CHAPTER II. “fixed Air” and “mephitic Air”--Their Discovery by Black and

“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: “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.

He began his medical studies in Edinburgh in the year 1751, and in 1755 he published, as his thesis for the degree of M.D., the work which has rendered his name famous. It appears that as early as 1752 he had been occupied with investigations on quicklime, which was then attracting attention as a remedy for urinary calculi. Opinion was divided regarding its virtue. In a manuscript copy of notes of Black’s lectures, which the author is so fortunate as to possess, he mentions that his attention was directed to the subject through the rival views of Drs. Alston and Whytt. It was not long before he proved that, in opposition to the commonly received notion, quicklime had gained nothing from the fire in which it was made, but that the limestone used for its preparation had lost nearly half its weight in becoming caustic. He also attempted successfully to trap the escaping gas, and again placed it in presence of lime, confining it over water. Instead of any escape of material when the lime became mild, “nothing escapes--the cup rises considerably by absorbing air.” And in his notes, a few pages farther on, he compares the loss of weight undergone by limestone on being calcined, with its loss on being dissolved in muriatic acid. These experiments appear from his journal to have been made before November 1752.

His thesis was not published, however, until 1755. Immediately after, in 1756, he succeeded Dr. Cullen as Professor in Glasgow, where he remained until 1766. During these ten years he began and made great progress with his well-known researches on the heat of fusion of ice, and the heat of vaporisation of water, or, as he termed them, the “latent heats” of water and of steam. In 1766, Dr. Cullen was appointed Professor of Medicine in the University of Edinburgh, and Black again succeeded him as Professor of Chemistry. There he lectured until 1797, when he retired from public life; he died as peacefully as he had lived, in 1799, in the seventy-first year of his age. Thomson, who relates these particulars, was one of his last students; he writes:--“I never listened to any lectures with so much pleasure as to his; and it was the elegant simplicity of his manner, the perfect clearness of his statements, and the vast quantity of information which he contrived in this way to communicate, that delighted me.... His illustrations were just sufficient to answer completely the object in view, and no more.”

Black’s original thesis for his degree was entitled Experiments upon Magnesia Alba, Quicklime, and other Alcaline Substances. It was published in 1755, and several times reprinted. It is now to be had in a convenient form as one of the “Alembic Club Reprints.”

It was the custom in those days to administer alkalies as a remedy for urinary calculi; and about the year 1750 lime-water was tried as a substitute. Opinion was divided as regarded its efficacy; and it was with the view of preparing a better remedy that Black undertook researches on magnesia alba. Black prepared magnesia from “bittern,” which remains in the pans after the crystallisation of salt from sea-water, and also from Epsom salts, “which is evidently composed of magnesia and the vitriolic acid.” The magnesia is thrown down from the sulphate as carbonate, by the addition of pearl ashes, at the temperature of ebullition, the soluble product being “vitriolated tartar,” or potassium sulphate. He describes how “magnesia is quickly dissolved with effervescence or explosion of air, by the acids of vitriol, nitre, and of common salt, and by distilled vinegar,” and gives an account of the properties of the sulphate, nitrate, chloride, and acetate. He subsequently heated this magnesia, and found that it lost “a remarkable proportion of its weight in the fire,” and his “attempts were directed to the investigation of this volatile part.” The residue in the retort did not effervesce on the addition of acids; hence the volatile part had been driven away by the heat. “Chemists have often observed, in their distillations, that part of the body has vanished from their senses, notwithstanding the utmost care to retain it; and they have always found, upon further inquiry, that subtile part to be air, which, having been imprisoned in the body, under a solid form, was set free, and rendered fluid and elastic by the fire. We may safely conclude that the volatile matter lost in the calcination of magnesia is mostly air; and hence the calcined magnesia does not emit air, or make an effervescence when mixed with acids.”

Magnesia, thus freed from “air” by ignition, was dissolved in “spirit of vitriol” and thrown down with an alkali. Its weight was nearly equal to that which it possessed before calcination, and it again effervesced with acids. “The air seems to have been furnished by the alkali, from which it was separated by the acid; for Dr. Hales has clearly proved that alkaline salts contain a large quantity of fixed air, which they emit in great abundance when joined to a pure acid. In the present case, the alkali is really joined to an acid, but without any visible emission of air: and yet the air is not retained in it; for the neutral salt, into which it is converted, is the same in quantity, and in every other respect, as if the acid employed had not been previously saturated with magnesia, but offered to the alkali in its pure state, and had driven the air out of it in their conflict. It seems, therefore, evident that the air was forced from the alkali by the acid, and lodged itself in the magnesia.”

