Transcriber Note
Text emphasis denoted by _Italics_.
[Illustration: JOHN TYNDALL]
THE FORMS OF WATER IN CLOUDS AND RIVERS ICE AND GLACIERS
BY
JOHN TYNDALL, LL.D., F. R. S.
_WITH TWENTY-FIVE ILLUSTRATIONS
DRAWN AND ENGRAVED UNDER THE DIRECTION
OF THE AUTHOR_
NEW YORK
D. APPLETON AND COMPANY
1899
Copyright, 1872,
By D. APPLETON AND COMPANY.
Electrotyped and Printed
at the Appleton Press, U. S. A.
AMERICAN PREFACE TO THE INTERNATIONAL SCIENTIFIC SERIES.
The rapid development of science in the present age, and the
increasing public interest in its results, make it desirable that the
most efficient measures should be adopted to elevate the character
of its popular literature. The tendency of careless and unscrupulous
book-makers to cater to public ignorance and love of the marvellous,
and to foist their crude productions upon those who are too little
instructed to judge of their real quality, has hitherto been so
strong as to cast discredit upon the idea of "popular science." It
is highly important to counteract this evil tendency by furnishing
the public with popular scientific books of a superior character.
The publication of the present volume is the first step in carrying
out a systematic enterprise of this kind. It initiates a series of
such works on a wide range of scientific subjects, to be prepared
by the leading thinkers of different countries, and known as the
"International Scientific Series."
It is designed to consist of compendious scientific treatises,
representing the latest advances of thought upon subjects of general
interest, theoretical and practical, to all classes of readers. The
familiar phenomena of surrounding Nature, in their physical and
chemical aspects, the knowledge of which has recently undergone
marked extension or revision, will be considered in their latest
interpretations. Biology, or the general science of life, which has
lately come into prominence, will be explained in its leading and
most important principles. The subject of mind, which, under the
inductive method and on the basis of its physical accompaniments
and conditions, is giving rise to a new psychology, will be treated
with the fulness to which it is entitled. The laws of man's social
development, or the natural history of society, which are now being
studied by the scientific method, will also receive a due share of
attention. While the books of this series are to deal with a wide
diversity of topics, it will be a leading object of the enterprise
to present the bearings of inquiry upon the higher questions of the
time, and to throw the latest light of science upon the phenomena of
human nature and the economy of human life.
As the first requisite of such a series of works is trustworthiness,
their preparation has been confided only to men of eminent ability,
and who are recognized authorities in their several departments.
As they are to address the non-scientific public, it is a further
requisite that they should be written in familiar and intelligible
language. It is not to be expected that the authors will all attain
to the same standard in this respect, but they are pledged to the
utmost simplicity of exposition that is possible consistently with
clear and accurate representation.
As science is now the supreme interest of civilization, and concerns
alike the people of every country, and as, moreover, it affords a
common ground upon which men of all races, tongues, faiths, and
nationalities, may work together in harmony, it seemed fitting that
an undertaking of this kind should be of comprehensive scope and
stand upon an international basis. With the growing sentiment of
sympathy and brotherhood among the most widely-separated students of
Nature, and the extensive facilities of business intercourse that now
exist, there appeared no reason why an international combination of
authors and publishers should not be effected that would be equally
favourable to their own private interests and advantageous to the
public. To gain this end and guarantee to authors better remuneration
for their work, is a distinctive purpose of the present enterprise.
But there was this difficulty in the way of any such arrangement,
that, while the rights of foreign authors are guarded by all other
civilized governments, they are not protected by the government of
the United States. To escape this difficulty, and secure American
coöperation, the first thing needed was to obtain the consent of an
American publishing-house to grant voluntarily to foreign authors
the justice which our government denies them. It was agreed by
Messrs. Appleton that they would pay the foreign contributors to
this series the full rates of copyright that are usually allowed
to American authors. When this was done, engagements were made
with distinguished scientists of England, France, Germany, and the
United States, to prepare works for the series, and with Henry S.
King & Co., of London, Germer Baillière, of Paris, and Messieurs
Brockhaus, of Leipsic, to publish them. Negotiations are pending for
the reproduction of the series in other countries, but the present
arrangements secure to the authors the benefits of the four leading
markets of the world.
It is a fact not without significance, that the proposal of this
enterprise was received with the most cordial favour by the eminent
scientific men who were solicited to aid in carrying it forward. Most
of them consented at once; but, while some were so heavily burdened
with work that they could enter into no immediate engagements, not
one of them declined to coöperate, and all promised to do so at the
earliest practicable opportunity. The feeling of the desirableness
of such an undertaking was strong and unanimous. The old dislike of
the cultivators of science to participate in the work of popular
teaching, seems very much to have passed away; and in England,
France, and Germany, alike it was freely acknowledged that _savants_
have an imperative duty to discharge in relation to the work of
general scientific education. As remarked by Prof. Virchow, of
Berlin, "the destiny of science is the service of humanity."
It was stipulated by the authors that they should have ample time
for the preparation of their books, and, as the arrangements were
recently made, only a few of the works are yet ready. Several,
however, are now in press, and will shortly appear.
Those interested in the series are under many obligations to Prof.
