh lines appear, and they are changed
into new spectra which are never so bright as those at ordinary
pressure. Fewer lines are visible in the rarefied spectra, and of these
four-tenths are not found in the spectra of atmospheric pressure. The
difference between the common pressure spectra, the transition, and the
rarefied spectra shows, that the character and even the existence of
certain lines depend upon the mere density of the media, the chemical
circumstances remaining unchanged. Dr. Robinson also observed that
spectra are not superposed without a change; the spectrum of atmospheric
air does not always exhibit all the lines of oxygen and nitrogen, and
occasionally there are some lines not visible in either of them. It
appears also that for certain lines the actions of bodies may be
antagonistic.
Metals do not always give the same spectrum, whatever may be the
combinations in which they are found. Among various instances M.
Mitscherlich mentions that the spectra of copper and the chloride and
iodide of copper present essential differences, and Mr. Roscoe has found
that a similar difference prevails in the spectra of carbon compounds
when in a state of incandescent gas, which have hitherto been supposed
to yield the same spectrum. ‘The spectrum obtained from the flame of
olefiant gas is different from that obtained by the electric discharge
through a vacuum of the same gas; while the spark passing through a
cyanogen vacuum produces a spectrum identical with that of the olefiant
gas flame, and through the carbonic oxide vacuum a spectrum coincident
with that of the spark through olefiant gas vacuum.’
The chlorides, bromides, and iodides are the most easily vaporized of
all the metallic salts, and give the most brilliant flames and the most
intense spectra, especially the chlorides. A small piece of the chloride
of barium volatilized by a colourless gas flame tinges the flame green,
and the red and green lines on the spectrum stand out with extreme
brilliancy. The scattered yellow light on the spectrum of the chloride
of sodium is comparatively dark by contrast with the bright lines, and
upon shading off the more luminous part of it, traces of lines are
visible in the more refrangible portion.
Chloride of lithium gives the red and orange lines on its spectrum; the
brilliant blue band discovered by Mr. Tyndall, and another more
refrangible blue line is seen when the ignition is at its greatest
intensity. Chloride of calcium gives a blue band very brightly, and
several other lines. The light of the chloride of copper is very vivid,
and its spectrum is remarkable for changing its appearance with the
decomposition of the chloride. The chlorides of lead and cadmium, also,
give very bright and definite spectra, and chloride of bismuth shows
numerous brilliant red and blue rays which quickly disappear. Thus the
chlorides give spectra with lines, such as the blue lithium and
strontium lines, hitherto only brought out by an intense electric
spark.[19]
M. Bunsen produced a beautiful effect by vaporizing a mixture of equal
parts of the chlorides of sodium, potassium, lithium, calcium,
strontium, and barium, and passing the light through the slit of his
apparatus. For on looking through the telescope the spectrum of each
substance with its characteristic coloured lines in all their brilliancy
came successively into view, and gradually faded away as each substance
was volatilized and driven off. The sequence showed the time required to
vaporize each metal, and by spectrum analysis each metal could be
recognized, although the mixture only contained the 1/1000 part of a
grain of each chloride.
The position, colour, and nature of the bright lines on the spectra of
more than thirty metals have been determined, besides those of the
elementary gases and that of the electric spark. To these M. Louis
Grandeau has added the spectrum of lightning. By a particular
arrangement the light passed at once through the slit in the instrument,
and a glass tube containing nitrogen and the vapour of water. The
general appearance of the lightning spectrum at first recalled that of
the electric spark, but on a closer examination, M. Grandeau noticed in
the spectrum of almost every flash the coincidence of a certain number
of the rays of the lightning spectrum with those of the spectra of
nitrogen and hydrogen. M. Grandeau remarks that this result is not
surprising, since all admit the production of ammonia and nitric acid
under the influence of electrical discharges. Besides the rays of
nitrogen and hydrogen, the lightning spectrum contains the ubiquitous
yellow ray of sodium.
Fraunhofer had noticed a coincidence between the double yellow sodium
line and the double dark line D of the solar spectrum, though he was not
aware to what it was due. This coincidence, observed by M. Kirchhoff
many years afterwards, was fully appreciated by him, and became the
foundation of one of the most brilliant discoveries of modern times.
During a systematic comparison between the spectra of volatilized
substances and the solar spectrum, he discovered a perfect coincidence
between Fraunhofer’s dark lines and all the bright and coloured lines on
the spectra of the volatilized substances, sodium, calcium, magnesium,
chromium, iron, and nickel. To these M. Angström has added aluminium and
manganese, and M. Plücker has very recently found that all the three
bright lines in the hydrogen spectrum are coincident with dark solar
lines, and that none of the potassium lines correspond with any solar
lines.
Drawings have been made of Fraunhofer’s spectrum placed above the
spectra of the principal metals and metallic salts, in which the
coincidence of the bright and dark lines is shown from the line A in the
extreme red to the line G in the indigo, and as the length of an
undulation of the extreme violet light of the solar spectrum is the
17/1,000,000 of an inch, and the length of an undulation of the extreme
red is the 26/1,000,000 of an inch, the length of the undulations of the
intermediate rays can be computed by the undulatory theory of light. The
length of the waves corresponding to Fraunhofer’s seven principal lines
and many of the intermediate ones have been computed, so that when a
bright or coloured line is coincident with any of these, the length of
its waves is at once known. There are other tables of Fraunhofer’s
lines, and the coincident bright ones in which each dark line is marked
by its own number, as the two principal lines in the double line D,
which are expressed by the numbers 1002·8 and 1006·8, and so with the
others; thus the coincidence of the spectra of volatilized substances
with the solar line forms a regular system.
