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in the original text, however, they are also used to designate a
subscript.
Equal signs "=" before and after a word or phrase indicate =bold=
in the original text.
Carat characters "^" are used to designate a superscript.
Small capitals have been converted to SOLID capitals.
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Typographical errors have been silently corrected but other variations
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THE GASES OF THE ATMOSPHERE
[Illustration]
[Illustration: STEPHEN HALES.]
THE GASES
OF
THE ATMOSPHERE
THE
HISTORY OF THEIR DISCOVERY
BY
WILLIAM RAMSAY, F.R.S.
PROFESSOR OF CHEMISTRY IN UNIVERSITY COLLEGE, LONDON
WITH PORTRAITS
London
MACMILLAN AND CO., LTD.
NEW YORK: THE MACMILLAN CO.
1896
_All rights reserved_
“Modern discoveries have not been made by large
collections of facts, with subsequent discussion,
separation, and resulting deduction of a truth thus
rendered perceptible. A few facts have suggested an
_hypothesis_, which means a _supposition_ proper
to explain them. The necessary results of this
supposition are worked out, and then, and not till
then, other facts are examined to see if their
ulterior results are found in Nature.”
--DE MORGAN, _A Budget of Paradoxes_, ed. 1872, p. 55.
PREFACE
The discovery of new elementary gas in the atmosphere in 1894 aroused
much interest, and public attention has again been directed to the
air, which was, for many centuries, a fruitful field for speculation
and conjecture. The account of this discovery, communicated to the
Royal Society in January 1895, was, however, necessarily couched in
scientific language; and many matters of interest to the chemist and
physicist were written in an abbreviated style, in the knowledge that
the passages describing them would be easily understood by the experts
to whom the communication was primarily addressed. But persons without
any special scientific training have frequently expressed to me the
hope that an account of the discovery would be published, in which the
conclusions drawn from the physical behaviour of argon should be
accompanied by a full account of the reasoning on which they are based.
An endeavour to fulfil this request is to be found in the following
pages. And as the history of the discovery of the better known
constituents of the atmosphere is of itself of great interest, and
leads up to an acquaintance with the new stranger, who has so long been
with us incognito, an effort has here been made to tell the tale of the
air in popular language.
CONTENTS
CHAPTER I PAGE
THE EXPERIMENTS AND SPECULATIONS OF BOYLE, MAYOW,
AND HALES 1
CHAPTER II
“FIXED AIR” AND “MEPHITIC AIR”--THEIR DISCOVERY
BY BLACK AND BY RUTHERFORD 38
CHAPTER III
THE DISCOVERY OF “DEPHLOGISTICATED AIR” BY
PRIESTLEY AND BY SCHEELE--THE OVERTHROW
OF THE PHLOGISTIC THEORY BY LAVOISIER 68
CHAPTER IV
“PHLOGISTICATED AIR” INVESTIGATED BY CAVENDISH--HIS
DISCOVERY OF THE COMPOSITION OF WATER 119
CHAPTER V
THE DISCOVERY OF ARGON 146
CHAPTER VI
THE PROPERTIES OF ARGON 181
CHAPTER VII
THE POSITION OF ARGON AMONG THE ELEMENTS 216
LIST OF PORTRAITS
STEPHEN HALES _Frontispiece_
ROBERT BOYLE _To face page_ 8
JOHN MAYOW " 17
JOSEPH BLACK " 48
DANIEL RUTHERFORD " 62
JOSEPH PRIESTLEY " 72
ANTOINE AUGUSTE LAVOISIER " 102
HON. HENRY CAVENDISH " 121
CHAPTER I
THE EXPERIMENTS AND SPECULATIONS OF BOYLE, MAYOW, AND HALES
To tell the story of the development of men’s ideas regarding the
nature of atmospheric air is in great part to write a history of
chemistry and physics. This history is an attractive and varied one:
in its early stages it was expressed in the quaint terms of ancient
mythology, while in its later developments it illustrates the advantage
of careful experimental inquiry. The human mind is apt to reason from
insufficient premisses; and we meet with many instances of incorrect
conclusions, based upon experiment, it is true, but upon experiment
inadequate to support their burden. Further research has often proved
the reasoning of the Schoolmen to be futile; not indeed from want of
logical method, but because important premisses had been overlooked.
Among the errors which misled the older speculators, three stand out
conspicuously. These are--
First, _The confusion of one gas with another_. Since gases are for the
most part colourless, and always transparent, they make less impression
on the senses than liquids or solids do. It was difficult to believe in
the substantiality of bodies which could not be seen, but the existence
of which had to be inferred from the testimony of other senses; indeed,
in certain instances only by the sense of touch, for many gases
possess neither smell nor taste. This peculiarity led, in past ages,
to the notion that air possessed a semi-spiritual nature; that its
substantiality was less than that of other objects more accessible to
our senses. We meet with a relic of this view in words still in common
use. Thus the Greek words =πνέω=, I blow, and =πνεῦμα=, a spirit or
ghost, are closely connected; in Latin we have _spiro_, I breathe, and
_spiritus_, the human spirit; in English, the words _ghost_ and _gust_
are cognate. And the same connection can be traced in similar words in
many other languages.