Tyndall for his kindness in consenting to furnish its commencing
volume. Being prepared in a short time, amid great pressure both
of laboratory and literary work, it contains somewhat less matter
than may be expected in the ensuing volumes. It treats of subjects
upon which he is perhaps the highest living authority; and it is an
admirable example of that vivid, stirring, impressive style for which
its author is so distinguished. Prof. Tyndall is not only a master in
the "scientific use of the imagination," but in kindling the action
of that faculty in his readers. He writes in pictures, so as to make
them see what he sees. In this volume he addresses himself directly
to his juvenile friends, groups them around him, takes them with him
to his favourite mountains, and thus adds a dramatic element and the
effect of personal sympathy to familiar colloquial exposition.
The "International Scientific Series" will form an elegant and
valuable library of popular science, fresh in treatment, attractive
in form, strong in character, moderate in price, and indispensable to
all who care for the acquisition of solid and serviceable knowledge;
and it is commended to American readers as a help in the important
work of sound public education.
E. L. Y.
New York, _September, 1872_.
AUTHOR'S PREFACE.
After an absence of twelve years, I visited the Mer de Glace last
June. It exhibited in a striking degree that excess of consumption
over supply which, if continued, would eventually reduce the Swiss
glaciers to the mere spectres of their former selves. When I first
saw the Mer de Glace its ice-cliffs towered over Les Mottets, and an
arm of the Arveiron, issuing from the cliffs, plunged as a powerful
cascade down the rocks. The ice has now shrunk far behind them. A
huge moraine, left behind by the retreating glacier, will mark, for
some time to come, its recent magnitude. The vault of the Arveiron
has dwindled considerably. The way up to the Chapeau lies on the
top of a lateral moraine, reached a few years ago by the surface
of the glacier, the present surface lying far below. The visible
and continual breaking away of the moraines, left thus stranded on
the mountain flank, explains the absence of ancient ridges on the
mountains where the slopes are steep. The ice-cascades of the Géant
has suffered much from the general waste. Its crevasses are still
wild, but the ice-cliffs and séracs of former days are but poorly
represented to-day.
The great Aletsch and its neighbours exhibit similar evidences of
diminution. I found moreover this year that the two ancient moraines
mentioned in paragraph 364 are parts of the same great lateral
moraine which flanked the glacier for a long period, during which its
magnitude must have remained practically constant. The place occupied
by the ancient ice-river is rendered strikingly conspicuous by this
well-preserved boundary.
During my residence at the Bel Alp this year, a catastrophe occurred
which renders, for the time being, the description of the Märgelin
See given in § 50 inappropriate. In company with two young friends I
had descended the glacier and passed through the gorge of the Massa.
On our return to the Bel Alp we found the domestics of the hotel
leaning out of the windows and looking excitedly towards the glacier.
From it proceeded a sound which resembled the roar of a cataract.
The servants remarked that the Märgelin See must have broken loose.
This was the case. For a time, however, the water flowed beneath the
glacier; but at a point about midway between the Bel Alp and the
Æggischhorn, it broke forth on the Æggischhorn side, and formed a
torrent between the glacier and the slope of the mountain. In some
places this river was more than sixty yards wide, at others it was
contracted to less than one-fifth of this width. Broken cascades of
great height were formed here and there by successive ledges of ice,
the torrent leaping with indescribable fury from ledge to ledge, and
sending a smoke of spray into the air. At one place the bottom of
the torrent was deep soft sand, which, after the water had passed,
could be seen to have been tortured into huge funnels by the whirling
eddies overhead.
Soon after we reached the Bel Alp, on the occasion just referred to,
the front of the torrent appeared at the opposite side of the valley
carrying everything movable before it, and immediately afterwards
swept through the hollow that we had traversed a little earlier in
the day. When at the end of the glacier I was struck by the force and
volume of the Massa, and the grandeur of its vault, but I could not
then account for the huge blocks of ice which it incessantly carried
down. Doubtless the eruption above had been partial before the grand
rush set in. The Rhone was considerably swollen, crops were damaged
or ruined, and the driver of the diligence was sorely perplexed to
find himself in three feet of water, without any apparent reason,
on the public highway. Two or three days subsequently I learned
at the Æggischhorn that an engineer had been sent up to report on
the possibility of opening a channel, so as to prevent any future
accumulation of water in the Märgelin See. If this be done a useful
end will be gained, by the abolition, however, of one of the most
beautiful objects in Switzerland.
J. Tyndall.
_September, 1872._
PREFACE TO THE FOURTH EDITION.
At a meeting of the Managers of the Royal Institution held on
December 12, 1825, "the Committee appointed to consider what lectures
should be delivered in the Institution in the next session," reported
"that they had consulted Mr. Faraday on the subject of engaging him
to take a part in the juvenile lectures proposed to be given during
the Christmas and Easter recesses, and they found his avocations were
such that it would be exceedingly inconvenient for him to engage in
such lectures."
At a general monthly meeting of the members of the Royal Institution,
held on December 4, 1826, the Managers reported "that they had
engaged Mr. Wallis to deliver a course of lectures on Astronomy,
adapted to a juvenile auditory, during the Christmas vacation."