After an account of some experiments showing that magnesia is not identical with lime or with alumina, he proceeds:--“It is sufficiently clear that the calcareous earths in their native state, and that the alkalis and magnesia in their ordinary condition, contain a large quantity of fixed air; and this air certainly adheres to them with considerable force, since a strong fire is necessary to separate it from magnesia, and the strongest is not sufficient to expel it entirely from fixed alkalis, or take away their power of effervescing with acid salts.

“These considerations led me to conclude that the relation between fixed air and alkaline substances was somewhat similar to the relation between these and acids: that as the calcareous earths and alkalis attract acids strongly, and can be saturated with them, so they also attract fixed air, and are, in their ordinary state, saturated with it; and when we mix an acid with an alkali, or with an absorbent earth, that the air is then set at liberty, and breaks out with violence; because the alkaline body attracts it more weakly than it does the acid, and because the acid and air cannot both be joined to the same body at the same time.... Crude lime was therefore considered as a peculiar acrid earth, rendered mild by its union with fixed air; and quicklime as the same earth, in which, by having separated the air, we discover that acrimony or attraction for water, for animal, vegetable, and for inflammable substances.”

The solubility of slaked lime in water is next discussed. If a solution of lime “be exposed to the open air, the particles of quicklime which are nearest the surface gradually attract the particles of fixed air which float in the atmosphere.”

Black next points out that, on mixing magnesia alba with lime-water, the air leaves the magnesia and joins itself to the lime; and as both magnesia and calcium carbonate are insoluble in water, the water is left pure. Similarly quicklime deprives alkalies of their air and renders them caustic. And it follows that if caustic alkali be added to a salt of magnesia or of lime, it will separate the magnesia or the calcareous earth from the acid, in a condition free from “air” but combined with water.

In order to show that the “air” which exists in combination with lime or alkalies is not the air which is contained in solution in water, lime-water was placed under an air-pump, along with an equal quantity of pure water; on making a vacuum, an approximately equal amount of air was evolved from each. “Quicklime, therefore, does not attract air when in its most ordinary form, but is capable of being joined to one particular species only, which is dispersed through the atmosphere, either in the shape of an exceedingly subtile powder, or more probably in that of an elastic fluid. To this I have given the name of fixed air, and perhaps very improperly; but I thought it better to use a word already familiar in philosophy than to invent a new name, before we be more fully acquainted with the nature and properties of this substance, which will probably be the subject of my further inquiry.”

The next proceeding was to render “mild alkali” caustic by means of lime, and to determine that nearly the same amount of acid is required to saturate the caustic alkali as to saturate the mild alkali from which the caustic alkali had been prepared. On exposure to air for a fortnight, the caustic alkali again became mild, owing to its absorption of fixed air. Careful experiments were made to prove that such caustic alkali contains no lime, and does not therefore owe its causticity and corrosive properties to the presence of that ingredient. The volatile alkali (ammonium carbonate) was also rendered caustic, and Black “obtained an exceedingly volatile and acrid spirit, which neither effervesced with acids nor altered in the least the transparency of lime-water; and although very strong was lighter than water, and floated upon it like spirit of wine.”

After a description of some unsuccessful attempts to render mild alkalies caustic by heat alone (i.e. to expel carbon dioxide from potassium carbonate), Black examines the action of the “sedative salt” or boracic acid on mild alkalies, by rubbing them together in presence of some water. At first there is no effervescence, but on adding successive quantities of boracic acid, brisk effervescence finally takes place, borax being formed. “This phenomenon may be explained by considering the fixed alkalis as not perfectly saturated with air ... if they expel a small quantity of air from some of the salt, this air is at the same time absorbed by such of the contiguous particles as are destitute of it.” And on “exposing a small quantity of a pure vegetable fixed alkali (carbonate of soda) to the air, in a broad and shallow vessel, for the space of two months,” crystals were obtained, which possessed a milder taste than that of ordinary salt of tartar, which effervesced with acids more violently than usual, and which could not be mixed with the smallest portion of boracic acid without emitting a sensible quantity of air (hydrogen sodium carbonate). It therefore follows that such alkaline substances have an attraction for fixed air; and this was proved by mixing magnesia alba in fine powder with caustic alkali, and shaking for some time. The magnesia was converted into the variety which did not effervesce with acids, and the alkali was rendered mild, like a solution of salt of tartar. These are the principal results of Black’s researches, and he concludes with a table of affinity of acids for fixed alkali, calcareous earth, volatile alkali, and magnesia, contrasting it with the affinity possessed by fixed air for the same bases.

