ANTOINE AUGUSTE LAVOISIER.
There is no account in Hales’ work of his reducing litharge in closed vessels. It is to be presumed that Lavoisier heated in a retort a mixture of litharge and charcoal, and that the air which he speaks of was a mixture of oxides of carbon. This account does not inform us of Lavoisier’s views on combustion, but merely shows the date at which he had first obtained what he supposed were results new to science. We recognise that Mayow had anticipated him in this.
It was not until Priestley, when dining with him in the autumn of 1774 (being in Paris with Lord Shelburne at the time), had informed Lavoisier of his discovery of “dephlogisticated” air, that the ideas of the latter upon the subject became precise. Priestley’s own words are:—“Having made the discovery some time before I was in Paris, in the year 1774, I mentioned it at the table of Mr. Lavoisier, when most of the philosophical people of the city were present, saying that it was a kind of air in which a candle burned much better than in common air, but I had not then given it any name. At this all the company, and Mr. and Mrs. Lavoisier as much as any, expressed great surprise. I told them I had gotten it from precipitate per se, and also from red-lead. Speaking French very imperfectly, and being little acquainted with the terms of chemistry, I said plombe rouge, which was not understood till Mr. Macquer said I must mean minium.”
Shortly after this, Lavoisier repeated Priestley’s experiments and confirmed their truth; and this led to the true explanation of experiments of which an account is given in the Memoirs of the French Academy for 1774, and which were fundamental in their character. They referred to the calcination of tin in hermetically-sealed retorts. The tin was placed in a retort which was heated on a sand-bath until the metal had melted. The beak of the retort, previously drawn out into a capillary, was then sealed, the air expelled having been collected and its weight noted. The retort was then cooled and weighed. It was again heated, and the temperature was maintained until the calcination of the tin stopped. With a large retort the calcination was more complete than when a smaller one was employed, this implying that the degree to which the calcination proceeded was dependent upon the amount of air present. After cooling the retort a second time, it was again weighed, when it was found to have undergone no change of weight. The beak was then broken, and air entered with a hissing noise. The gain in weight was now about 10 grains with a large retort. The tin and its calx were next weighed, and it was found that the gain in weight of the tin was always equal to the loss of weight of the air in the retort, measured by the quantity of air which entered on breaking the beak of the retort, less the air driven out of the retort before hermetically sealing it. From this Lavoisier concluded that calx of tin is a compound of tin and air.
Lavoisier’s next research, communicated to the Academy in 1775, and published in 1778, was entitled “On the Nature of the Principle which combines with Metals during their Calcination, and which increases their Weight.” In this he describes experiments showing that when metallic calces are converted into metals by heating with charcoal, a quantity of fixed air is expelled; and here for the first time he points out that fixed air is a compound of carbon with the elastic fluid contained in the calx. He then describes the preparation of oxygen by Priestley’s process of heating red oxide of mercury (mercurius precipitatus per se), and shows that the red oxide, when heated with charcoal, manifests the properties of a true calx, inasmuch as metallic mercury is formed, and a large quantity of fixed air is produced.
His next paper, which appeared in 1777 in the Mémoires of the Academy, deals with the combustion of phosphorus; and here he recapitulates Rutherford’s experiments, and shows that one-fifth of the air disappears, and that the residue, to which he gave the name “mouffette atmosphérique,” is incapable of supporting combustion. It will be remembered that Rutherford named this residue “phlogisticated air,” inasmuch as he imagined it to have absorbed phlogiston from the burning phosphorus; Scheele, too, had made a similar experiment with a similar result. From these observations, Lavoisier concluded that air consists of a mixture or compound of two gases, one capable of absorption by burning bodies, the other incapable of supporting combustion.
This paper was immediately followed by another, also published in 1777. Its title is, “On the Combustion of Candles in Atmospheric Air, and in Air eminently respirable.” In this memoir he distinguishes between four kinds of air:—1, Atmospheric air, in which we live and which we breathe. 2, Pure air, alone fit for breathing, constituting about one-fourth of atmospheric air, and termed by Priestley “dephlogisticated air.” 3, Azotic gas, identical with Rutherford’s “mephitic air,” and of which the properties were then unknown. 4, Fixed air, which he proposed to call “acide crayeux,” or acid of chalk, discovered twenty-five years previously by Black.
By this time his theory was well developed. He accounted for the phenomena of combustion without having recourse to the phlogistic hypothesis: the calx was produced by the union of the metal with the active constituent of air; and when carbonaceous material burned, the carbon united with this same constituent, producing fixed air. But there were still difficulties in his way: it was known that in dissolving metals in dilute vitriol or muriatic acid, a combustible and very light air was evolved; and that the metals were thereby converted into calces in combination with the respective acids. This fact was not explained even by the supporters of the phlogistic theory, but it had the effect of preventing them from accepting Lavoisier’s views. Some considered that hydrogen and phlogiston were identical, and that on dissolving a metal the calx was formed by the escape of the phlogiston; while others had a hazy idea that hydrogen was a compound of water and phlogiston; but of this more hereafter.
Lavoisier’s objection to such a theory was that the calx was heavier than the metal, and that hydrogen, though light, still possessed weight.[22] Moreover, he had ascertained that the calces of mercury, tin, and lead are compounds of these metals with active air, and that as fixed air is produced by heating such calces with carbon, fixed air must be a compound of carbon and vital air, or, as he named it, the “oxygine principle,” inasmuch as its combination with phosphorus, sulphur, and carbon resulted in the formation of acids (ὀξύς, an acid).
In 1777 he read another memoir, “On the Solution of Mercury in Vitriolic Acid, and on the Resolution of that Acid into Aeriform Sulphurous Acid, and into Air eminently respirable.” Priestley had already shown that this process yielded sulphur dioxide; Lavoisier carried the temperature higher, and, decomposing the sulphate of mercury, produced metallic mercury, sulphur dioxide, and oxygen. It appeared therefore that sulphurous differed from sulphuric acid in containing a smaller proportion of oxygen.