In a report dated April 16, 1827, the Board of Visitors express
"their satisfaction at finding that the plan of juvenile courses of
lectures had been resorted to. They feel sure that the influence of
the Institution cannot be extended too far, and that the system of
instructing the younger portion of the community is one of the most
effective means which the Institution possesses for the diffusion of
science."
Faraday's holding aloof was but temporary, for at Christmas 1827 we
find him giving a "Course of Six Elementary Lectures on Chemistry,
adapted to a Juvenile Auditory."[A]
[A] There is no record to show that Mr. Wallis gave the Astronomical
lectures referred to, and our librarian believes that the _Christmas_
courses were opened by Faraday.
The Easter lectures were soon abandoned; but from the date here
referred to to the present time the Christmas lectures have been a
marked feature of the Royal Institution.
In 1871 it fell to my lot to give one of these courses. I had been
frequently invited to write on Glaciers in encyclopædias, journals,
and magazines, but had always declined to do so. I had also abstained
from making them the subject of a course of lectures, wishing to take
no advantage of my position here, and indeed to avoid writing a line
or uttering a sentence on the subject for which I could not be held
personally responsible. In view of the discussions which the subject
had provoked, I thought this the fairest course.
But, in 1871, the time (I imagined) had come when, without risk of
offence, I might tell our young people something about the labours
of those who had unravelled for their instruction the various
problems of the ice-world. My lamented friend and ever-helpful
counsellor, Dr. Bence Jones, thought the subject a good one, and
accordingly it was chosen. Strong in my sympathy with youth, and
remembering the damage done by defective exposition to my own young
mind, I sought, to the best of my ability, to confer upon these
lectures clearness, thoroughness, and life.
Wishing, moreover, to render them of permanent value, I wrote out
copious Notes of the course, and had them distributed among the boys
and girls. In preparing these Notes I aimed at nothing less than
presenting to my youthful audience, in a concentrated but perfectly
digestible form, every essential point embraced in the literature of
the glaciers, and some things in addition, which, derived as they
were from my own recent researches, no book previously published on
the subject contained.
But my theory of education agrees with that of Emerson, according
to which instruction is only half the battle, what he calls
_provocation_ being the other half. By this he means that power of
the teacher, through the force of his character and the vitality
of his thought, to bring out all the latent strength of his pupil,
and to invest with interest even the driest matters of detail. In
the present instance I was determined to shirk nothing essential,
however dry; and, to keep my mind alive to the requirements of my
pupil, I proposed a series of ideal ramblings, in which he should be
always at my side. Oddly enough, though I was here dealing with what
might be called the abstract idea of a boy, I realised his presence
so fully as to entertain for him, before our excursions ended, an
affection consciously warm and real.
The "Notes" here referred to were at first intended for the use of my
audience alone. At the urgent request of a friend I slightly expanded
them, and converted them into the little book here presented to the
reader.
The amount of attention bestowed upon the volume induces me to give
this brief history of its origin.
A German critic, whom I have no reason to regard as specially
favourable to me or it, makes the following remark on the style of
the book: "This passion [for the mountains] tempts him frequently
to reveal more of his Alpine wanderings than is necessary for
his demonstrations. The reader, however, will not find this a
disagreeable interruption of the course of thought; for the book
thereby gains wonderfully in vividness." This, I would say, was the
express aim of the breaks referred to. I desired to keep my companion
fresh as well as instructed, and these interruptions were so many
breathing-places where the intellectual tension was purposely
relaxed and the mind of the pupil braced to fresh action.
Of other criticisms, flattering and otherwise, I forbear to speak.
As regards some of them, indeed, it would be a reproach to that
manliness which I have sought to encourage in my pupil to return
blow for blow. If the reader be acquainted with them, this will let
him know how I regard them; and if he be not acquainted with them,
I would recommend him to ignore them, and to form his own judgment
of this book. No fair-minded person who reads it will dream that I,
in writing it, had a thought of acting otherwise than justly and
generously towards my predecessors, the last of whom, to the grief of
all who knew him, has recently passed away.
John Tyndall.
_April, 1874._
CONTENTS.
