The Gases of the Atmosphere: The History of Their Discovery

by William Ramsay

He also experimented with iron pyrites, and his experiments recall those of Boyle. Boyle found that “marcasite,” a disulphide of iron, on exposure to air, gained in weight, while vitriol of iron was formed. Lavoisier performed the same experiment, not “in a very pure air,” as Boyle did when he left the pyrites exposed in a quiet dust-free room, but in a confined quantity of ordinary air; and he found that the air was rendered incapable of supporting combustion, or, in other words, its oxygen was removed.

In the same volume of the Memoirs of the Academy for 1778, another of Lavoisier’s papers—“On Combustion in General”—is to be found. In this he showed that oxygen gas is the only substance which supports combustion; that during the burning of combustible substances in air a portion of the oxygen disappears, and converts the burning substance into one of two kinds of compounds: either an acid, such as sulphuric acid from sulphur, phosphoric acid from phosphorus, or carbonic acid from carbon (for in those days the term “acid” was applied to what we now term an anhydride); or in the case of metals a calx, or compound of oxygen with the metal. The processes are analogous, but differ in the rate at which they take place; for the calcination of metals is a much slower operation than the combustion of sulphur or phosphorus. It is the rapidity of the action which leads to actual inflammation. He next examined and attacked the theory of phlogiston, and maintained that the existence of phlogiston is purely hypothetical, and quite unnecessary for the explanation of the phenomena. But his papers were received with doubt. The change demanded was too great; the trammels of custom were too firmly bound. He gained no converts.

Until the true nature of hydrogen had been explained, the attack on the phlogistic theory could not be said to be complete. This combination of hydrogen and oxygen to form water was first proved by Cavendish. And as soon as Sir Charles Blagden, in 1783, had communicated Cavendish’s results to Lavoisier, the latter at once saw their bearing on the new theory which he was endeavouring to uphold, and perceived how they would give a final blow to the adherents of the theory of phlogiston. For it had been frequently adduced as an objection to his new views, that they were incapable of explaining why hydrogen should be evolved during the solution of metals in acids, or why it should be absorbed during the reduction of calces to the metallic state. Lavoisier at once repeated Cavendish’s experiments on a large scale, and was assisted on that occasion by Laplace, Sir Charles Blagden also being present. A considerable quantity of water was produced, and the volumes of the combining gases were found to be 1 of oxygen to 1·91 of hydrogen. Shortly after, in conjunction with Meunier, he performed the converse operation, in decomposing steam by passing it over iron wire heated to redness in a porcelain tube. The iron withdrew the oxygen from the water, while the hydrogen passed on and was collected in the gasholder.

The explanation of the solution of metals in acids was now easy: it depended on the decomposition of water. While the oxygen united with the metal to form a calx, the hydrogen was evolved; the calx dissolved in the acid, forming a salt of the metal. And the operation of producing hydrogen by the action of steam on red-hot iron met with an equally simple explanation: the oxygen and iron united to form an oxide—the ancient ethiops martial—while the hydrogen escaped. The converse took place during the reduction of a calx to the metallic state by hydrogen. Here the hydrogen seized on the oxygen of the calx, removed it in the form of water, and the metal was left. These experiments were due to Cavendish; all that Lavoisier did was to show the true nature of the phenomena. The opponents of the new doctrines, Priestley chief among them, did their best to disprove the view that water was a compound of oxygen and hydrogen. But in vain. Many of Lavoisier’s opponents had to admit the justice of his views; and in 1787 De Morveau, Berthollet, and Fourcroy joined Lavoisier in reconstructing the nomenclature of chemistry on a new basis, which is substantially that in use at the present day. Black, too, was a convert, but Priestley and Cavendish remained true to their old faith, and one of Priestley’s last acts was to publish a defence of the phlogistic theory. We shall see later how Cavendish carefully considered the rival theories, and what reasons induced him to cast his vote for the older one.