Professor J. P. Cooke, junior, has recently constructed a spectroscope
which shows that the lines of the solar spectrum are as innumerable as
the stars of heaven, that at least ten times as many are distinctly seen
as are given by Kirchhoff in his chart, besides an infinitude of
nebulous bands just on the point of being resolved. Yet even with this
greatly increased power, the coincidences between the bright lines of
the metallic spectra and the dark lines of the solar spectrum remain
perfect. M. Kirchhoff had seen a fine yellow line between the double
lines D of the sodium spectrum. M. Merz of Munich found four additional
lines, but Professor Cooke has discovered that there are in all seven
intermediate lines and a nebulous band. Although the two members of the
sodium line D could be spread so far apart that the 1/2000 part of the
intermediate space could be readily distinguished, yet the coincidence
with the two dark Fraunhofer lines was absolute. The spectroscope ‘shows
that many of the bands of the metallic spectra are broad coloured spaces
crossed themselves by bright lines. This is the case with the orange
band of the strontium spectrum, and with the whole of the calcium and
barium spectra to a remarkable extent.’[20]
As early as the year 1849, M. Foucault discovered that the sun’s light
when shining through the electric light gives black bands on that part
of the spectrum where the electric light alone would have produced
bright bands, so that the black and bright bands could be produced
alternately by admitting or excluding the solar light; whence he
concluded that the electric arc emits the same lines which it absorbs
when they come from another luminous source. M. Angström also observed
that the bright lines on the spectra of volatilized metals could be
reversed by a stronger light shining through their flames. Neither of
these gentlemen was aware of the importance of a discovery which enabled
M. Kirchhoff to apply his delicate and refined analysis of terrestrial
matter to the sun and stars.
He had already determined the coincidence of the double yellow sodium
line with Fraunhofer’s dark line D, but while looking with a prism at a
bright solar beam passing through a yellow sodium flame, he was
surprised to see a strong and well-defined double dark line instead of
the double yellow sodium line which he expected. He obtained the very
same result, more strongly, with Drummond’s lime light, which is
brighter than the flame of any volatilized metal, and as he found that
he could produce the dark and yellow lines alternately, by admitting and
shutting out the brighter light, he concluded that the sodium flame is
subject to the law of exchange, in consequence of which it absorbs rays
of the same refrangibility with those that it emits. In fact, the soda
flame is pervious to all the rays in solar light and Drummond’s flame,
except those of the same refrangibility with its own; these it absorbs
and it may be supposed changes them into heat. Hence M. Kirchhoff came
finally to the conclusion, that the double dark line in the solar
spectrum is the reverse or negative of the double yellow line seen on
the spectrum of the sodium flame.
Quite recently, M. Fizeau has discovered that the spectrum of sodium
burning in air is reversed during the combustion. At first it is black,
with the usual double yellow line; at last, when the light is at its
maximum, the double yellow line becomes black on a continuous spectrum
with all the seven colours.
After M. Kirchhoff had ascertained that the bright lines in the spectra
of calcium, chromium, magnesium, iron and nickel coincide with dark
lines in the solar spectrum, he reversed them by sending Drummond’s
light through their respective flames, thus proving that the coloured
flames of these six metals are subject, like the sodium light, to the
law of exchanges.
M. Kirchhoff infers by analogy that the vapours of all these six metals
exist in the luminous atmosphere of the sun, and that they absorb and
change into heat such rays of the continuous light of the incandescent
solar globe as have the same refrangibility with their own, so that the
corresponding dark rayless lines on the solar spectrum are the reverses
of the bright lines in the spectra which these vapours would give were
it not for the brighter light of the sun shining through his luminous
atmosphere.
The dazzling white light of the incandescent body of the sun containing
rays of all refrangibilities would give a continuous spectrum shaded
with all the seven colours, but for his luminous absorbent atmosphere,
which comes like a veil between him and the earth, and crosses his
spectrum with thousands of dark lines, which are the reverses or
negatives of the bright lines in the spectra of the innumerable vapours
it contains, all of which must doubtless be the gases of substances
existing in the solar mass itself and vaporized by his intense heat.
Every metal, and almost every elementary substance in a state of gaseous
combustion, gives its own peculiar luminous lines to its spectrum, but
no volatilized matter can be proved to exist in the sun’s atmosphere
except such as have bright lines in their spectra coincident with some
of its dark lines.
The bright lines in the spectrum of iron, coincident with the dark lines
of the solar spectrum, are so numerous that many yet remain unknown. M.
Kirchhoff counted seventy in the small space between Fraunhofer’s lines
D and F, in which the coincidence extends even to shade, the deepest
dark lines corresponding to the most brilliant bright ones, and he
computed that the chances are as 1 to the ninth power of 10, that the
coincidence of these seventy lines is not fortuitous, but owing to a
definite cause, whence he concluded that the presence of iron vapour in
the solar atmosphere is proved with as much certainty as can be attained
in any question of natural science.
In a later publication, M. Angström observes that, although the
coincident iron lines between D and F are not so numerous as M.
Kirchhoff affirmed, they are quite sufficient to establish beyond a
doubt the presence of iron in the solar atmosphere. The iron lines are
the most characteristic in the whole solar spectrum, and if a magnifying
power be used, or if the light be refracted through several prisms,
these lines, or at any rate the stronger ones among them, appear to be
perfectly black. M. Angström noticed that on a careful examination of
the solar spectrum, certain lines can be discovered, imbedded in a mass
of fainter ones, which, with increased illumination, seem to withdraw
themselves and disappear, while the first mentioned lines, on the
contrary, only stand out in a stronger relief. These are metallic lines
of high fusion temperature; the most remarkable among them almost
invariably belong to iron.
The substances common on earth that have their vapours in the atmosphere
of the sun, though they have fewer bright lines in their spectra than
that of iron, are quite as characteristic, and quite as distinctly
coincident with their reverses, whether they be single, in groups, or
double, as the sodium line, which is brighter and its reverses darker
than that of any other substance, because volatilized sodium gives out a
greater quantity of light, and consequently absorbs a greater quantity.
M. Angström has added aluminium and manganese to the seven metals whose
vapours M. Kirchhoff has shown to exist in the atmosphere of the sun,
but he thinks it doubtful whether barium, zinc, or copper are solar
metals, for although their brighter lines correspond with distinct dark
solar lines, their weaker lines do not. Strontium is doubtful also, for
one of its strongest bright lines is not coincident with any dark line.
Though both iron and nickel are decidedly solar metals, yet as cobalt is
doubtful, it cannot be presumed that meteorites are of solar origin.