Our sense of smell is affected by extremely minute traces of gases
and vapours--traces so small as to be unrecognisable by any other
method of perception, direct or indirect. A piece of musk retains
its fragrant odour for years, and the most delicate balance fails to
detect any appreciable loss of weight in it. We are capable of smelling
gases only: liquids and solids, if introduced into the nostrils,
irritate the olfactory nerves, but do not stimulate them so as to
incite the sense of smell; yet the admixture of a minute trace of some
odorous vapour with air appears entirely to change its properties.
The effect of inhaling such air, although sometimes pleasant, is very
different from the sensation produced by pure inodorous air, and such
admixtures were in olden times naturally taken to be air modified in
its properties. But such modifications are obviously almost infinite
in number, for varieties of scent are excessively numerous; and it
was therefore perhaps deemed useless to attempt to investigate such a
substance as air, whose properties could change in so inexplicable and
mysterious a manner. Owing, therefore, to its elusive and, as it were,
semi-spiritual properties, and to its unexpected changes of character,
it was long before its true nature was discovered. It had not escaped
observation that “air” obtained by distilling animal and vegetable
matter, or by the action of acids on iron and zinc, differed from
ordinary air by being inflammable; but such “airs” were regarded as
atmospheric air, modified in some manner, as it is modified when
perfumed. And “airs” escaping from fermenting liquids, or produced by
the action of acids on carbonates, were neglected. For long no attempt
was made to catch them; and the frothing and bubbling were regarded
as a species of boiling, as is still seen in the use of our word
“fermentation” (_fervere_, to boil).
Second, _Erroneous ideas regarding the phenomena of combustion_. While
it was recognised that a burning candle was extinguished if placed in
a confined space, its extinction was attributed not to the absence of
air, but to the impossibility of the escape of flame. Indeed, flame was
regarded as possessing the same semi-spiritual, semi-material nature as
air. Together with earth and water, air and flame or fire formed the
four elementary principles of the Ancients; and all substances--stones,
metals, animals, and vegetables--were regarded as partaking of the
properties of these elements, and often as being constituted of the
latter in varying proportions, according as they were cold and dry
(earth), cold and moist (water), hot and moist (air), or hot and dry
(fire). It is not within the scope of this book to enter into details
regarding such ancient views. Those who are interested in the matter
will find them expounded in Kopps’ _History of Chemistry_, Rodwell’s
_Dawn of Chemistry_, E. von Mayer’s _History of Chemistry_, and in
other similar works. But we shall be obliged to consider the later
developments of such ideas in the _phlogistic theory_, by means of
which all chemical changes connected with combustion were interpreted
from the latter part of the seventeenth to the end of the eighteenth
century. With erroneous views regarding the nature of combustion, and
ignorance as to the part played by the atmosphere in the phenomena of
burning, the true nature of air was undiscoverable.
Third, _The lack of attention to gain or loss of weight_. It was in
past times not recognised that nothing could be created and nothing
destroyed. In popular language, a candle is destroyed when it is
burned, nothing visible being produced from it. The products, we now
know, are gaseous and invisible, and possessed of greater weight than
the unburnt candle; but for want of careful experiment, it was formerly
concluded that the candle, when burnt, was annihilated. The formation
of a cloud in a cloudless sky; the growth of vegetables in earth,
from which, apparently, they did not derive their substance; and the
reputed growth of metalliferous lodes in mines--these were all adduced
as proofs of the creative power of Nature. With such ideas, therefore,
the necessity of controlling the gain or loss of material during
experiment, by determining gain or loss of weight, did not appear
imperative; and hence but few quantitative experiments were made, and
little importance was attached to these few. It had, for example,
long been noticed that certain metals gained weight when burned and
converted into a “calx,” or, as we should now say, a metallic oxide,
but such gain in weight was not regarded as of any consequence. When
considered in relation to the supposed loss of “phlogiston” suffered
by a metal on being converted into a calx, it was explained by the
hypothesis that phlogiston possessed “levity”--the antithesis of
gravity--and that the calx weighed more than the metal, owing to its
having lost a principle which was repelled instead of being attracted
by the earth.
Among the most remarkable early attempts to elucidate the true nature
of air, we meet with one by the Hon. Robert Boyle, who published about
the middle of the seventeenth century his _Memoirs for a General
History of the Air_. Boyle was one of the most distinguished scientific
men of his own, or indeed of any, age, and in his spirit of calm
philosophical inquiry he was far in advance of his contemporaries.
He was born in the early part of the year 1626, in Ireland, whither
his father, Richard Boyle, had emigrated at the age of twenty-two.
Boyle’s mother, daughter of Sir Geoffrey Fenton, principal Secretary
of State for Ireland, died while he was still a child. Yet she must
have lived in the recollection of her son Robert, for he wrote: “To
be such parents’ son, and not their eldest, was a happiness that
our Philarethes (himself) would mention with great expressions of
gratitude; his birth so suiting his inclinations and designs, that had
he been permitted an election, his choice would scarce have altered
God’s discernment.”