Among the numerous memoirs which Lavoisier communicated to the Academy during the ten years between 1772 and 1782, one still remains to be mentioned. It was published as early as 1777, but it must be remembered that many of these memoirs were antedated. It referred to the respiration of animals; and Lavoisier concluded, on the ground that the phenomena of respiration are essentially similar to those of combustion and calcination, that the only portion of the air which supports animal life is the oxygen. The azote or nitrogen is inhaled along with the oxygen, but is exhaled unaltered. The oxygen, however, is gradually converted into carbonic acid; and when a certain amount, but by no means the whole, has been thus changed, the air becomes unfit for respiration. If the carbonic acid is withdrawn by means of lime-water or caustic alkali, the residue is air poor in oxygen, and the azote is the same as that left after the calcination of metals, or the burning of a candle, in air.

At the time of his impeachment, Lavoisier was engaged in experiments on perspiration, along with Séguin. He had nearly finished his experimental work, but had drawn up no account of it. His request that his life might be prolonged until he had compiled a statement of his results was refused; but Séguin, who was fortunately spared, undertook the task. The facts collected do not, however, bear directly on our subject, and shall not be further alluded to here.

This account of Lavoisier’s researches would be incomplete without a reference to his text-book of chemistry, Traité élémentaire de Chimie, in which his views are stated in order, and with great clearness. The nomenclature current at the time was so cumbrous, that it was almost, if not quite, impossible for the supporters of the new theory to express their meaning in an intelligible manner. De Morveau had suggested a nomenclature for salts; Black, too, had invented one; but neither of these systems was adapted to represent the new views. It was partly with the object of avoiding such embarrassment that Lavoisier wrote his Treatise.

He begins with a clear statement of what is generally termed “the states of matter”—solid, liquid, and gaseous—and points out that solids and liquids are almost all capable of change into the aeriform state by the addition of “caloric.” Proceeding next to the consideration of the nature of air, he shows that it must necessarily contain all those gases capable of existence at the ordinary temperature; and he explains how water-vapour must be one of them, seeing that even though water is a liquid at the ordinary temperature, it is capable, like many other liquids, of existing as vapour, when mixed with other gases. He next treats of the analysis of air, and describes his classical experiment of heating four ounces of mercury for twelve days in a retort communicating with a bell-shaped receiver, standing in a mercury trough. Having marked the initial height of the air in the jar by means of a piece of gummed paper, he found that, after twelve days’ heating close to the boiling-point, the air had diminished in volume by about one-sixth, and that the mercury had become covered with a red deposit of mercurius calcinatus per se, which, when collected, weighed 45 grains. The residual air in the retort and in the jar was incapable of supporting life or combustion; but the red precipitate, when heated, lost 3½ grains of its weight, yielding 41½ grains of metallic mercury, while it evolved 7 or 8 cubic inches of oxygen, capable of supporting the combustion of a candle vividly, and of causing charcoal to burn with a crackling noise, throwing out sparks. Oxygen was thus successfully separated from air, and obtained from it in a pure condition for the first time, in a single series of operations.

In Lavoisier we see a master mind, not only capable of devising and executing beautiful experiments, but of assimilating those of others, and deducing from them their true meaning. Although his additions to the known chemical compounds were few in number, and cannot be compared with those of Scheele or of Priestley, yet his reasoning in disproof of the phlogistic theory was so accurate and so exact that it rapidly secured conviction. With the exceptions already mentioned, almost all the eminent chemists of the day accepted his conclusions; and one, Kirwan, who had written a formal treatise in defence of the phlogistic theory, was so fair-minded, that after his work had been translated into French and published with comments, he acknowledged that the old theory was dead, and that truth had conquered.

It will be interesting now to trace Cavendish’s part in developing the history of the discovery of the constituents of air, and to note his arguments in favour of the phlogistic theory. Although Cavendish never publicly acknowledged its insufficiency, yet he had ceased to occupy himself with chemical problems at the time when its adoption was universal, and his true opinions have never been recorded.