The spectrum of luminous magnesium has many green lines perfectly
coincident with those in the solar spectrum, so there is no doubt of
that metal being a constituent of the sun’s atmosphere. But there are
magnesium rays as well as some of iron of such high refrangibility that
in Mr. Stokes’s long spectrum they are situated ten times as far from H
as the whole length of the visible spectrum from A to H. These highly
refrangible rays only become visible at the exalted temperature of the
electric spark, and as they are not found in the solar spectrum, it is
inferred that the heat of the sun is inferior to that of the electric
spark.[21] Mr. Roscoe observes that this conclusion would only be
legitimate if we knew that these rays of high refrangibility are not
absorbed in passing through the atmosphere.
These are some of the most striking results of the numerous
investigations that have been made since M. Kirchhoff published his
discoveries, for the subject is anything but exhausted.
The intensely vivid light of a magnesium flame is rich in violet and
extra-violet rays, partly due to the incandescent vapour of magnesium,
and partly to the intensely heated magnesia formed by the combustion.
The properties of this light having been examined and compared with
those of the sun by Professors Roscoe and Bunsen, with a view to
photographic purposes, they came to the conclusion that ‘the steady and
equable light evolved by magnesium wire, burning in the air, and the
immense chemical action thus produced, render this source of light
valuable as a simple means of obtaining a given amount of chemical
illumination, and that the combustion of this metal constitutes a
definite and simple source of light for the purpose of photochemical
measurement.’
Bright lines of two different metals sometimes coincide with the same
black line, that is, they appear to have the same reverse as an iron and
a magnesian line, an iron and a nickel line, and some others; but it is
not known whether the coincidence be real or apparent.
M. Kirchhoff has proved that neither gold, silver, tin, lead, antimony,
arsenic, mercury, lithium, cadmium, and some others are constituents of
the sun, because none of their bright lines are coincident with any of
the dark lines of the solar spectrum. This negative discovery does no
less honour to M. Kirchhoff than the proof of so many substances being
common to the earth and sun.
Since all incandescent solid and liquid bodies give a continuous
spectrum which exhibits no dark lines, M. Kirchhoff conceives that the
sun consists of a solid or liquid nucleus, heated to the temperature of
the most dazzling whiteness, and that it is surrounded by a luminous
gaseous atmosphere of somewhat lower temperature, endowed with the law
of exchanges. The spectra of Arcturus, Capella, and many other fixed
stars are crossed by dark lines similar to, and often coincident with,
the dark lines in the solar spectrum; therefore, it may be concluded
that their structure is to a certain extent the same with that of the
sun.
Numerous observations have been made on the spectra of the fixed stars,
both in Britain and on the Continent. In England, Mr. Huggins and
Professor W. A. Miller have published tables of the measures of about
ninety dark lines in the spectrum of Aldebaran, nearly eighty in that of
α Orionis or Betelgeux, and fifteen in that of β Pegasi, with diagrams
of the two first which include the results of a comparison of the
spectra of various terrestrial elements with those of the stars. Thus
coloured lines of sodium, magnesium, calcium, hydrogen, iron, bismuth,
tellurium, antimony and mercury were found to be coincident with some of
the dark lines in the spectrum of Aldebaran, and besides these there are
numerous lines in the spectrum of this star which are probably due to
forms of matter unknown to us. Coloured lines of sodium, magnesium,
calcium, iron and bismuth, coincided with dark lines in the spectrum of
α Orionis; and β Pegasi had a spectrum closely resembling that of α
Orionis, but much fainter.
Between forty and fifty stars were examined, and it was observed that
the solar lines C and F corresponding to hydrogen, which are present in
the spectra of nearly all the stars, are wanting in those of α Orionis
and β Pegasi. With a few exceptions, the terrestrial elements hydrogen,
sodium, magnesium, and iron, which appear to be most widely diffused
through the stars, are precisely those which with the exception of
magnesium are essential to life as it exists upon the earth. Besides,
the elements hydrogen, sodium, and magnesium, represent the ocean, which
is an essential part of a world similar to the earth. Should any planets
revolve round α Orionis and β Pegasi, they probably would have no
hydrogen, consequently, no ocean and no water: therefore, they could not
be inhabited by beings constituted as we are.
Padre Secchi, the Roman astronomer, divides the stars into three types;
the first and most dominant type includes Sirius, α Lyræ, and other
white stars, which invariably contain hydrogen of high temperature, and
are denoted by a black line in their spectra, which coincides with the
solar line F; and there is another band also probably due to hydrogen in
the violet half of the stars visible to the naked eye belonging to this
group. A singular modification of this group, however, occurs in the
stars of the constellation Orion, which so rarely show any deviation
from one type, that, with the exception of α Orionis or Betelgeux, they
may be said to form a family distinguished from all the other stars in
the sky; their spectra are crossed by fine lines, faint in the violet,
with a band more or less visible in F. γ Cassiopeiæ and β Lyræ differ
from the stars of the first type in having a bright band near the solar
line F, instead of a black one.[22]
Padre Secchi’s second type includes α Orionis, α Tauri, Antares, β
Pegasi, &c., which have coloured bands in the red and orange. According
to M. Secchi, the most remarkable star in this section is α Herculis. It
gives a spectrum which has the appearance of columns illuminated on one
side; ‘the stereoscopic effect of the convexity of these bands due to
the shading is so surprising, that it cannot be beheld without
astonishment.’ The spectrum of the star δ^2 Lyræ has a similar
appearance, only instead of convex it has concave bands.
The third type consists of stars whose spectra are crossed by fine
lines, as Arcturus, Capella and our own sun.
The colours of the stars are produced by vapours existing in their
atmospheres, one colour predominating over the others, which are
absorbed by the number of dark lines.
Messrs. Huggins and Miller obtained extraordinary results from the
examination of temporary and periodic stars. Temporary stars suddenly
shine forth with great brilliancy and soon vanish or nearly vanish. A
temporary star which suddenly appeared on the night of May 12, 1866,
when examined with a spectroscope, had two spectra, showing that its
light emanated from two distinct sources. One spectrum, analogous to
that of the sun, was formed by the light of an incandescent solid or
liquid photosphere, which suffered absorption by the vapours of an
envelope cooler than itself. The second spectrum consisted of a few
bright lines, indicating that the light by which it was formed was
emitted by luminous gas: the position of some of the lines denoted
hydrogen; whence the observers believed the phenomena to result from the
burning of hydrogen with some other element, and that the photosphere
was heated to incandescence by the resulting temperature.