It was the habit of the Scottish students to pass down notes taken during the lectures of their professors from one generation to another. As the lectures were generally read, and not delivered extempore, the process resulted in an almost verbatim report of the actual words of the lecturer. One of these copies of lectures, bearing the date 1778, gives an account of the experiments which have been described, in words almost identical with those used in the thesis of 1755. Black appears to have shown his class this air, made, however, according to Hales’ plan, by heating magnesium carbonate in a bent gun-barrel, and collected over water in the usual way. He demonstrated its weight by pouring it from one vessel to another, and showed that it extinguished the flame of a candle. He mentions also that in 1752 he discovered that this air is the same as choke-damp, and that it is fatal to animal life. He speaks of the Grotto del Cane, and observes that fixed air is produced by fermentation, and by the burning of charcoal, and showed to his class experiments in which air from each source is shaken with lime-water, giving a turbidity of carbonate. The well-known experiment of inspiring air through lime-water, which, owing to the small amount of carbonic anhydride it contains, does not produce a turbidity, and expiring through lime-water, showing the formation of carbon dioxide in the lungs, is described and performed. He next describes Cavendish’s experiments on the solubility of fixed air and its density, and researches by Dr. Brownrigg and Dr. Gahn of Sweden on its occurrence in mineral waters. He also explains how calcareous petrifactions are produced by the escape of fixed air from water, which then deposits its dissolved calcium carbonate, present in solution as bicarbonate. The deposit of iron from chalybeate waters is ascribed to the same cause, and the explanation is attributed to Mr. Lane.

“Upon the whole,” these manuscript notes relate, “this sort of air is quite distinct from common air, though it is commonly mixed with it in small quantity.” “With regard to its origin, when treating of inflammable substances and metals I shall consider this more completely. I shall now only hint that it is a vital air, changed by some matter, seemingly the principle of inflammability. This appears from several phenomena when an animal or burning body is enclosed with a certain quantity of this air, until it is changed as much as possible.” The air is diminished in volume by the breathing of the animal or by the burning of the candle. And Dr. Priestley has found that “growing vegetables had the power of restoring this sort of air to common or vital air again, which must be by their taking away some matter which it had received from the burning body or animal.”

Black’s account of fixed air and its properties is the first example we possess of a clear and well-reasoned series of experimental researches, where nothing was taken on trust, but everything was made the subject of careful quantitative measurement. It was not long since Hales had pronounced air to be a chaotic mixture of effluvia. Black showed that common air contains a small amount of fixed air, and that fixed air must be considered as a fluid differing in many of its properties from common air, especially in its being absorbed by quicklime and by alkalies. It must be remembered that at that time carbon was not recognised as an element; and hence, though Black knew that fixed air was a product of the combustion of charcoal, he did not attribute it to the union of carbon with oxygen, although the sentence quoted above closely approaches to the truth.

The discovery of nitrogen was next in the order of time. It was made by Daniel Rutherford, a pupil of Black’s, and at his instigation, and its description formed a thesis for his degree of Doctor of Medicine.

Daniel Rutherford was born at Edinburgh on November 3rd, 1749. He was the son of a medical man, Dr. John Rutherford, one of the founders of the Medical School in that city. He was educated at Edinburgh University, and after graduating in Arts, became a medical student, taking his degree of M.D. in 1772. His diploma was obtained on 12th September. He then travelled for three years in England, France, and Italy, and in 1775 he returned to his native town, where he practised his profession. In 1786 he succeeded Dr. John Hope in the Chair of Botany in his University, but he did not on that account resign his practice. He was president of the Royal College of Physicians of Edinburgh from 1796 to 1798. During the greater part of his life he suffered from gout; he died in 1819, at the age of seventy.

Rutherford does not seem to have pursued the study of chemistry further: his duties led him into other fields. His genial, pleasant face, seen in the portrait by Raeburn, shows him to have possessed a happy disposition; and he is said to have maintained until his death his friendship with Black, and his interest in the progress which science was then rapidly making.

The title of Rutherford’s dissertation, of which I have been able to find a copy only in the British Museum, is Dissertatio Inauguralis de aere fixo dicto, aut mephitico. It was published at Edinburgh in 1772, seventeen years after Black’s memorable dissertation on Fixed Air. As will be seen shortly, it precedes Priestley’s and Scheele’s writings by a year or two. Evidently Black had noticed that a residue was left after the combustion of carbonaceous bodies in air, and absorption of the fixed air produced by the combustion, and had suggested to Rutherford, then a student of his, the advantage of further investigating the matter, and ascertaining the properties of the residual gas.