Cloud-banner of the Aiguille du Dru _Frontispiece_
PAGE
§ 1, 2. Clouds, Rains, and Rivers 1, 6
3. The Waves of Light 8
4. The Waves of Heat which produce the Vapour of our
Atmosphere and melt our Glaciers 11
5. Experiments to prove the foregoing statements 14
6. Oceanic Distillation 19
7. Tropical Rains 23
8. Mountain Condensers 27
9. Architecture of Snow 29
10. Atomic Poles 32
11. Architecture of Lake Ice 35
12. The Source of the Arveiron. Ice Pinnacles, Towers, and
Chasms of the Glacier des Bois. Passage to the Montanvert 38
13. The Mer de Glace and its Sources. Our First Climb to the
Cleft Station 43
14. Ice-cascade and Snows of the Col du Géant 46
15. Questioning the Glaciers 48
16. Branches and Medial Moraines of the Mer de Glace from the
Cleft Station 51
17. The Talèfre and the Jardin. Work among the Crevasses 52
18. First Questions regarding Glacier Motion. Drifting of
Bodies buried in a Crevasse 54
19. The Motion of Glaciers. Measurements by Hugi and Agassiz.
Drifting of Huts on the Ice 69
20. Precise Measurements of Agassiz and Forbes. Motion of a
Glacier proved to resemble the Motion of a River 60
21. The Theodolite and its Use. Our own Measurements 62
22. Motion of the Mer de Glace 66
23. Unequal Motion of the two Sides of the Mer de Glace 70
24. Suggestion of a new Likeness of Glacier Motion to River
Motion. Conjecture tested 72
25. New Law of Glacier Motion 76
26. Motion of Axis of Mer de Glace 78
27. Motion of Tributary Glaciers 79
28. Motion of Top and Bottom of Glacier 80
29. Lateral Compression of a Glacier 81
30. Longitudinal Compression of a Glacier 84
31. Sliding and Flowing. Hard Ice and Soft Ice 86
32. Winter on the Mer de Glace 88
33. Winter Motion of the Mer de Glace 93
34. Motion of the Grindelwald and Aletsch Glacier 93
35. Motion of Morteratsch Glacier 95
36. Birth of a Crevasse: Reflections 98
37. Icicles 99
38. The Bergschrund 102
39. Transverse Crevasses 103
40. Marginal Crevasses 105
41. Longitudinal Crevasses 109
42. Crevasses in relation to Curvature of Glacier 110
43. Moraine-ridges, Glacier Tables, and Sand-Cones 112
44. The Glacier Mills or Moulins 116
45. The Changes of Volume of Water by Heat and Cold 118
46. Consequences flowing from the foregoing Properties of
Water. Correction of Errors 122
47. The Molecular Mechanism of Water-Congelation 125
48. The Dirt Bands of the Mer de Glace 127
49. Sea-ice and Icebergs 132
50. The Æggischhorn, the Märgelin See and its Icebergs 136
51. The Bel Alp 139
52. The Riffelberg and Görner Glacier 140
53. Ancient Glaciers of Switzerland 145
54. Erratic Blocks 147
55. Ancient Glaciers of England, Ireland, Scotland, and Wales 150
56. The Glacier Epoch 152
57. Glacial Theories 155
58. Dilatation and Sliding Theories 155
59. Plastic Theory 156
60. Viscous Theory 161
61. Regelation Theory 163
62. Cause of Regelation 167
63. Faraday's View of Regelation 171
64. The Blue Veins of Glaciers 176
65. Relation of Structure to Pressure 183
66. Slate Cleavage and Glacier Lamination 187
67. Conclusion 191
[Illustration: CLOUD-BANNER OF THE AIGUILLE DU DRU (par. 84 and 227).]
THE FORMS OF WATER
IN CLOUDS AND RIVERS, ICE AND GLACIERS.
§ 1. _Clouds, Rains, and Rivers._
1. Every occurrence in Nature is preceded by other occurrences which
are its causes, and succeeded by others which are its effects. The
human mind is not satisfied with observing and studying any natural
occurrence alone, but takes pleasure in connecting every natural fact
with what has gone before it, and with what is to come after it.
2. Thus, when we enter upon the study of rivers and glaciers, our
interest will be greatly augmented by taking into account not only
their actual appearances, but also their causes and effects.
3. Let us trace a river to its source. Beginning where it empties
itself into the sea, and following it backwards, we find it from
time to time joined by tributaries which swell its waters. The river
of course becomes smaller as these tributaries are passed. It shrinks
first to a brook, then to a stream; this again divides itself into a
number of smaller streamlets, ending in mere threads of water. These
constitute the source of the river, and are usually found among hills.
4. Thus the Severn has its source in the Welsh Mountains; the Thames
in the Cotswold Hills; the Danube in the hills of the Black Forest;
the Rhine and the Rhone in the Alps; the Ganges in the Himalaya
Mountains; the Euphrates near Mount Ararat; the Garonne in the
Pyrenees; the Elbe in the Giant Mountains of Bohemia; the Missouri in
the Rocky Mountains, and the Amazon in the Andes of Peru.
5. But it is quite plain that we have not yet reached the real
beginning of the rivers. Whence do the earliest streams derive their
water? A brief residence among the mountains would prove to you that
they are fed by rains. In dry weather you would find the streams
feeble, sometimes indeed quite dried up. In wet weather you would see
them foaming torrents. In general these streams lose themselves as
little threads of water upon the hill sides; but sometimes you may
trace a river to a definite spring. The river Albula in Switzerland,
for instance, rushes at its origin in considerable volume from a
mountain side. But you very soon assure yourself that such springs
are also fed by rain, which has percolated through the rocks or soil,
and which, through some orifice that it has found or formed, comes to
the light of day.
6. But we cannot end here. Whence comes the rain which forms the
mountain streams? Observation enables you to answer the question.
Rain does not come from a clear sky. It comes from clouds. But what
are clouds? Is there nothing you are acquainted with which they
resemble? You discover at once a likeness between them and the
condensed steam of a locomotive. At every puff of the engine a cloud
is projected into the air. Watch the cloud sharply: you notice that
it first forms at a little distance from the top of the funnel. Give
close attention and you will sometimes see a perfectly clear space
between the funnel and the cloud. Through that clear space the thing
which makes the cloud must pass. What, then, is this thing which
at one moment is transparent and invisible, and at the next moment
visible as a dense opaque cloud?