The variation in the brightness of periodic stars has by some been
supposed to be due to an opaque body periodically obscuring the light.
Should that body be surrounded by an atmosphere like our planet’s, its
presence would be revealed by the absence or presence of additional
lines of absorption in the spectrum of the star. Now three lines
determined in the spectrum of Betelgeux were no longer found when the
star arrived at its maximum of brightness, indicating it may be the
presence of an atmosphere round the opaque body.
With regard to our own planets, Jupiter has lines in his spectrum which
indicate the existence of an absorptive atmosphere; one band indicates
the presence of vapours similar to those existing in our atmosphere,
another band has no counterpart among the lines of absorption of the
earth’s atmosphere, and tells of some gas which it does not contain.
In the feeble spectrum of Saturn there are lines similar to those in the
spectrum of Jupiter. These lines are less strongly marked in the ansæ of
the rings, and show that the absorptive power of the atmosphere about
the rings is less than that of the atmosphere which surrounds the ball.
M. Jansen has found lines denoting aqueous vapour in the atmospheres of
both Jupiter and Saturn. Some very remarkable lines have been seen in
the more refrangible part of the spectrum of Mars supposed to be
connected with his red colour. Though the spectrum of Venus is
brilliant, and the dark lines distinct, no additional lines indicate the
existence of an atmosphere differing from our own.
The phenomena resulting from an examination of the nebulæ are most
wonderful; their light is very feeble, even that of the brightest. ‘The
total light of the whole nebula in Orion, the largest and brightest of
them, makes so small an impression on the naked eye, that you may look
twenty times at its place and not perceive any nebulous light at
all.’[23] Besides, the brightness of a surface cannot be increased by a
telescope, however good. Notwithstanding difficulties which seem to be
almost insurmountable, Mr. Huggins in England, and Padre Secchi at Rome,
have been, and still are, engaged in these researches.
The planetary nebulæ are beautiful objects; they are like planets with a
round or oval disc, equable, slightly mottled and of enormous magnitude;
one near γ Aquarii is twenty seconds, and another is twelve seconds in
diameter. Sir John Herschel computed that if these objects be as far
from us as the nearest of the fixed stars, their magnitude, on the
lowest estimation, would fill the orbit of Uranus. He discovered
twenty-eight or twenty-nine of them, some of a beautiful blue tint, in
the southern hemisphere; and from the uniformity of the discs in both
hemispheres, and their apparent want of condensation, he presumed that
they may be hollow shells emitting a feeble light from their surfaces
only. The spectrum analysis of that light, by Mr. Huggins, in six of the
planetary nebulæ, showed that their structure is utterly unlike anything
else in creation,[24] for instead of an ordinary spectrum he found, to
his infinite surprise, that the spectra of the feeble light of these
bodies consist only of three bright lines, such as those which proceed
from an intensely heated gas, and that the lines exhibited some of those
of the hydrogen and nitrogen spectra and an unknown gaseous substance:
whence he draws the astounding conclusion, that planetary nebulæ are
probably composed of hydrogen, nitrogen, and some unknown gas, without
any solid nucleus whatever.
The annular nebula in Lyra, which is probably nearest to the earth, and
the dumb-bell nebula, gave a spectrum indicating matter in a gaseous
form. The annular nebula appears to be a hollow elliptical ring of
nebulous matter of enormous magnitude. The interior opening of the ring
is not entirely dark, but filled with a faint hazy light, like fine
gauze stretched over a hoop. The dumb-bell nebula in the constellation
Vulpecula is like an hour-glass of bright matter surrounded by a thin
hazy atmosphere, which gives the whole the form of an oblate spheroid.
Both of these nebulæ when viewed with a very high telescopic power seem
to consist of minute clustering stars, but the spectra of these two
nebulæ have one bright line, the structure of both being of the same
gaseous constitution.
The great nebula on the sword handle of Orion was then examined. The
spectrum of the light from the brightest parts of this nebula, near the
trapezium, was crossed by three bright lines, in all respects similar to
those on the spectra of the planetary and other nebulæ. Other portions
of the great nebula were then brought successively under examination,
but the spectra of the whole of those portions which still were
sufficiently bright for this method of observation remained unchanged,
and exhibited the three bright lines only. The whole of the great
nebula, as far as it lay within the power of Mr. Huggins’ instrument,
emits light which is identical in its characters; the light from one
part differs from the light from another part in intensity alone. The
brighter portions of this nebula have been to a certain extent resolved
into stars, by the powerful telescopes of Lord Rosse and Professor Bond,
of the United States of America; the whole, or the greater part, of the
light from that portion of the nebula must therefore be regarded as the
united radiation of numerous stellar points. The spectrum of this
radiation being crossed by the three bright lines reveals its gaseous
source; Mr. Huggins therefore infers that at least some of these stellar
points are merely denser parts of a gaseous matter, and that the nebulæ
which he examined are enormous gaseous systems.
The spectrum of the great nebula in Orion was subsequently examined by
Padre Secchi. He describes the light of the spectrum as of a uniform
green, crossed by three bright lines; one tolerably wide and perfectly
sharp, a very slender one close to it, and the third at a little
distance from the latter. This spectrum afforded a striking contrast to
the spectra of the small stars in the brighter parts of the nebula. As
soon as the light from one of these stars entered the slit of the
instrument, its continuous spectrum was seen to flash across the field
of vision in a long coloured band. This shows that the mass of matter in
this immense nebula is in a different state from that of the stars
themselves, as Mr. Huggins had already observed. Padre Secchi does not
draw any inference from his observations as to the structure of nebulæ
in general, probably thinking it premature, but he expresses
astonishment at their results.
Since the preceding lines were written, Mr. Huggins and Professor W. A.
Miller have continued their researches on the constitution of the
celestial bodies by a method of direct simultaneous comparison of the
lines in their spectra with the lines in the spectra of many of the
terrestrial elements. The spectra for comparison were obtained from the
spark of the induction coil taken between points of various metals; and
sometimes a platinum wire was used, surrounded with cotton, moistened
with a solution of the substance required. The telescope of the
instrument was mounted equatorially, and followed the star by clockwork.
