By this investigation Lavoisier destroyed part of the experimental basis of alchemy, and established the one and only method by which chemical changes can be investigated; the method wherein constant use is made of the balance.
Lavoisier now turned his attention to the calcination of metals, and particularly the calcination of tin. Boyle supposed that the increase in weight which accompanies the calcination of a metal is due to the fixation of "matter of fire" by the calcining metal; Rey regarded the increase in weight as the result of the combination of the air with the metal; Mayow thought that the atmosphere contains two different kinds of "airs," and one of these unites with the heated metal. Lavoisier proposed to test these suppositions by calcining a weighed quantity of tin in a closed glass vessel, which had been weighed before, and should be weighed after, the calcination. If Boyle's view was correct, the weight of the vessel and the tin would be greater at the end than it was at the beginning of the operation; for "matter of fire" would pass through the vessel and unite with the metal. If there was no change in the total weight of the apparatus and its contents, and if air rushed in when the vessel was opened after the calcination, and the total weight was then greater than at the beginning of the process, it would be necessary to adopt either the supposition of Rey or that of Mayow.
Lavoisier made a series of experiments. The results were these: there was no change in the total weight of the apparatus and its contents; when the vessel was opened after the calcination was finished, air rushed in, and the whole apparatus now weighed more than it did before the vessel was opened; the weight of the air which rushed in was exactly equal to the increase in the weight of the tin produced by the calcination, in other words, the weight of the inrushing air was exactly equal to the difference between the weights of the tin and the calx formed by calcining the tin. Lavoisier concluded that to calcine tin is to cause it to combine with a portion of the air wherein it is calcined. The weighings he made showed that about one-fifth of the whole weight of air in the closed flask wherein he calcined tin had disappeared during the operation.
Other experiments led Lavoisier to suspect that the portion of the air which had united with the tin was different from the portion which had not combined with that metal. He, therefore, set himself to discover whether there are different kinds of "airs" in the atmosphere, and, if there is more than one kind of "air," what is the nature of that "air" which combines with a metal in the process of calcination. He proposed to cause a metallic calx (that is, the substance formed by calcining a metal in the air) to give up the "air" which had been absorbed in its formation, and to compare this "air" with atmospheric air.
About this time Priestley visited Paris, saw Lavoisier, and told him of the new "air" he had obtained by heating calcined mercury. Lavoisier saw the great importance of Priestley's discovery; he repeated Priestley's experiment, and concluded that the air, or gas, which he refers to in his Laboratory Journal as "l'air dephlogistique de M. Priestley" was nothing else than the purest portion of the air we breathe. He prepared this "air" and burned various substances in it. Finding that very many of the products of these combustions had the properties of acids, he gave to the new "air" the name oxygen, which means the acid-producer.
At a later time, Lavoisier devised and conducted an experiment which laid bare the change of composition that happens when mercury is calcined in the air. He calcined a weighed quantity of mercury for many days in a measured volume of air, in an apparatus arranged so that he was able to determine how much of the air disappeared during the process; he collected and weighed the red solid which formed on the surface of the heated mercury; finally he heated this red solid to a high temperature, collected and measured the gas which was given off, and weighed the mercury which was produced. The sum of the weights of the mercury and the gas which were produced by heating the calcined mercury was equal to the weight of the calcined mercury; and the weight of the gas produced by heating the calcined mercury was equal to the weight of the portion of the air which had disappeared during the formation of the calcined mercury. This experiment proved that the calcination of mercury in the air consists in the combination of a constituent of the air with the mercury. Fig. XVII. (reduced from an illustration in Lavoisier's Memoir) represents the apparatus used by Lavoisier. Mayow's supposition was confirmed.
FIG. XVII.
Lavoisier made many more experiments on combustion, and proved that in every case the component of the atmosphere which he had named oxygen combined with the substance, or with some part of the substance, which was burned. By these experiments the theory of Phlogiston was destroyed; and with its destruction, the whole alchemical apparatus of Principles and Elements, Essences and Qualities, Souls and Spirits, disappeared.