7. It is the _steam_ or _vapour of water_ from the boiler. Within
the boiler this steam is transparent and invisible; but to keep it
in this invisible state a heat would be required as great as that
within the boiler. When the vapour mingles with the cold air above
the hot funnel it ceases to be vapour. Every bit of steam shrinks
when chilled, to a much more minute particle of water. The liquid
particles thus produced form a kind of _water-dust_ of exceeding
fineness, which floats in the air, and is called _a cloud_.
8. Watch the cloud-banner from the funnel of a running locomotive;
you see it growing gradually less dense. It finally melts away
altogether, and if you continue your observations you will not fail
to notice that the speed of its disappearance depends upon the
character of the day. In humid weather the cloud hangs long and
lazily in the air; in dry weather it is rapidly licked up. What has
become of it? It has been reconverted into true invisible vapour.
9. The _drier_ the air, and the _hotter_ the air, the greater is the
amount of cloud which can be thus dissolved in it. When the cloud
first forms, its quantity is far greater than the air is able to
maintain in an invisible state. But as the cloud mixes gradually
with a larger mass of air it is more and more dissolved, and finally
passes altogether from the condition of a finely-divided liquid into
that of transparent vapour or gas.
10. Make the lid of a kettle air-tight, and permit the steam to issue
from the pipe; a cloud is precipitated in all respects similar to
that issuing from the funnel of the locomotive.
11. Permit the steam as it issues from the pipe to pass through the
flame of a spirit-lamp, the cloud is instantly dissolved by the heat,
and is not again precipitated. With a special boiler and a special
nozzle the experiment may be made more striking, but not more
instructive, than with the kettle.
12. Look to your bedroom windows when the weather is very cold
outside; they sometimes stream with water derived from the
condensation of the aqueous vapour from your own lungs. The windows
of railway carriages in winter show this condensation in a striking
manner. Pour cold water into a dry drinking-glass on a summer's day:
the outside surface of the glass becomes instantly dimmed by the
precipitation of moisture. On a warm day you notice no vapour in
front of your mouth, but on a cold day you form there a little cloud
derived from the condensation of the aqueous vapour from the lungs.
13. You may notice in a ball-room that as long as the door and
windows are kept closed, and the room remains hot, the air remains
clear; but when the doors or windows are opened a dimness is visible,
caused by the precipitation to fog of the aqueous vapour of the
ball-room. If the weather be intensely cold the entrance of fresh
air may even cause _snow_ to fall. This has been observed in Russian
ball-rooms; and also in the subterranean stables at Erzeroom, when
the doors are opened and the cold morning air is permitted to enter.
14. Even on the driest day this vapour is never absent from our
atmosphere. The vapour diffused through the air of this room may be
congealed to hoar-frost in your presence. This is done by filling
a vessel with a mixture of pounded ice and salt, which is colder
than the ice itself, and which, therefore, condenses and freezes the
aqueous vapour. The surface of the vessel is finally coated with a
frozen fur, so thick that it may be scraped away and formed into a
snow-ball.
15. To produce the cloud, in the case of the locomotive and the
kettle, _heat_ is necessary. By heating the water we first convert
it into steam, and then by chilling the steam we convert it into
cloud. Is there any fire in nature which produces the clouds of our
atmosphere? There is: the fire of the sun.
16. Thus, by tracing backward, without any break in the chain of
occurrences, our river from its end to its real beginnings, we come
at length to the sun.
§ 2.
17. There are, however, rivers which have sources somewhat different
from those just mentioned. They do not begin by driblets on a hill
side, nor can they be traced to a spring. Go, for example, to the
mouth of the river Rhone, and trace it backwards to Lyons, where it
turns to the east. Bending round by Chambery, you come at length
to the Lake of Geneva, from which the river rushes, and which you
might be disposed to regard as the source of the Rhone. But go to
the head of the lake, and you find that the Rhone there enters it,
that the lake is in fact a kind of expansion of the river. Follow
this upwards; you find it joined by smaller rivers from the mountains
right and left. Pass these, and push your journey higher still. You
come at length to a huge mass of ice the--end of a glacier--which
fills the Rhone valley, and from the bottom of the glacier the river
rushes. In the glacier of the Rhone you thus find the source of the
river Rhone.
18. But again we have not reached the real beginning of the river.
You soon convince yourself that this earliest water of the Rhone is
produced by the melting of the ice. You get upon the glacier and walk
upwards along it. After a time the ice disappears and you come upon
snow. If you are a competent mountaineer you may go to the very top
of this great snow-field, and if you cross the top and descend at the
other side you finally quit the snow, and get upon another glacier
called the Trift, from the end of which rushes a river smaller than
the Rhone.