By this arrangement the spectrum of the star, and the spectrum of the
metal compared with it, are seen in juxtaposition; and the coincidence
or relative position of a dark line in the stellar spectrum with a
bright line in the metallic spectrum can be determined with great
precision.
It was found that Jupiter’s atmosphere has a much greater absorptive
power than the terrestrial atmosphere; that they have some gases or
vapours in common, but that they are not identical.
Some of the lines seen in the atmosphere of Saturn appear to be
identical with those seen in the spectrum of Jupiter.
‘The lines characterizing the atmospheres of Jupiter and Saturn are not
present in the spectrum of Mars. Groups of lines appear in the blue
portion of the spectrum; and these, by causing the predominance of the
red rays, may be the cause of the red colour which distinguishes the
light of this planet.’[25]
All the stronger lines of the solar spectrum were seen in the brilliant
light of Venus; but no additional lines indicating an absorptive action
of the planet’s atmosphere.
The authors are of the opinion that in most of the planets the light is
probably reflected from clouds floating at some distance from the
surface, so that it is not subject to the strong absorptive action of
the lower and denser strata of the planet’s atmosphere, which, like our
own, are most effective in producing atmospheric lines.
The results of the observations on the fixed stars are exceedingly
interesting, for they show that their elementary constituents are
similar, but not identical; and that although they contain many of the
sixty-five terrestrial elements, there are probably new unknown
substances also.
When seventy dark lines on the spectrum of the star Aldebaran, and
eighty on that of α Orionis (Betelgeux) were compared with the bright
lines on the spectra of the vapours of a variety of the terrestrial
simple elements, it was found that Aldebaran contained nine terrestrial
substances and α Orionis five: that is, there were only nine out of
seventy of the dark lines of Aldebaran coincident with bright lines, and
five out of eighty of those of α Orionis. Yet the seventy and eighty
dark lines that were compared represented some of the strongest only of
the numerous lines which were seen on the spectra of these stars. Some
of those remaining were probably due to the vapours of other terrestrial
elements which were not compared with these stars, but Mr. Huggins
concludes that many of those dark lines are due to new unknown elements
existing in these stars, and that we cannot assume that the sixty-five
simple terrestrial elements constitute the entire primary material of
the universe. A community of matter, however, exists throughout the
visible creation; for the stars contain many of the elements common to
the sun and earth. ‘It is remarkable that the elements most widely
diffused through the host of stars are some of those most closely
connected with the living organism of our globe, including hydrogen,
sodium, magnesium and iron. May it not be that, at least, the brighter
stars are like our sun, the upholding and energizing centres of systems
of worlds adapted to the abode of living beings?’
With regard to the nebulæ Mr. Huggins’s observations show that nine are
gaseous, the spectra of six exhibiting three bright lines, one shows an
additional faint line also, while the spectra of the dumb-bell nebula
and the annular nebula in Lyra show the brightness of three green lines
only. The spectra of eight other nebulæ were continuous, showing that
their light has not undergone any modification on its way to us.
Mr. Huggins has been able to discriminate between the light of the
nucleus of a comet and that of its tail. The nucleus is self-luminous,
and its substance is in the form of ignited gas. The coma shines by
reflected light as clouds do, and observations of the spectra give
reason to believe that comets chiefly consist of nitrogen and another
elementary body different from nitrogen combined with it.
The terrestrial elements found in the fixed stars show that, like the
sun, they have an intensely luminous nucleus: but if it be taken for
granted that highly heated gases are non-luminous internally, the
planetary nebula and the great nebula in Orion itself being thus
considered to be gaseous, must emit their feeble light from their
surfaces alone. All the true clusters of stars which are resolved by the
telescope into distinct bright points of light, give a spectrum which
does not consist of separate bright lines, but is apparently continuous
in its light. The great nebula in Andromeda, which is visible to the
naked eye, has an apparently continuous spectrum, but the whole of the
red and orange part is wanting, and the brighter parts have a mottled
appearance. The easily resolvable cluster in Hercules has a similar
spectrum; Lord Rosse discovered dark streaks or lines in both.
There is a striking correspondence between the results of prismatic and
telescopic observations; half of the nebulæ which have a continuous
spectrum have been resolved into stars, while none of the gaseous nebulæ
have been resolved even by Lord Rosse’s telescope. Thus it appears
probable that primordial nebulous matter does exist, according to the
theories of Sir William Herschel and La Place.
The structure of the sun himself, which forms one amidst the multitude
of stars which constitute the Milky Way; and the maintenance of his
light and heat without apparent waste, are still in various respects
involved in mystery.
The luminous gaseous atmosphere of the sun is of great extent and of
lower temperature, at least in its upper regions, than the photosphere
on which it rests. Mr. De la Rue’s photographs of the sun show that the
light from the border of the solar disc is less intense than that from
the equator, on account of the greater depth of solar atmosphere it has
to pass through before it reaches the earth, by which a larger portion
of the light is absorbed.
The photosphere of the sun has a mottled appearance, exhibiting minute
masses, which must be of enormous magnitude to be visible at such a
distance. They have been examined with a very high telescopic power by
Mr. Nasmyth, who describes them as lens-shaped bodies of wonderful
uniformity, and likens them to willow leaves crossing each other in all
directions, and moving irregularly among themselves. Mr. De la Rue and
Padre Secchi say they have seen something similar, and others liken them
to rice grains. Sir John Herschel[26] is of opinion that they consist of
incandescent matter sustained at a level corresponding to their density
in the solar atmosphere, an atmosphere which he considers as varying
from a liquid state below to the highest tenuity of a rarefied gas
above. In a memoir read at the Institute of Paris,[27] by M. Faye,
something of the same kind is suggested.
There are comparatively brighter waves of the sun’s disc, called faculæ,
which are portions of the sun’s photosphere thrown up into the higher
regions of his atmosphere; for Mr. De la Rue took a stereoscopic
impression of a solar spot and some faculæ, in which the spot appeared
to be a hollow and the faculæ elevated ridges. Being elevated above the
photosphere, their light is less absorbed by the sun’s atmosphere, and
by contrast they are brighter at the less luminous border of the solar
disc than at the equator.