In those days of early development, Boyle had finished his school-days
at Eton by his twelfth year. He informs us that he devoured books
omnivorously, and could hardly be induced to join in games. The next
six years of his life he spent on the Continent with his elder brother;
and on his father’s death, which happened when he was abroad, he
returned to England, and settled at Stalbridge, in Dorsetshire, where
he had inherited a manor. Here he passed most of his life in great
retirement, with only an occasional visit to London; for though he
lived through troublous times, he avoided politics. Indeed, he is
known only to have appeared once on a public platform, and that was in
defence of the Royal Society, then in its infancy, from attacks made
upon it by some too scrupulously loyal Churchmen.
[Illustration: ROBERT BOYLE.]
Boyle did not confine his attention exclusively to scientific pursuits:
he interested himself deeply in theology, and published numerous tracts
on religious subjects. He wrote with equal ease in English, French,
and Latin, and his books appeared simultaneously in the first and last
of these languages. His researches are remarkable for their wide range
and for the boldness of his conceptions. But Boyle, ingenious though
he was, was unable to fathom the mystery of atmospheric air. His views
regarding it are succinctly stated by him in his _Memoirs for a General
History of the Air_, and in the same work he sums up the views of the
Ancients. His words are:
“The Schools teach the air to be a warm and moist
element, and consequently a simple and homogeneous body.
Many modern philosophers have, indeed, justly given up
this elementary purity in the air, yet few seem to think
it a body so greatly compounded as it really appears to
be. The atmosphere, they allow, is not absolutely pure,
but with them it differs from true and simple air only as
turbid water from clear. Our atmosphere, in my opinion,
consists not wholly of purer aether, or subtile matter
which is diffused thro’ the universe, but in great number
of numberless exhalations of the terraqueous globe; and the
various materials that go to compose it, with perhaps some
substantial emanations from the celestial bodies, make up
together, not a bare indetermined feculancy, but a confused
aggregate of different effluvia. One principal sort of
these effluvia in the atmosphere I take to be saline, which
float variously among the rest in that vast ocean; for they
seem not to be equally mixed therein, but are to be found
of different kinds, in different quantities and places,
in different seasons.... Many men talk much of a volatile
nitre in the air, as the only salt wherewith that fluid is
impregnated. I must own the air, in many places, seems to
abound in corpuscles of a nitrous nature; but I don’t find
it proved by experiments to possess a volatile nitre. In
all my experiments upon salt-peter, I found it difficult
to raise that salt by a gentle heat; and spirits of nitre,
which is drawn by means of a vehement one, has quite
different properties from crude nitre, or the supposed
volatile kind in the air, for ’tis exceeding corrosive.”[1]
Boyle then proceeds to collect and comment on the effluvia from
volcanoes and from decaying vegetables and animals, and proposes tests
for the presence of such ingredients. He even attributes the darkening
of silver chloride to its being a reagent for certain salts present
in air at one time and not at another, and draws attention to the
sulphurous smell produced by “thunder.” Farther on (p. 61) he writes:
“The generality of men are so accustomed to judge of things by their
senses, that because the air is invisible they ascribe but little to
it, and think it but one remove from nothing. And this fluid is even
by the Schoolmen considered only as a receptacle of visible bodies,
without exerting any action on them unless by its manifest qualities,
heat and moisture; tho’, for my part, I allow it other faculties,
and among them, such as are generative, maturative, and corruptive;
and that, too, in respect not only of animals and bodies of a light
texture, but even of salts and minerals.”
In another place (p. 17) he states:
“I conjecture that the atmospherical air consists of
three different kinds of corpuscles: the first, those
numberless particles which, in the form of vapours or
dry exhalations, ascend from the earth, water, minerals,
vegetables, animals, etc.; in a word, whatever substances
are elevated by the celestial or subterraneal heat, and
thence diffused into the atmosphere. The second may be
yet more subtile, and consist of those exceedingly minute
atoms the magnetical effluvia of the earth, with other
innumerable particles sent out from the bodies of the
celestial luminaries, and causing, by their impulse, the
idea of light in us. The third sort is its characteristic
and essential property, I mean permanently elastic parts.”
Boyle also relates experiments designed to “produce what appears to be
air”; and he describes the production, by the action of oil-of-vitriol
on steel filings, of “air” (now known as hydrogen) which possessed
the property of elasticity; although he failed to notice its
inflammability. He further obtained carbon dioxide by the fermentation
of raisins, and probably also hydrogen chloride in the gaseous form
by breaking a bulb containing “some good spirit-of-salt” in a vacuous
receiver.
The result of shrewd reasoning power, applied, however, to imperfect
observations, is well illustrated by the following passages:
“For tho’, by reason of its great thinness and of its
being, in its usual state, devoid both of taste and smell,
air seems wholly unfit to be a menstruum [or solvent]; yet
it may have a dissolving, or at least a consuming, power
on many bodies, especially such as are peculiarly disposed
to admit its operations. For the air has a great advantage
by the vast quantity of it that may come to work, in
proportion to the bodies exposed thereto.... Thus we find a
rust on copper that has been long exposed to the air.”[2]
Boyle, shortly after, describes the production of “an efflorescence of
a vitriolic nature” on marcasite (or sulphide of iron) which has been
exposed to the air; and he relates that the “ore of alum, robb’d of its
salt, will in tract of time recover it by being exposed to the air, as
we are assured by the experienced _Agricola_.”