19. You soon learn that the mountain snow feeds the glacier. By some
means or other the snow is converted into ice. But whence comes the
snow? Like the rain, it comes from the clouds, which, as before, can
be traced to vapour raised by the sun. Without solar fire we could
have no atmospheric vapour, without vapour no clouds, without clouds
no snow, and without snow no glaciers. Curious then as the conclusion
may be, the cold ice of the Alps has its origin in the heat of the
sun.
§ 3. _The Waves of Light._
20. But what is the sun? We know its size and its weight. We also
know that it is a globe of fire far hotter than any fire upon earth.
But we now enter upon another enquiry. We have to learn definitely
what is the meaning of solar light and solar heat; in what way they
make themselves known to our senses; by what means they get from the
sun to the earth, and how, when there, they produce the clouds of our
atmosphere, and thus originate our rivers and our glaciers.
21. If in a dark room you close your eyes and press the eyelid
with your finger-nail, a circle of light will be seen opposite to
the point pressed, while a sharp blow upon the eye produces the
impression of a flash of light. There is a nerve specially devoted to
the purposes of vision which comes from the brain to the back of the
eye, and there divide into fine filaments, which are woven together
to a kind of screen called the _retina_. The retina can be excited
in various ways so as to produce the consciousness of light; it may,
as we have seen, be excited by the rude mechanical action of a blow
imparted to the eye.
22. There is no spontaneous creation of light by the healthy eye.
To excite vision the retina must be affected by something coming
from without. What is that something? In some way or other luminous
bodies have the power of affecting the retina--but _how?_
23. It was long supposed that from such bodies issued, with
inconceivable rapidity, an inconceivably fine matter, which flew
through space, passed through the pores supposed to exist in the
humours of the eye, reached the retina behind, and by their shock
against the retina, aroused the sensation of light.
24. This theory, which was supported by the greatest men, among
others by Sir Isaac Newton, was found competent to explain a great
number of the phenomena of light, but it was not found competent
to explain _all_ the phenomena. As the skill and knowledge of
experimenters increased, large classes of facts were revealed which
could only be explained by assuming that light was produced, not by a
fine matter flying through space and hitting the retina, but by the
shock of minute waves against the retina.
25. Dip your finger into a basin of water, and cause it to quiver
rapidly to and fro. From the point of disturbance issue small ripples
which are carried forward by the water, and which finally strike the
basin. Here, in the vibrating finger, you have a source of agitation;
in the water you have a vehicle through which the finger's motion
is transmitted, and you have finally the side of the basin which
receives the shock of the little waves.
26. In like manner, according to the _wave theory_ of light, you
have a source of agitation in the vibrating atoms, or smallest
particles, of the luminous body; you have a vehicle of transmission
in a substance which is supposed to fill all space, and to be
diffused through the humours of the eye; and finally, you have the
retina, which receives the successive shocks of the waves. These
shocks are supposed to produce the sensation of light.
27. We are here dealing for the most part with suppositions and
assumptions merely. We have never seen the atoms of a luminous body,
nor their motions. We have never seen the medium which transmits
their motions, nor the waves of that medium. How, then, do we come to
assume their existence?
28. Before such an idea could have taken any real root in the human
mind, it must have been well disciplined and prepared by observations
and calculations of ordinary wave-motion. It was necessary to know
how both water-waves and sound-waves are formed and propagated. It
was above all things necessary to know how waves, passing through
the same medium, act upon each other. Thus disciplined, the mind was
prepared to detect any resemblance presenting itself between the
action of light and that of waves. Great classes of optical phenomena
accordingly appeared which could be accounted for in the most
complete and satisfactory manner by assuming them to be produced by
waves, and which could not be otherwise accounted for. It is because
of its competence to explain all the phenomena of light that the wave
theory now receives universal acceptance on the part of scientific
men.
Let me use an illustration. We infer from the flint implements
recently found in such profusion all over England and in other
countries that they were produced by men, and also that the Pyramids
of Egypt were built by men, because, as far as our experience goes,
nothing but men could form such implements or build such Pyramids.
In like manner, we infer from the phenomena of light the agency of
waves, because, as far as our experience goes, no other agency could
produce the phenomena.
§ 4. _The Waves of Heat which produce the Vapour of our Atmosphere
and melt our Glaciers._
29. Thus, in a general way, I have given you the conception and
the grounds of the conception, which regards light as the product
of wave-motion; but we must go farther than this, and follow the
conception into some of its details. We have all seen the waves of
water, and we know they are of different sizes different in length
and different in height. When, therefore, you are told that the
atoms of the sun, and of almost all other luminous bodies, vibrate
at different rates, and produce waves of different sizes, your
experience of water-waves will enable you to form a tolerably clear
notion of what is meant.
30. As observed above, we have never seen the light-waves, but we
judge of their presence, their position, and their magnitude, by
their effects. Their lengths have been thus determined, and found to
vary from about 1/30000th to 1/60000th of an inch.
31. But besides those which produce light, the sun sends forth
incessantly a multitude of waves which produce no light. The largest
waves which the sun sends forth are of this non-luminous character,
though they possess the highest heating power.