It appears that the red flames and protuberances seen round the edge of
the sun during a total eclipse are gaseous or vaporous luminous bodies
which certainly belong to the sun; for during the total eclipse in 1860
it was observed, that as the moon moved over the sun’s disc, the red
flames and part of the corona discovered themselves at the side which
she had left, and were covered by her disc at the side towards which she
was approaching. Besides, the illuminating effect of the red light of
these flames is so inferior to its photographic power, that Mr. De la
Rue photographed one of the protuberances, although it was invisible to
the naked eye.
The sun spots which are situated in that region of the sun which lies
below the photosphere consist of a central darkness or umbra, surrounded
by a penumbra which is less dark. Professor Wilson, of Glasgow, proved
that the spots are cavities, of which the umbra or darkest part forms
the bottom, and the penumbra the sloping sides, by observing that the
umbra encroaches on that side of the penumbra which is next to the
visual centre of the sun. Hence the umbra of a spot is at a lower level
than the penumbra; and since luminous ridges and sometimes detached
portions of luminous matter cross over the spots, it is concluded that
the whole phenomenon is below the surface. The spots have an apparent
motion from east to west, due to the rotation of the sun; and Mr.
Carrington discovered that they have a proper motion also from east to
west, those nearest the solar equator moving fastest. They are confined
to the equatorial regions.
No reason has yet been assigned for the periodicity of the spots, which
go through a cycle of maxima and minima every ten years nearly. They are
singularly connected with terrestrial magnetism; the maximum of the
spots coincides with the period of the greatest disturbance of
terrestrial magnetism. The spots seem to be influenced by the planet
Venus in such a manner that when a spot comes round by rotation to the
ecliptical neighbourhood of this planet, it has a tendency to dissolve;
and, on the other hand, as the sun’s surface recedes from the planet it
has a tendency to break out into spots.[28]
PART II.
VEGETABLE ORGANISMS.
SECTION I.
MICROSCOPIC STRUCTURE OF THE VEGETABLE WORLD.
THE STUDY of the indefinitely small in the vegetable and animal
creation, is as interesting as the relation between the powers of nature
and the particles of matter.
The intimate organic structure of the vegetable world consists of a
great variety of different textures indeterminable by the naked eye, and
for the most part requiring a very high magnifying power to
discriminate. But ultimate analysis has shown that vegetables are
chemical combinations of a few very simple substances. Carbon and the
three elementary gases constitute the bases of all. No part contains
fewer than three of these universal elements, hence the great uniformity
observed in the chemical structure of vegetables. The elements unite
according to the same laws within the living plant as in the inorganic
creation, and the chemical laws acting upon them are the same. For, as
already mentioned, M. Berthelot having combined carbon and hydrogen into
acetylene, which no plant is capable of doing, he assumed it as a base
from which he deduced, by the common laws of synthetic chemistry,
hundreds of substances precisely similar to those produced by
vegetables. Although it may be inferred from this that chemical action
is the same within the vegetable as it is in the inorganic world, yet it
is accomplished within the plant under the control of the occult
principle of plant-life. No mere physical powers are capable of forming
directly out of inorganic elements, the living organism whose passage
through the cycle of germination, growth, reproduction and decay, serves
so pre-eminently to distinguish between it and inert matter. Plants,
indeed, borrow materials from the inorganic, and powers from the
physical world, to mould them into living structures, but both are
returned at death to the great storehouse of nature.
All other circumstances being the same, the vigour and richness of
vegetation are proportionate to the quantity of light and heat received.
The functions of light and heat are different, but their combined and
continued action is indispensable for the perfect development of
vegetation. Light enables plants to decompose, change into living
matter, and consolidate, the inorganic elements of carbonic acid gas,
water, and ammonia, which are absorbed by the leaves and roots, from the
atmosphere and the earth; the quantity of carbon consolidated being
exactly in proportion to the intensity of the illumination, which
accounts for the darker green tint of the tropical forests. Light acting
in its chemical character is a deoxidizing principle, by which the
numerous neutral compounds common to vegetables are formed. It is the
principal agent in preparing the food of plants, and in all the
combinations and decompositions the law of definite quantitative
proportions is maintained. It is during these chemical changes that the
specific heat of plants is slowly evolved, which, though generally
feeble, is sometimes very sensible, especially when the flowers and
fruit are forming, on account of the increase of chemical energy at that
time. To the same cause, the phosphorescence of certain flowering plants
and a few fungi, is supposed to be due.
The action of heat is manifested through the whole course of vegetable
life, but its manifestations take various forms suited to the period and
circumstances of growth. Upon it depends the formation of protein and
nitrogenous substances, which abound in the seeds, buds, the points of
the roots, and all those organs of plants which are either in a state of
activity, or are destined to future development. The heat received,
acting throughout the entire organism of a plant, may augment its
structure to an indefinite extent, and thus supply new instruments for
the chemical agency of light, and the production of new organic
compounds. The whole energy of vegetable life is manifested in this
production, and, in effecting it, each organ is not only drawing
materials, but power, from the universe around it. The organizing power
of plants bears a relation of equivalence to the light and heat which
act upon them. The same annual plant from germination to the maturation
of its seed receives about the same amount of light and heat, whatever
be the latitude, its rate of growth being in an inverse ratio to the
amount it receives in any given time. For one of the same species, the
more rapid the growth, the shorter the life.
