THE GASES OF THE ATMOSPHERE

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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

LIST OF PORTRAITS

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.

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.

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.

CHAPTER II

“FIXED AIR” AND “MEPHITIC AIR”—THEIR DISCOVERY BY
BLACK AND BY RUTHERFORD

Before relating the history of the discoveries of Black, Rutherford, and Priestley, it will be appropriate to give an account of a theory which professed to explain the phenomena of combustion, and with it the conversion of metals into calces, and the reduction of these calces to the reguline or metallic state. Like other theories, it was slow in developing. Its germ is to be traced to the writings of Johann Baptist van Helmont of Brabant, Seigneur of Merode, Royenboch, Oorshot, and Pellines, who was born in Brussels in 1577. He adopted a fantastical creation of Paracelsus, the archaeus, a kind of demon which, by means of fermentation, draws together all the particles of matter. Believing that water was the true principle and origin of everything (for he had succeeded in producing a willow tree, weighing 164 lbs., from water alone, the earth in which it grew having neither gained nor lost appreciably in weight), he conceived that it was acted on by a ferment or principle pre-existing in the seed developed by it, and exhaling an odour by which the archaeus was attracted. Water undergoing the action of this ferment developed a vapour, to which van Helmont gave the name of “gas.” A “gas” was a substance intermediate between spirit and matter, and the word was probably derived from Geist, the common German word for spirit. Another word introduced by him to denote the life-principle of the stars was Blas, connected probably with blasen, to blow, and our English word blast.

It is curious to notice how the idea of an archaeus survived down to later times under the name of a “life-principle”—a conception that all organic substances must necessarily owe their origin to life itself, and not to the usual chemical and physical transformations.

Van Helmont was acquainted with various kinds of gases, as appears from his treatise “De Flatibus.” His gas sylvestre was evolved from fermenting liquors, and he knew that it was formed during the combustion of charcoal, and also that it was present in the Grotto del Cane near Naples. He was likewise acquainted with combustible gases, which he named gas pingue, gas siccum, or gas fuliginosum.

These principles of van Helmont’s apparently suggested to his successors, Becher and Stahl, the notion of a principle inherent in every combustible substance, which was lost during combustion. The development of this—the phlogistic—theory is almost wholly due to the latter chemist, and indeed it is difficult to trace Becher’s share in it.

George Ernest Stahl was born at Anspach in 1660; he studied and graduated in medicine at Halle, and in 1694 he was appointed second professor of medicine at that University, where he continued to teach for twenty-two years. His most important work was his Fundamenta chymiae dogmaticae et experimentale. His theoretical views are contained in the last part of this work. He there treats of zymotechnia, or fermentation; halotechnia, or the production of salts; and pyrotechnia, or the doctrine of combustion. It is the last of these sections which gives an account of the doctrine of phlogiston.

The fundamental conception of this doctrine is that all combustible bodies are compounds. During combustion one of these constituents, common to all, was dissipated and escaped, while the other, sometimes an acid, sometimes an earthy powder or calx, remained behind. Thus sulphur and phosphorus, when burnt, give acids; and the metals form calces. Non-combustible substances, such as lime, were imagined to be calces, and it was supposed that if phlogiston were restored to them, they too would be converted into metals. This combustible principle was thought to be inherent in all combustible bodies whatsoever; it corresponds in kind with the “sulphur” of more ancient writers, but differs from the latter inasmuch as no very precise ideas were entertained of the identity of the “sulphur” which conferred on the substances containing it as a constituent, or possessing it as a property, their power of combustion. It was also made more definite by Stahl that substances capable of burning or conversion into calces are compounds containing phlogiston in combination with other substances.

Stahl can hardly be credited with more than the invention of the term “phlogiston,” and with bringing the subject in a clear and definite form before his contemporaries. For Stahl wrote in 1720; and we find Mayow, in 1674, entering into an elaborate argument to prove that sulphuric acid is not contained in sulphur, but that it is produced by the union of the sulphur with his fire-air particles. But Stahl amplified the doctrine which Mayow had controverted, in pointing out that if such substances as phosphorus, sulphur, or metals are heated, they burn, and are changed into phosphoric acid, sulphuric acid, or “calces”; and reciprocally, if phosphoric acid, sulphuric acid, or a calx such as that of tin or lead, is heated with matter rich in phlogiston, such as charcoal, pitcoal, sugar, flour, etc., phlogiston is restored to the burnt substance, and the original material, phosphorus, sulphur, tin, or lead, is reproduced. The idea at once captivated the minds of the chemists of that age, who received it with approbation, and devised experiments designed to extend the applications of the theory and to confirm its truth.

Substances were not supposed always to be completely deprived of phlogiston by combustion. Indeed, if the phlogiston were removed wholly, or nearly so, it was by no means easy to restore it. Thus the calx of zinc, or of iron, which was regarded as nearly devoid of phlogiston, is difficult to reduce to the metallic state by ignition with substances rich in phlogiston, such as coal or charcoal. The addition of phlogiston alters the appearance of the substance as regards colour or metallic lustre, and these vary according to the proportion of phlogiston present.