To account for such actions, and for combustion, he proceeds (p. 81):
“The difficulty we find in keeping flame and fire
alive, tho’ but for a little time, without air, renders
it suspicious that there may be dispersed thro’ the rest
of the atmosphere some odd substance, either of a solar,
astral, or other foreign nature; on account whereof the
air is so necessary to the subsistance of flame.... It
also seems by the sudden wasting or spoiling of this
fine substance, whatever it be, that the bulk of it is
but very small in proportion to the air it impregnates
with its vertue; for after the extinction of the flame,
the air in the receiver was not visibly alter’d; and for
ought I could perceive by several ways of judging, the air
retained either all, or at least the far greatest part, of
its elasticity; which I take to be its most genuine and
distinguishing property. And this undestroyed springyness
of the air, with the necessity of fresh air to the life of
hot animals, suggest a great suspicion of some vital
substance, if I may so call it, diffused thro’ the air;
whether it be a volatile nitre, or rather some anonymous
substance, sidereal or subterraneal; tho’ not improbably of
kin to that which seems so necessary to the maintenance of
the other flames.”
The experimental part of Boyle’s work relates to the oxidation of
cuprous to cupric compounds, with the change of colour from brown to
blue or green, either in ammoniacal or in hydrochloric acid solution;
and he goes so far as to prove that two ounces of marcasites broken
into small lumps, and kept in a room “freely accessible to the air,
which was esteemed to be very pure,” for somewhat less than seven
weeks, gained above twelve grains by oxidation.
In his _Memoirs for a General History of the Air_, Boyle draws up a
programme of research, of the carrying out of which, however, there is
no record. He proposes (p. 23):
“1. To produce air by fermentation in well clos’d receivers.
“To produce air by fermentation in sealed glasses.
“To separate air from liquors by boiling.
“To separate air from liquors by the air-pump.
“To produce air by corrosion, especially with spirit of vinegar.
“To separate air by animal and sulphureous dissolvants.
“To obtain air in an exhausted receiver by burning-glasses
and red-hot irons.
“To produce air out of gunpowder and other nitrous bodies.
“2. To examine the produced aerial substances by their preserving
or reviving animals, flame, fire, the light of rotten wood,
and of fish.
“To examine it by its elasticity, and the duration thereof.
“To do the same by its weight, and its elevating the fumes
of liquors.”
We shall all agree that if Boyle had successfully carried out such
experiments, our knowledge of the true nature of air would have come
quite a century before it did. Some of these experiments were indeed
made by John Mayow, his contemporary, whose work and speculations we
shall now proceed to consider.
John Mayow was born in the parish of St. Dunstan, London, in 1645. His
family was originally Cornish, having come from Bree, in Cornwall. He
entered Wadham College, Oxford, at the early age of sixteen, and was
shortly afterwards made a probationer-fellow of All Souls’ College.
After the usual three years of study, he took his degree in Law; but
not being attracted by the legal profession, he turned his attention
to medicine, and became a medical practitioner at Bath, where he lived
during the fashionable season. When not more than twenty-three years of
age, he wrote two essays on Respiration, ascribing the inflation of the
lungs to the action of the intercostal muscles. These “Tractatus duo”
were published in 1668. Some years later he produced the treatise on
which his fame rests; it is entitled “Tractatus quinque medico-physici,
quorum primus agit de sal-nitro et spiritu nitro-aëreo; secundus, de
respiratione; tertius, de respiratione foetus in utero et ovo; quartus,
de motu musculari, et spiritibus animalibus; ultimus, de rhachitide;
studio Joh. Mayow, LL.D. & Medici, nec non Coll. Omn. Anim. in Univ.
Oxon. Socii. Oxonii e Theatro Sheldoniano, An. Dom. MDCLXXIV.”
The work was dedicated to Sir Henry Coventry. It was inserted in
an abridged form in the _Philosophical Transactions_ of the Royal
Society, some time after its publication, but received only scant
recognition, for the fame of Newton and Boyle overshadowed the
labours of less well-known investigators. And Mayow did not live to
press his discoveries on the attention of his contemporaries, for he
died in 1679, five years after the publication of his tracts, in his
thirty-fourth year. Little is known of Mayow’s domestic life, save
that he married shortly before his death. His scientific work proves
that if he had been granted the usual span of life, his extraordinary
genius would have furthered the knowledge of the true explanation
of the nature of air, and its function in supporting combustion and
respiration, and that his views would have been accepted more than
a century before Lavoisier--with fuller knowledge, and with the
scientific position which at once gained a hearing--forced precisely
similar doctrines upon the attention of the scientific world.
[Illustration: John Mayow.]