Rutherford begins his essay with an apt quotation from Lucretius:--

Denique res omnes debent in corpore habere Aëra, quandoquidem rara sunt corpora et aër Omnibus est rebus circumdatus appositusque.

He next proceeds to define the atmosphere as a pellucid thin fluid, in which clouds float and vapours rise. Its necessity for animal and vegetable life is acknowledged by all. It possesses weight and elasticity. It can be fixed by other bodies; but the air obtained from them by distillation differs from ordinary vital, salubrious air, and is often termed mephitic or poisonous.

After acknowledging his debt to his illustrious preceptor Black, he proceeds to quote from the latter to the effect that mephitic or fixed air is the air which proves fatal to animals and extinguishes fire; which is easily absorbed by quicklime and by alkaline salts; which occurs in the Grotto del Cane, and in mineral waters; and which is produced during exhalation from the lungs, by combustion, and during certain kinds of fermentation. Its density, compared with that of ordinary air, is as 15½ or 16 to 9; hence it can be kept for some time in an open glass, and a candle lowered into it is extinguished. It has an agreeable taste and smell; and it changes the colour of syrup of violets from blue to purple. It prevents putrefaction, but putrefied bodies are not made fresh by it. It possesses the power of combining with lime, which acquires new properties as the result of its action. Rutherford then recalls Black’s experiments on lime and on magnesia, pointing out how these bases absorb fixed air, and how it can be recovered from them and from its compounds with alkalies, sometimes by heat, and always by the action of acids.

Rutherford next describes experiments which show that a mouse, placed in atmospheric air, and left till dead, diminishes the volume of the air by one-tenth; and that the residual air, on treatment with alkali, loses one-eleventh of its volume. The residue extinguishes the flame of a candle; but tinder continues to smoulder in it for a short time. It is thus proved that after the whole of the fixed air has been withdrawn by alkalies, the residue is still incapable of supporting life and combustion.

Some burning bodies deprive air of its “salubrity” more easily than others. The phosphorus of urine continues to glow in air in which a candle has ceased to burn, or in which charcoal has burned until it is extinguished. Even after the absorption of all fixed air by alkalies, phosphorus burns, emitting clouds of the dry acid of phosphorus, which can be absorbed by lime-water.

“It therefore appears that pure air is not converted into mephitic air by force of combustion, but that this air rather takes its rise or is thrown out from the body thus resolved. And from this it is permissible to draw the conclusion that that unwholesome air is composed of atmospheric air in union with, and, so to say, saturated with, phlogiston. And this conjecture is confirmed by the fact that air which has served for the calcination of metals is similar, and has clearly taken away from them their phlogiston.” Such air differs from the air evolved from metals by the action of acids, which is more thoroughly impregnated with phlogiston; and also from that from decaying flesh, which is a mixture of mephitic air and combustible air.

He proceeds:--“I had intended to add something regarding the composition of mephitic air, and to seek for a reason for its unwholesome effects, but I have not been able to find out anything with certainty. Certain experiments appear to show, however, that it consists of atmospheric air in union with phlogistic material; for it is never produced except from bodies which abound in inflammable parts: the phlogiston appears to escape from such bodies when they become converted into the calces of metals. I say from phlogistic material, because, as already mentioned, pure phlogiston, in combination with common air, can be seen to yield another kind of air [viz. hydrogen].... I have lately heard that Priestley believes that vegetables growing in mephitic air dispel its noxious ingredients, or, as it were, extract them, and restore its original wholesomeness; and that mephitic air, added to air from putrid flesh, partly mitigates its unwholesome character. But I have been unable to try such experiments.”

We see, then, that Rutherford’s claims to the discovery of nitrogen amount to this:--he removed the oxygen from ordinary air by combustibles such as charcoal, phosphorus, or a candle; and having got rid of the carbon dioxide, in those cases when it was formed, by alkali or lime, he obtained a residue, now known as nitrogen. His view of the nature of this gas, in the phlogistic language of the time, was that the burning bodies had given up some of their “phlogistic material” to the air, which was thus altered. Nitrogen was “phlogisticated air,” even though incombustible; hydrogen, too, was phlogisticated air, but air produced by the union of pure phlogiston with atmospheric air. The step taken by Rutherford, under Black’s guidance, was an advance, though not a great one, in the development of the theory of the true nature of air; and he may be well credited with the discovery of nitrogen.

A Free Inquiry into the Vulgar Notion of Nature; Prefatory remarks.