32. A common sunbeam contains waves of all kinds, but it is possible
to _sift_ or _filter_ the beam so as to intercept all its light, and
to allow its obscure heat to pass unimpeded. For substances have been
discovered which, while intensely opaque to the light-waves, are
almost perfectly transparent to the others. On the other hand, it is
possible, by the choice of proper substances, to intercept in a great
degree the pure heat-waves, and to allow the pure light-waves free
transmission. This last separation is, however, not so perfect as the
first.
33. We shall learn presently how to detach the one class of waves
from the other class, and to prove that waves competent to light a
fire, fuse metal, or burn the hand like a hot solid, may exist in a
perfectly dark place.
34. Supposing, then, that we withdraw, in the first instance the
large heat-waves, and allow the light-waves alone to pass. These
may be concentrated by suitable lenses and sent into water without
sensibly warming it. Let the light-waves now be withdrawn, and the
larger heat-waves concentrated in the same manner; they may be caused
to boil the water almost instantaneously.
35. This is the point to which I wished to lead you, and which
without due preparation could not be understood. You now perceive the
important part played by these large darkness-waves, if I may use
the term, in the work of evaporation. When they plunge into seas,
lakes, and rivers, they are intercepted close to the surface, and
they heat the water at the surface, thus causing it to evaporate;
the light-waves at the same time entering to great depths without
sensibly heating the water through which they pass. Not only,
therefore, is it the sun's fire which produces evaporation, but a
particular constituent of that fire, the existence of which you
probably were not aware of.
36. Further, it is these selfsame lightless waves which, falling upon
the glaciers of the Alps, melt the ice and produce all the rivers
flowing from the glaciers; for I shall prove to you presently that
the light-waves, even when concentrated to the uttermost, are unable
to melt the most delicate hoar-frost; much less would they be able to
produce the copious liquefaction observed upon the glaciers.
37. These large lightless waves of the sun, as well as the
heat-waves issuing from non-luminous hot bodies, are frequently
called obscure or invisible heat.
We have here an example of the manner in which phenomena, apparently
remote, are connected together in this wonderful system of things
that we call Nature. You cannot study a snow-flake profoundly without
being led back by it step by step to the constitution of the sun. It
is thus throughout Nature. All its parts are interdependent, and the
study of any one part completely would really involve the study of
all.
§ 5. _Experiments to prove the foregoing Statements._
38. Heat issuing from any source not visibly red cannot be
concentrated so as to produce the intense effects just referred to.
To produce these it is necessary to employ the obscure heat of a body
raised to the highest possible state of incandescence. The sun is
such a body, and its dark heat is therefore suitable for experiments
of this nature.
39. But in the atmosphere of London, and for experiments such as
ours, the heat-waves emitted by coke raised to intense whiteness by a
current of electricity are much more manageable than the sun's waves.
The electric light has also the advantage that its dark radiation
embraces a larger proportion of the total radiation than the dark
heat of the sun. In fact, the force or energy, if I may use the term,
of the dark waves of the electric light is fully seven times that of
its light-waves. The electric light, therefore, shall be employed in
our experimental demonstrations.
40. From this source a powerful beam is sent through the room,
revealing its track by the motes floating in the air of the room; for
were the motes entirely absent the beam would be unseen. It falls
upon a concave mirror (a glass one silvered behind will answer) and
is gathered up by the mirror into a cone of reflected rays; the
luminous apex of the cone, which is the _focus_ of the mirror, being
about fifteen inches distant from its reflecting surface. Let us mark
the focus accurately by a pointer.
41. And now let us place in the path of the beam a substance
perfectly opaque to light. This substance is iodine dissolved in a
liquid called bisulphide of carbon. The light at the focus instantly
vanishes when the dark solution is introduced. But the solution is
intensely transparent to the dark waves, and a focus of such waves
remains in the air of the room after the light has been abolished.
You may feel the heat of these waves with your hand; you may let them
fall upon a thermometer, and thus prove their presence; or, best of
all, you may cause them to produce a current of electricity, which
deflects a large magnetic needle. The magnitude of the deflection is
a measure of the heat.
42. Our object now is, by the use of a more powerful lamp, and
a better mirror (one silvered in front and with a shorter focal
distance), to intensify the action here rendered so sensible. As
before, the focus is rendered strikingly visible by the intense
illumination of the dust particles. We will first filter the beam so
as to intercept its dark waves, and then permit the purely luminous
waves to exert their utmost power on a small bundle of gun-cotton
placed at the focus.
43. No effect whatever is produced. The gun-cotton might remain there
for a week without ignition. Let us now permit the unfiltered beam
to act upon the cotton. It is instantly dissipated in an explosive
flash. This experiment proves that the light-waves are incompetent to
explode the cotton, while the waves of the full beam are competent to
do so; hence we may conclude that the dark waves are the real agents
in the explosion.
44. But this conclusion would be only probable; for it might be urged
that the _mixture_ of the dark waves and the light-waves is necessary
to produce the result. Let us then, by means of our opaque solution,
isolate our dark waves and converge them on the cotton. It explodes
as before.
45. Hence it is the dark waves, and they only, that are concerned in
the ignition of the cotton.
46. At the same dark focus sheets of platinum are raised to vivid
redness; zinc is burnt up; paper instantly blazes; magnesium wire is
ignited; charcoal within a receiver containing oxygen is set burning;
a diamond similarly placed is caused to glow like a star, being
afterward gradually dissipated. And all this while the _air_ at the
focus remains as cool as in any other part of the room.