Mayow was a contemporary of Boyle, and frequently made use of Boyle’s
experiments in support of the theories which he advanced. Curiously
enough, while Boyle seems to have read Mayow’s work, he does not appear
to have been favourably impressed by his conclusions. Boyle, at the age
of fifty-two, had doubtless formed his own opinions, and was unwilling
that they should be disturbed by the speculations, well founded though
they were, of so young a man. And shortly after Mayow’s death, the
views of Becher, one of his contemporaries, expounded and made definite
by Stahl, regarding the nature of combustion, were universally received.
After Lavoisier’s theories had overthrown these false views, attention
was again directed to Mayow’s tracts by Johann Andreas Scherer, in
a work published at Vienna in 1793, and also by Dr. Yeats in 1798.
Scherer gives a careful analysis of Mayow’s work, somewhat altering the
order of his paragraphs, with a paraphrase in German of the Latin text,
which he quotes in full. Yeats’ treatise is more especially concerned
with the medical aspect of Mayow’s work, although it also deals with
the purely chemical portion at considerable length. In the following
account of Mayow’s researches, free use has been made of both of these
works, as well as of his own “Tracts.”
Mayow’s contributions to the chemistry of the atmosphere may be
classified thus:--
1. The atmosphere consists of particles of two kinds of gases at least:
one of these, termed “nitro-aerial particles,” is necessary for the
support of life and for the combustion of inflammable bodies; while the
other, left after this constituent has been removed, is incapable of
supporting either life or combustion. The portion which is necessary
for life enters, during respiration, into the blood. It is the chief
cause of motion in animals and in plants.
2. These “nitro-aerial particles” are also present in saltpetre or
nitre, as can be shown by mixing inflammable substances, such as
sulphur and charcoal, with nitre to form gunpowder, filling a tube
with the powder, and, after setting it on fire, immediately plunging
the open end of the tube under water. The sulphur and charcoal will be
as completely consumed as if burned in the open air. Such combustion
might, however, be ascribed to a “sulphureous” constituent in
saltpetre; by “sulphureous” is to be understood combustible, for those
substances capable of burning were imagined to contain a “sulphur”
which gave them that property. That nitre does not contain such
“sulphur” can be shown by exposing it alone to heat, when no change
takes place, except fusion. Besides, nitre is compounded of “spirit
of nitre” or nitric acid and pure alkali, neither of which contains a
combustible sulphur; hence the particles of fire-air must be present in
nitre in no small amount. But it is probable that it is the spirit of
nitre which contains such fire-air particles, because, as will be shown
later, they are not present in the alkali.
One difficulty occurs to Mayow. How is it that so large a quantity
of gas as is necessary to support combustion can be contained in a
relatively small bulk of saltpetre? He tries whether a solution of
saltpetre evolves air-bubbles when placed in a vacuum, and finds that
it effervesces less than pure water does. He also prepares saltpetre
by mixing nitric acid and alkali in a vacuum; a brisk effervescence
occurs, and the dried-up salt is ordinary saltpetre. Hence saltpetre
cannot contain elastic air. Mayow consequently draws a distinction
between “air” and “air-particles.”
The residue left after the “fire-air,” or _spiritus igneo-aerius_, has
been removed from ordinary air by breathing or by combustion is proved
to be lighter than the fire-air itself; because a mouse dies sooner if
kept at the top of air in a confined bell-jar than at the bottom; and
a candle goes out sooner. Here the conclusion is right, although the
reason given is wrong; for it is the temperature of the respired air
which makes it rise, and not the fact that it is specifically lighter
than the oxygen.
Metallic antimony gains in weight when it is set on fire by a lens,
and burns; if this gain in weight, Mayow remarks, is not due to the
absorption of nitro-aerial particles and to the fire, it is difficult
to say to what it is due.
The reason why substances burn so violently in nitre compared with air,
is because of the proximity of the fire-air particles; and these are
evidently due to the nitric acid, because the residue--the alkali--if
mixed with sulphur and inflamed, does not produce ignition.
3. All acids contain fire-air particles, for acids have great
similarity to each other. This is shown as follows:--Antimony made into
a calx by the sun’s rays with a burning-glass gives the same calx as
when it is evaporated repeatedly with nitric acid and converted into
“Bezoar-mineral,” _i.e._ oxide of antimony. And iron-rust obtained from
sulphide of iron appears to be formed by the union of the fire-air
particles with the metallic “sulphur” of the iron.
It has up till now been believed that sulphuric acid is an ingredient
of common sulphur. But this is unlikely, for sulphur has a sweetish,
and not an acid taste. Moreover, quite a different substance from
a vitriol (or sulphate) is obtained by melting together alkali and
sulphur; and no effervescence takes place during its preparation.
Sulphur, too, is precipitated out of the “liver of sulphur” (potassium
persulphide) by the addition of sulphuric acid. Now, were sulphuric
acid contained in sulphur, it would hinder the union of the sulphur
with the alkali.
It is to be noticed that the volatile sulphuric acid, from the
combustion of sulphur, is produced in the following way:--“The flame
of the burning sulphur consists, like every other flame, in the
violent motion of the sulphur particles with that of the nitro-aerial
particles; hence the sulphur particles, at first solid, become sharp
and acid, and probably form the ordinary ‘spirit of sulphur’ (sulphuric
acid). If this be not so, I know not in what manner this acid can be
produced; for, as has been shown, it is very improbable that it
previously existed in the mass of the sulphur before its deflagration.