The living medium which possesses the marvellous property of being
roused into energy by the action of light and heat, and which either
forms the whole or the greatest part of every plant, is in its simplest
form a minute globe consisting of two colourless transparent concentric
cells in the closest contact, yet differing essentially in character and
properties. The external one, which is the strongest, is formed of one
or more concentric globular layers of cellulose, a substance nearly
allied to starch, being a chemical compound of carbon, hydrogen, and
oxygen in the proportions of 12, 10, and 10, respectively.[29] It forms
the universal framework or skeleton of the vegetable world, but it has
no share whatever in the vital functions of vegetation. It only serves
as a protection to the globular cell within it, which is called the
primordial cell because it is first formed, and because it pre-eminently
constitutes the living part, since the whole phenomena of growth and
reproduction depend upon it. In its earliest stage the primordial cell
is a globular mass of an azotized colourless organizable liquid, called
protoplasm, the life blood of vegetation, containing albuminous matter
and dextrine or starch-gum. It is sufficiently viscid to maintain its
globular form, but its surface becomes slightly consolidated into a
delicate soft film. The viscid albuminous liquid within it is mixed with
highly coloured semi-transparent particles containing starch; besides
cavities or vacuoles full of a watery vegetable sap of highly refractive
power are imbedded in it. By degrees the coloured particles become more
and more condensed within a globule of mucus, which constitutes the
nucleus of the primordial cell. The watery sap in the cavities increases
so much as ultimately to fill nearly the whole of the cell at the
expense of the viscid protoplasm, which then merely forms a lining to
the cell either coloured or hyaline. The primordial cell then secretes
and envelopes itself with the strong protecting coats of cellulose
already described. On account of its high colour, which is chiefly
green, the whole contents of the primordial cell are called the
endochrome. The minute globular nucleus contains a liquid of high
refractive power, and is coated with a delicate film. Its structure,
which is best seen in the hairs and young parts of plants, is not always
the same, nor is it always in the centre of the primordial cell, being
sometimes attached to the internal cell wall.[30] On the minute but
complicated organ, the primordial cell, vegetable life depends.
It will be shown afterwards that the primordial cell sometimes
constitutes the whole plant, with or without its cellular coat. By its
continual bisection when so coated, linear plants, such as the confervæ,
are formed and lengthened (fig. 3). When bisection is about to take
place, the cell increases in length; the nucleus, which always plays an
important part in cell formation, spontaneously divides into halves; at
the same time the cell wall becomes constricted in the middle and
gradually folds between them, and divides the original cell into two new
ones, in which the nuclei become perfect and assume their normal
position. The terminal cell may undergo the same process, so that the
plant may be lengthened indefinitely.
[Illustration: Fig. 3. Development of Ulva:—A, isolated cells; B, C,
clustered subdivided cells; D, E, confervoid filaments; F, G, frond-like
expansions.]
Plants which spread in two directions are formed and increased by the
successive division of the cells into four equal parts, as in some of
the fuci, and the solid vegetable mass is formed and augmented upon the
same principle, so that it consists of a congeries of primordial cells
or globules coated with cellulose, which by mutual pressure take a
many-sided cellular form. Six or eight sides are most common; when
six-sided, a section of the solid is like honeycomb, but it frequently
resembles a very irregular fine lace or network. The form of the cells,
which not only depends upon the number of sides but on the direction of
the pressure, varies exceedingly in different plants, and in different
parts of the same plant. The size of the cells averages from the three
to the five hundredth part of an inch in diameter. Some are very large,
as in the pulp of the orange and lemon; but in the pollen of flowering
plants, and other cases, they are not more than the thousandth part of
an inch in diameter, consequently invisible to the naked eye.
Occasionally the cells are elongated in the direction of least pressure,
as in the stems and hairs of plants, or sometimes they have a stellar
form. In the looser and fleshy parts they retain their globular form and
only touch one another, leaving triangular spaces between them filled
with air in water plants; but in general the cells are held together by
a viscid liquid. When these intercellular spaces, whether left by
globular or polyangular cells, are placed the one over the other for
some distance, they constitute intercellular passages or channels, and
sometimes they form lacunæ or large empty spaces.
Notwithstanding its great variety of forms, this solid congeries of
cells, called cellular tissue, is the universal basis of vegetable
structure; it forms the principal part of all plants, and the entire
mass of many. Though often highly coloured, as in flowers, green leaves
and young shoots, it is frequently hyaline and colourless. The dark
cells in fig. 4 represent the green part of a leaf, the white ones are
those of the colourless skin. Since the primordial cell is the medium in
which light and heat act, cellular tissue is present wherever growth is
in progress, for all the vital operations take place within its cells.
All the organs of plants in their earliest stage consist entirely of
cellular tissue, and even in full grown trees the bark and pith of the
stem, as well as the soft parts of leaves and flowers, are generally
composed of the cells of this tissue, which though assuming a great
variety of forms never deviates far from the original type. Every
important change in the structure of the cell diminishes or destroys its
power of contributing to the nourishment of the plant, as appears in all
the tissues derived from it, and which, according to M. von Mohl, is a
necessary consequence of the disappearance of the vital part of the
primordial cell from those parts of the cellular tissue destined to
undergo the change.
[Illustration: Fig. 4. Vertical section of the cuticle of Iris
germanica:—_a_, cells of the cuticle; _b_, cells at the sides of the
stomata; _c_, small green cells placed within these; _d_, openings of
the stomata; _e_, lacunæ of the parenchyma; _f_, cells of the
parenchyma.]
The fibro-vascular bundles which constitute the wood of trees, and form
consecutive cylinders round the stem between the pith and the bark,
consist of vascular ducts and woody fibre. The vascular ducts are formed
of large wide cells each standing upon the other’s flattened end, their
cavities being thus separated from each other by septa or partitions
directed at right angles to their longitudinal axis. This vascular
tissue when young conveys the sap from the roots through the stem and
branches to the leaves. It forms part of the stems of all climbing and
quick-growing plants, in which the circulation of the sap is rapid, and
the perspiration great. The sap in this crude state passes freely
through the partitions, being probably a dialysable liquid; but in the
autumn, when the sap ceases to rise, the septa are either absorbed or
destroyed, as appears from the fragments of them that sometimes remain,
and then the ducts become filled with air, which they convey to mature
the sap in the leaves and all parts of the plant. Some of the vascular
ducts have very narrow parallel fibres of a bluish colour twisted in a
more or less elastic spiral from end to end of their internal surface,
which in by far the greater number of cases turns in the same direction
as a left-handed screw. In some ducts they merely cross the inner wall
of the cell at regular distances as circles. The reticulated form from
the crossing of right and left-handed spirals is still more frequent
than the simple spiral; there is scarcely a plant, from the mosses
upwards, in which that structure cannot be found.
In a vast majority of cases the secondary internal membrane of some of
the ducts is perforated by orifices of numerous forms, sometimes
irregularly, and sometimes in a regular pattern like a sieve, and on
that account they are called the pitted tissue.