47. To obtain the light-waves we employ a clear solution of alum in
water; to obtain the dark waves we employ the solution of iodine
above referred to. But as before stated (32), the alum is not so
perfect a filter as the iodine; for it transmits a portion of the
obscure heat.
48. Though the light-waves here prove their incompetence to ignite
gun-cotton, they are able to burn up black paper; or, indeed, to
explode the cotton when it is blackened. The white cotton does not
absorb the light, and without absorption we have no heating. The
blackened cotton absorbs, is heated, and explodes.
49. Instead of a solution of alum, we will employ for our next
experiment a cell of pure water, through which the light passes
without sensible absorption. At the focus is placed a test-tube also
containing water, the full force of the light being concentrated upon
it. The water is not sensibly warmed by the concentrated waves. We
now remove the cell of water; no change is visible in the beam, but
the water contained in the test-tube now boils.
50. The light-waves being thus proved ineffectual, and the full beam
effectual, we may infer that it is the dark waves that do the work of
heating. But we clench our inference by employing our opaque iodine
filter. Placing it on the path of the beam, the light is entirely
stopped, but the water boils exactly as it did when the full beam
fell upon it.
51. The truth of the statement made in paragraph (34) is thus
demonstrated.
52. And now with regard to the melting of ice. On the surface of
a flask containing a freezing mixture we obtain a thick fur of
hoar-frost. Sending the beam through a water-cell, its luminous waves
are concentrated upon the surface of the flask. Not a spicula of the
frost is dissolved. We now remove the water-cell, and in a moment a
patch of the frozen fur as large as half-a-crown is melted. Hence,
inasmuch as the full beam produces this effect, and the luminous part
of the beam does not produce it, we fix upon the dark portion the
melting of the frost.
53. As before, we clench this inference by concentrating the dark
waves alone upon the flask. The frost is dissipated exactly as it was
by the full beam.
54. These effects are rendered strikingly visible by darkening with
ink the freezing mixture within the flask. When the hoar-frost is
removed, the blackness of the surface from which it had been melted
comes out in strong contrast with the adjacent snowy whiteness. When
the flask itself, instead of the freezing mixture, is blackened, the
purely luminous waves being absorbed by the glass, warm it; the glass
reacts upon the frost, and melts it. Hence the wisdom of darkening,
instead of the flask itself, the mixture within the flask.
55. This experiment proves to demonstration the statement in
paragraph (36): that it is the dark waves of the sun that melt the
mountain snow and ice, and originate all the rivers derived from
glaciers.
There are writers who seem to regard science as an aggregate of
facts, and hence doubt its efficacy as an exercise of the reasoning
powers. But all that I have here taught you is the result of reason,
taking its stand, however, upon the sure basis of observation and
experiment. And this is the spirit in which our further studies are
to be pursued.
§ 6. _Oceanic Distillation._
56. The sun, you know, is never exactly overhead in England. But at
the equator, and within certain limits north and south of it, the sun
at certain periods of the year is directly overhead at noon. These
limits are called the Tropics of Cancer and of Capricorn. Upon the
belt comprised between these two circles the sun's rays fall with
their mightiest power; for here they shoot directly downward, and
heat both earth and sea more than when they strike slantingly.
57. When the vertical sunbeams strike the land they heat it, and the
air in contact with the hot soil becomes heated in turn. But when
heated the air expands, and when it expands it becomes lighter. This
lighter air rises, like wood plunged into water, through the heavier
air overhead.
58. When the sunbeams fall upon the sea the water is warmed, though
not so much as the land. The warmed water expands, becomes thereby
lighter, and therefore continues to float upon the top. This upper
layer of water warms to some extent the air in contact with it,
but it also sends up a quantity of aqueous vapour, which being far
lighter than air, helps the latter to rise. Thus both from the land
and from the sea we have ascending currents established by the action
of the sun.
59. When they reach a certain elevation in the atmosphere, these
currents divide and flow, part towards the north and part towards
the south; while from the north and the south a flow of heavier and
colder air sets in to supply the place of the ascending warm air.
60. Incessant circulation is thus established in the atmosphere. The
equatorial air and vapour flow above towards the north and south
poles, while the polar air flows below towards the equator. The two
currents of air thus established are called the upper and the lower
trade winds.
61. But before the air returns from the poles great changes have
occurred. For the air as it quitted the equatorial regions was laden
with aqueous vapour, which could not subsist in the cold polar
regions. It is there precipitated, falling sometimes as rain, or
more commonly as snow. The land near the pole is covered with this
snow, which gives birth to vast glaciers in a manner hereafter to be
explained.
62. It is necessary that you should have a perfectly clear view of
this process, for great mistakes have been made regarding the manner
in which glaciers are related to the heat of the sun.
63. It was supposed that if the sun's heat were diminished, greater
glaciers than those now existing would be produced. But the lessening
of the sun's heat would infallibly diminish the quantity of aqueous
vapour, and thus cut off the glaciers at their source. A brief
illustration will complete your knowledge here.