Such a change also, in all probability, takes place in pyrites, when
it is converted to green vitriol; because pyrites yields sulphur on
distillation; and the green vitriol on distillation gives sulphuric
acid, leaving red colcothar (iron oxide) behind.”
Similarly, nitre appears to be a triple salt, formed by the union of
the fiery part of air with a salt-like substance existing in the
earthy material, together forming nitric acid; and this added to earthy
salts (alkali) yields ordinary nitre. “I have tried to show that
all acids consist of certain saline particles rendered fluid by the
nitro-aerial particles.”
4. Boyle has shown that a flame is extinguished more rapidly in
a vacuous space than in a confined space containing air; this is
obviously due to absence of nourishment in the air, rather than to
its choking by its own vapours; for in the vacuous vessel there is
evidently more space for such noxious vapours than in the air-filled
vessel, and yet the flame is more rapidly extinguished. Moreover, no
combustible matter can be kindled in a vacuum by means of a
burning-glass. But it must not be concluded that this fire-air
constitutes the whole of ordinary air; because a candle goes out in air
confined in a glass while a large quantity of air is still contained in
it.
While gunpowder burns owing to the fire-air particles which it
contains, and requires no sustenance from external air, the combustion
of vegetables is supported partly by the igno-aerial particles which
they themselves contain, partly by those of the external air.
Air which has supported combustion loses to some extent its elasticity
(_i.e._ diminishes in volume), as shown by the burning of a candle in
air confined over water. This is to be ascribed partly to actual loss
of elasticity, partly to the absorption of the fire-air. The loss of
volume amounts to about three per cent of the whole quantity of air
taken.
All this is exceedingly clear, and in accordance with our modern views,
but Mayow’s mind is somewhat confused with reference to flame and heat,
since he imagined that the diminution of the volume of air in which
combustible substances have been burned is due to the escape of heat;
and inasmuch as a rise of temperature was known to increase the volume
of air, so a loss of heat should, in his opinion, produce the opposite
effect. The fire-air particles are apparently regarded as a sort of
compound of heat with matter (as indeed in a certain sense they are);
and by combustion or by respiration both are removed. The loss of
volume is to be explained by the removal of both from the air, and the
gain in weight by the union of the matter with the combustible body,
such as antimony.
Such is a brief account of Mayow’s views on the nature of atmospheric
air. But the tale would be incomplete without mention of the fact that
he prepared a gas by the action of nitric acid on iron, viz. nitric
oxide, which, when introduced into ordinary air confined over water,
decreased its volume; and he found that further admission of nitric
oxide produced no further diminution in the volume of the air. A very
little more, and he would have recognised in this a means of analysing
air, and depriving it wholly of its oxygen. He goes so far as to
speculate that a compound is formed between the nitric oxide and the
oxygen, but the solubility of gases in water appears not to have struck
him as important. He notices, however, that the combination of the two
gases is attended by rise of temperature, and is in so far analogous to
combustion.
It would lead us too far to consider in detail Mayow’s theories of
fermentation and of respiration. Suffice it to say that he ascribes the
production of animal heat to the consumption of his fire-air particles
by the animal, and remarks that the pulse is heightened by respiration.
This view was in opposition to that held by his contemporaries, viz.
that the purpose of respiration was to cool the blood.
It is impossible to avoid being impressed with the clearness and
justice of Mayow’s inferential reasoning. All that was wanting was the
discovery of oxygen and carbon dioxide, and the identification of the
first with his fire-air, and of the second with one of the products
of combustion. But these discoveries were not made until a century
after his death. Had he lived, there can be little doubt that, unless
discouraged by the want of appreciation with which his ideas were
received, he would have continued to labour in the fruitful fields from
which he had already reaped so rich a harvest.
Before leaving the seventeenth century, it is perhaps fitting to
mention the name of Jean Rey, a French physician, who wrote in 1630
concerning the gain in weight of tin and lead when calcined. While Rey
exhibited some leaning towards the modern methods of experimentation,
he still lay fettered in the bonds of mediæval scholasticism. In
discussing the weight of air and fire, he finds occasion to consider
the question whether a vacuum can exist. His words are so quaint that
they are worth quoting: “It is quite certain that in the bounds of
Nature a vacuum, which is nothing, can find no place. There is no power
in Nature from which nothing could have made the universe, and none
which could reduce the universe to nothing: that requires the same
virtue. Now the matter would be otherwise if there could be a vacuum.
For if it could be here, it could also be there; and being here and
there, why not elsewhere? and why not everywhere? Thus the universe
could reach annihilation by its own forces; but to Him alone who could
make it is the glory of being able to compass its destruction.” And
since air cannot be drawn down by a vacuum, it must descend by virtue
of its own weight when it fills a hole. And hence, as air has weight,
tin and lead gain in weight when they combine with air. It will be
admitted that this is very inferior to the speculations and deductions
of Boyle and Mayow.