In the stem of a tree the vascular cylinders alternate with cylinders of
woody fibre, as may be seen in a section perpendicular to the axis, in
which the two tissues form a series of alternate rings. The woody or
ligneous tissue, which gives strength and solidity to all vegetable
structures, consists of bundles of nearly parallel spindle-shaped
tubular fibres, having their attenuated extremities applied end to end
to the extremities of those above and below them, so that they form
groups of nearly straight lines; but although the ends of these tubular
fibres overlap each other they do not prevent a free circulation of the
sap. The different layers of these combined tissues which form the wood
do not convey the rising sap in equal quantity. The outermost layers
that are nearest the bark, which are always the last formed or youngest,
convey it in greatest quantity, and on that account are called the
sap-wood: the older the layers the less they convey, because the
interior walls of the cells of both tissues are coated with successive
layers of a mucilaginous substance which is the colouring matter of the
wood (lignin), and is called sclerogen, which becomes hard, is
ultimately united to the cell walls, and fills or nearly fills the
tubular fibres and vascular ducts, so that those nearest the centre of
the tree lose or nearly lose the power of conducting the sap, as in hard
wood like the oak, though in softer wood, as the lime tree, it is not
entirely lost. Ligneous tissue forms the chief part of the stems,
branches, and shoots of trees and shrubs; it gives firmness to leaves,
flowers, and all their parts, and strength to the stems and skins of
herbaceous plants; it is found in the bark of all trees, and constitutes
the strong fibre of hemp, flax, the agave, and many other plants, whence
linen, canvas, and cordage are made. Cells lined with sclerogen form the
shells of nuts, cocoa-nuts, and walnuts, as well as cherry, peach, and
plum stones, the brown coat of apple and pear seeds, the gritty
particles in the heart of the pear, the white coats of the pips of the
orange and lemon, the husks of peas, &c.
All the tissues are represented in fig. 5, which is a longitudinal
section of the Italian reed, much magnified. It consists of three parts:
at _a_ the cellular tissue of the pith is represented; _b_ is a
fibro-vascular bundle containing annular ducts (1), spiral ducts (2), a
pitted duct (3), besides the long spindle-shaped threads of woody fibre;
_c_ is the exterior part of the reed, which consists of cellular tissue,
the two surface rows being rather compressed and filled with coloured
particles.
Besides the spiral vessels that are attached to the interior walls of
the vascular ducts, there are groups of independent spiral vessels of
great beauty and elasticity, of which the seeds of the wild clary afford
a remarkable instance. They consist of cylindrical tubes with conical
extremities twisted into a right or left-handed screw, which can be
unrolled without breaking. They are found in the leaves of almost all
plants, in the petals and stamens of flowers, in the stalks of all
fruits, even in the minutest seeds; large parallel bundles of them
imbedded in hexagonal cellular tissue may be seen in the veins of the
kernel of the hazel nut, and they constitute the medullary sheath which
surrounds the pith in trees. They are all hollow, and capable of
conducting liquids.
[Illustration: Fig. 5. Longitudinal section of stem of Italian
reed:—_a_, pith; _b_, fibro-vascular bundles; _c_, cuticle.]
The laticiferous vessels or vasa propria, those which contain the proper
juices of plants whether milky or coloured, are exceedingly varied in
their forms and arrangement in different plants, and in different parts
of the same plant. In the leaves they generally form a delicate
capillary network, in the bark they constitute a system of long vascular
ducts forming an elongated irregular network pervious to the proper
juices throughout; sometimes they are formed of cells joined end to end,
and frequently they are thin branching flexuous tubes, meandering
through the passages or interstices of the cellular tissue, and
occasionally filling the lacunæ.[31]
Every one of the preceding tissues may be found in many of the highest
class of vegetables—those which are distinguished by having seeds with
two lobes or seed leaves, such as our common trees, shrubs, and most of
the herbaceous plants. Palms, the cereals, grasses, canes, and all
plants having seeds with but one lobe, which form the second class,
consist of cellular tissue mixed with fibro-vascular bundles; whilst in
the third or flowerless spore-bearing class, there is a general tendency
to a more and more simple structure, from the tree fern to the lichens
and algæ, which last consist of cellular tissue alone, and contain the
lowest germs of vegetable life.
In seeds the miniature plant is enclosed between the two lobes, as in
peas and beans, or in a cavity of a lobe, as in a grain of wheat or
barley; and all the parts of the embryo are merely developed into the
perfect plant during the progress of vegetation. A spore, on the
contrary, which is the seed of a Cryptogam, or flowerless class of
plants, is a most minute globular cell, full of granular matter, in
which no embryo has yet been discovered, so that the parts of the future
plant are supposed to be formed during the progress of vegetation,
instead of being developed. Seeds, and spores also, sometimes produce
new varieties, while buds and offsets only transmit the parent plant,
with all its peculiarities. In the higher classes, the organs of
nutrition and reproduction are always separate; in the lowest grades of
vegetable life they are often the same. Seeds bear no proportion to
spores either in size or number; the latter are often so extremely small
that they are invisible to the unaided eye, and are not to be counted
even by thousands. It appears that beings, whether animal or vegetable,
are prolific in the inverse ratio of their size. The incredible
multitudes of the lowest grades of vegetable life, the rapidity of their
growth, the shortness of their existence, and their enormous
fruitfulness, make them powerful agents in preparing soil for the higher
classes which are nourished by their decay. But no sooner do even the
monarchs of the forest fall than the work of destruction begins; the
light and heat which in their chemical form brought them to maturity,
now in their physical character accelerate their decay; the moss and the
lichen resume their empire, and live at the expense of the dying and the
dead, a cycle which perpetuates the green mantle of the earth.
Notwithstanding the important part these inferior beings perform in the
economy of nature, they were imperfectly known till they became a test
for the power of the microscope. Then indeed not only were the most
wonderful organisms discovered in the ostensible tribes of the
Cryptogamia, but a new and unseen creation was brought under mortal eye,
so varied, astonishing, and inexhaustible, that no limit can be assigned
to it. This invisible creation teems in the earth, in the air, and in
the waters, innumerable as the sand on the seashore. These beings have a
beauty of their own, and are adorned and finished with as much care as
the creatures of a higher order. The deeper the research, the more does
the inexpressible perfection of God’s works appear, whether in the
majesty of the heavens, or in the infinitesimal beings on the earth.