There existed no very definite idea regarding the appearance or properties of phlogiston itself. Becher’s name for it was terra pinguis, and it was represented by Becher and by Stahl as a dry substance of an earthy nature, consisting of very fine particles, which were capable of being set into violent motion; this idea was derived partly from the fact that combustion is usually accompanied by flame, which was supposed to be produced by the motion of the particles of the body, communicated to it by the phlogiston.

It must not be forgotten that at this time it was perfectly well known that metals gain weight on calcination. Jean Rey was quite aware of this, and Boyle relates an experiment to show that tin gains weight when converted into calx; and it will be remembered that Mayow made experiments on the ignition of antimony by the aid of a burning-glass, and rightly conjectured that the substance produced was the same as that formed by treating it with nitric acid, and subsequent ignition. Boyle’s view was that calx of tin was a compound of tin and heat; Mayow’s more correct view was that calx of antimony was a compound of antimony and fire-air. But in spite of these well-proved facts, the adherents of the theory of phlogiston ignored them, and it does not appear to have occurred to Becher or to Stahl that they were inconsistent with their theories.

When this difficulty was stated, which was not until a much later date, a lame explanation of a metaphysical nature, and in itself contradictory, was all that could be offered. It was that phlogiston is endowed with the contrary of gravity or weight, i.e. levity or absolute lightness. This means, of course, that it is repelled by the earth. But if repelled by matter, how comes it that it enters into combination with matter? For it could not remain united if its property were to repel and not to attract. Notwithstanding this, however, the idea satisfied some as to the gain in weight which metals undergo in changing into calces.

It is indeed astonishing that men of such great ability and acumen as Black and Cavendish should have so long lain under the yoke of this absurd theory. It is probable that, in the case of these two great chemists, they stated their results in terms of the theory, partly because they were content to express the facts to which they wished to call attention in this manner, partly because they were not in a position to replace the theory by a more rational one. It is not easy to revolutionise a language, even though its vocabulary be a restricted one. The object of writing is to convey thoughts to others; and it is certainly more convenient to make use of terms understood by others, even if they only imperfectly convey the meaning which it is desired to express, than to attempt a revolution which will probably be unsuccessful, and even if successful, will at all events take time. It is not so difficult to understand Priestley’s attitude, which we shall have to consider later; for Priestley was first of all an experimentalist, and was captivated more by the acquisition of a new fact than by assigning to that fact its proper place in the cosmogony of nature.

The influence of the phlogistic theory on the knowledge of the nature of air was of such a kind as to retard its progress. For how could that knowledge be furthered, when the most active constituent of air was represented by a negation? It may be said that it is easy to be wise after the event,—in this case the discovery of oxygen; but here was a theory which was in contradiction to many known facts, and which furnished but a lame explanation of phenomena, and which had been anticipated by another theory, subsequently proved to be correct. Its sole support was the authority of its inventors or adapters, and the deeply-ingrained notions of centuries. We may read from it a lesson that it is wiser to seek out facts which test and prove a theory rather than those which support it, and we may learn for the hundredth time the folly of relying on authority, however ancient and associated with famous names it may be. This was happily expressed by Boyle when he wrote:[3] “For I am wont to judge of opinions as of coins: I consider much less in any one that I am to receive, whose inscription it bears, than what metal ’tis made of. ’Tis indifferent enough to me whether ’twas stamped many years or ages since, or came but yesterday from the mint. Nor do I regard how many or how few hands it has passed through, provided I know by the touchstone whether or no it be genuine, and does or does not deserve to have been current. For if, on due proof, it appears to be good, its having been long, and by many, received for such, will not tempt me to refuse it. But if I find it counterfeit, neither the prince’s image nor superscription, nor the multitude of hands it has passed through, will engage me to receive it. And one disfavouring trial, well made, will much more discredit it with me than all these spurious things I have named can recommend it.”

It has been necessary to enter at some length into the nature of the phlogistic theory, because the discoveries of the time were expressed in its language. The fire-air or vital air of Mayow was termed dephlogisticated air; i.e. air wholly deprived of the power of burning, or air more capable of supporting combustion than ordinary air; while airs capable of burning were supposed to be more or less highly charged with phlogiston; indeed, at one time, it was imagined that hydrogen was phlogiston itself.

It is to Joseph Black that the discovery of carbon dioxide, that constituent of air first to be definitely recognised, if we except Mayow’s early work, is generally ascribed. But we must remember that it had been prepared by Becher and by Hales, and had been doubtless obtained in an impure state by many others. It will be seen that Black’s work was so complete, and established the identity of this gas in so definite a manner, that his right to be named as its true discoverer can hardly be questioned.

Black was born near Bordeaux in 1728. His father, a wine-merchant, was originally a native of Belfast, being descended from a Scottish family which had been settled there for some time. When twelve years of age. Black returned to Belfast, and received his education in the local grammar-school, afterwards proceeding to the University of Glasgow in 1746, at the age of eighteen. He was a pupil of Dr. Cullen, then Lecturer on Chemistry at the College there, who is mentioned by Professor Thomas Thomson, in his History of Chemistry, as an excellent and instructive lecturer. Black intended to choose the career of medicine, and he indeed practised occasionally as a medical man during the greater part of his life.