The next stage in the history of our subject is the consideration
of the work of Stephen Hales and of Joseph Priestley. Both of these
philosophers were essentially experimentalists. While both discovered
gases and prepared them in a more or less pure state, Hales had no
theory to guide him, and concluded as the result of his researches
that air was possessed of “a chaotic nature”; for he did not recognise
his gases as different kinds of matter, but supposed them all to be
modified air. Priestley, on the other hand, was an adherent of the
theory of phlogiston, and interpreted all his experiments by its help.
Hales was a country clergyman, interested in botany, and undertook
researches on air in order to gain knowledge of the growth and
development of plants. Priestley was also a divine, who amused himself
with experiments during the intervals of composing sermons or writing
controversial pamphlets on disputed doctrines. Both possessed the
experimental faculty, and both employed it to good purpose.
Hales’ chief work is entitled “Statical Essays, containing Vegetable
Staticks; or an account of Statical Experiments on the Sap in
Vegetables, being an Essay towards a Natural History of Vegetation:
of use to those who are curious in the Culture and Improvement of
Gardening, etc.: Also, a specimen of an attempt to analyse the air by a
great Variety of Chymiostatical Experiments, which were read at several
meetings before the Royal Society. By Stephen Hales, D.D., F.R.S.,
Rector of Farringdon, Hampshire, and Minister of Teddington, Middlesex.”
In his “Introduction” Hales reveals his method of research. The
determination of weight and volume was at that date especially
necessary; for want of numerical data the experimental researches of
the time were of a somewhat vague character, and it often happened that
the conclusions drawn from them were incorrect. Hence it is with a
feeling of satisfaction that we read (vol. i. p. 2):--
“And since we are assured that the all-wise Creator has observed the
most exact proportions of _number_, _weight_, and _measure_ in the
make of all things, the most likely way, therefore, to get any insight
into the nature of those parts of the creation which come within our
observation must in all reason be to number, weigh, and measure. And
we have much encouragement to pursue this method of searching into
the nature of things, from the great success which has attended any
attempts of this kind.” For God has “comprehended the dust of the earth
in a measure, and weighed the mountains in scales, and the hills in a
balance.”
From experiments on the rise of sap in plants, many of them very
ingenious and well adapted to secure their end, and which are still
regarded by botanists as classic, Hales noticed that a quantity of
air was inspired by plants. In order to ascertain the composition and
amount of this air, the process of distillation was resorted to; for
Hales remarks: “That elasticity is no immutable property of air is
further evident from these experiments; because it were impossible for
such great quantities of it to be confined in the substances of animals
and vegetables, in an elastick state, without rending their constituent
parts with a vast explosion” (Preface, p. viii.). Hence, concluding
that the air absorbed by plants and animals could be recovered by their
distillation, Hales proceeded to distil a great number of substances of
animal and vegetable origin, such as hogs’ blood, tallow, a
fallow-deer’s horn, oystershell, oak, wheat, peas, amber, tobacco,
camphor, aniseed oil, honey, beeswax, sugar, Newcastle coal, earth,
chalk, pyrites, a mixture of salt and bone-ash, of nitre and bone-ash,
tartar, compound aquafortis, and a number of other substances. He
collected the “air” in each case over water, and gave numerical data to
show what proportion the air bore by weight to the substance from which
it had been obtained. He even tried to compare the weight of ordinary
air with that of air from distilled tartar; but his experiment led to
no positive conclusion, because of the crudeness of his appliances. The
compressibility or “elasticity” of the air from tartar, however, was
found to be identical with that of common air.
Hales does not appear to have made any special experiments on the
properties of his various airs, by trying whether they supported
combustion, whether they were themselves combustible, etc. We see from
this list that he had under his hands mixtures of hydrocarbons, carbon
dioxide, probably sulphur dioxide, hydrochloric acid and ammonia (both,
however, dissolving in water as they were formed), oxides of nitrogen,
possibly chlorine, and, as minium or red-lead was one of the substances
he tried, oxygen in a more or less pure state. It must be remembered
that in all cases the gas obtained was mixed with the air originally
present in the retort. He next proceeded to produce “air” by the
fermentation of grain, of raisins, and of other fruits; this “air”
obviously was carbon dioxide more or less pure.
It is curious to note here that he anticipated Lord Kelvin in devising
a sounding-lead which should register the depth of the sea by the
compression of air, the distance to which the air had receded along the
tube being shown by the entry of treacle. He successfully carried out a
sounding by means of his apparatus.
The next series of experiments related to the generation of “air” by
the action of acids on metals. _Aqua-regia_ and gold, _aqua-regia_
and antimony, _aquafortis_ and iron, dilute oil-of-vitriol and iron,
yielded gases which contracted on standing in contact with water.
This, in the case of the oxides of nitrogen, is to be ascribed to
their reacting with the oxygen of the air accidentally present in the
receiver; but in the last case Hales noticed that the gas absorbed in
cold weather was re-evolved on rise of temperature, as one would expect
with hydrogen.
These experiments led him to investigate the action of certain mixtures
on ordinary air. Thus a mixture of spirits of hartshorn (or ammonia)
with iron filings absorbed 1½ cubic inches of air, and one with
copper filings, twice as much. Further, a mixture of iron filings and
brimstone absorbed in two days no less than 19 cubic inches of air.
But it is disappointing to find that, in spite of all the experimental
facts which Hales accumulated, he was unable to make use of them. The
prejudice in favour of the unity and identity of all these “airs” was
too great for him to overcome. True, he sometimes theorises a little,
as for example when he remarks (p. 285):--“If fire was a particular
kind of body inherent in sulphur (_i.e._ combustible matter of all
kinds), as _Mr. Homberg_, _Mr. Lemery_, and some others imagine, then
such sulphureous bodies, when ignited, should rarefy and dilute all
the circumambient air; whereas it is found by many of the preceding
experiments, that acid sulphureous fuel constantly attracts and
condenses a considerable part of the circumambient elastick air: an
argument that there is no fire endued with peculiar properties inherent
in sulphur; and also that the heat of fire consists principally in the
brisk vibrating action and re-action between the elastick repelling air
and the strongly attracting acid sulphur, which sulphur in its Analysis
is found to contain an inflammable oil, an acid salt, a very fix’d
earth, and a little metal.”
Enough has now been said to give a fair idea of Stephen Hales’
researches. It will suffice if his conclusions be stated in his own
words (p. 314):--
“Thus, upon the whole, we see that air abounds in animal, vegetable,
and mineral substances; in all which it bears a considerable part; if
all the parts of matter were only endued with a strongly attracting
power, whole nature would then immediately become one unactive cohering
lump; wherefore it was absolutely necessary, in order to the actuating
and enlivening this vast mass of attracting matter, that there should
be everywhere intermix’d with it a due proportion of strongly repelling
elastick particles, which might enliven the whole mass, by the
incessant action between them and the attracting particles; and since
these elastick particles are continually in great abundance reduced by
the power of the strong attracters, from an elastick to a fixt state,
it was therefore necessary that these particles should be endued with a
property of resuming their elastick state, whenever they were
disengaged from that mass in which they were fixt, that thereby this
beautiful frame of things might be maintained in a continual round of
the production and dissolution of animal and vegetable bodies.
“The air is very instrumental in the production and growth of animals
and vegetables, both by invigorating their several juices while in an
elastick active state, and also by greatly contributing in a fix’d
state to the union and firm connection of several constituent parts of
those bodies, viz. their water, salt, sulphur, and earth. This band of
union, in conjunction with the external air, is also a very powerful
agent in the dissolution and corruption of the same bodies; for it
makes one in every fermenting mixture; the action and re-action of the
aerial and sulphureous particles is, in many fermenting mixtures, so
great as to excite a burning heat, and in others a sudden flame; and it
is, we see, by the like action and re-action of the same principles, in
fuel and the ambient air, that common culinary fires are produced and
maintained.
“Tho’ the force of its elasticity is so great as to be able to bear a
prodigious pressure, without losing that elasticity, yet we have, from
the foregoing Experiments, evident proof that its elasticity is easily,
and in great abundance destroyed; and is thereby reduced to a fixt
state by the strong attraction of the acid sulphureous particles which
arise either from fire or from fermentation; and therefore _elasticity_
is not an _essential immutable property_ of air-particles; but they
are, we see, easily changed from an elastick to a fixt state, by the
strong attraction of the acid, sulphureous, and saline particles
which abound in air. Whence it is reasonable to conclude that our
atmosphere is a _Chaos_, consisting not only of elastick, but also
of unelastick air-particles, which in plenty float in it, as well as
the sulphureous, saline, watery, and earthy particles, which are no
ways capable of being thrown off into a permanently elastick state,
like those particles which constitute true permanent air. Since, then,
air is found so manifestly to abound in almost all natural bodies;
since we find it so operative and active a principle in every chymical
operation; since its constituent parts are of so durable a nature, that
the most violent action of fire or fermentation cannot induce such an
alteration of its texture as thereby to disqualify it from resuming,
either by the means of fire or fermentation, its former elastick state;
unless in the case of vitrification, when, with the vegetable Salt and
Nitre in which it is incorporated, it may, perhaps, some of it, with
other chymical principles, be immutably fixt,--since then this is the
case, may we not with good reason adopt this now fixt, now volatile
_Proteus_ among the chymical principles, and that a very active one, as
well as acid sulphur; notwithstanding it has hitherto been overlooked
and rejected by chymists, as no way intitled to that denomination?”
This quotation shows us how little Mayow’s shrewd reasoning and
well-devised experiments had impressed the thinkers of his age. While
Hales quotes frequently from Boyle’s and Newton’s works, his reference
to Mayow is meagre; nor does he adopt any one of Mayow’s conclusions.
One would have thought that, having prepared so many gases by means
of apparatus well adapted to their purpose, and having observed that
certain substances introduced into air produced contraction, he would
have drawn the conclusion that such “airs” were essentially different
kinds of matter. But the “Proteus” was too much for him; and he left
the subject practically in the same state of “Chaos” in which he found
it.
[1] _Memoirs for a General History of the Air_; Shaw’s Abridgment of
Boyle’s works, edition 1725, vol. iii. p. 26.
[2] “Suspicions about some hidden qualities of the Air,” _ibid._ p. 77.