Most of those who noticed this said that the gases came from the dissolving metals; Lavoisier said they were produced by the decomposition of the acids.
In order to study the interaction of nitric acid and mercury, Lavoisier caused a weighed quantity of the metal to react with a weighed quantity of the acid, and collected the gas which was produced; when all the metal had dissolved, he evaporated the liquid until a white solid was obtained; he heated this solid until it was changed to the red substance called, at that time, _red precipitate_, and collected the gas produced.

Finally, Lavoisier strongly heated the red precipitate; it changed to a gas, which he collected, and mercury, which he weighed.
The weight of the mercury obtained by Lavoisier at the end of this series of changes was the same, less a few grains, as the weight of the mercury which he had caused to react with the nitric acid.

The gas obtained during the solution of the metal in the acid, and during the decomposition of the white solid by heat, was the same as a gas which had been prepared by Priestley and called by him _nitrous air_; and the gas obtained by heating the red precipitate was found to be oxygen.

Lavoisier then mixed measured volumes of oxygen and "nitrous air," standing over water; a red gas was formed, and dissolved in the water, and Lavoisier proved that the water now contained nitric acid.
The conclusions regarding the composition of nitric acid drawn by Lavoisier from these experiments was, that "nitric acid is nothing else than _nitrous air_, combined with almost its own volume of the purest part of atmospheric air, and a considerable quantity of water." Lavoisier supposed that the stages in the complete reaction between mercury and nitric acid were these: the withdrawal of oxygen from the acid by the mercury, and the union of the compound of mercury and oxygen thus formed with the constituents of the acid which remained when part of its oxygen was taken away.

The removal of oxygen from nitric acid by the mercury produced _nitrous air_; when the product of the union of the oxide of mercury and the nitric acid deprived of part of its oxygen was heated, more nitrous air was given off, and oxide of mercury remained, and was decomposed, at a higher temperature, into mercury and oxygen.
Lavoisier thought of these reactions as the tearing asunder, by mercury, of nitric acid into definite quantities of its three components, themselves distinct substances, nitrous air, water, and oxygen; and the combination of the mercury with a certain measurable quantity of one of these components, namely, oxygen, followed by the union of this compound of mercury and oxygen with what remained of the components of nitric acid.
Lavoisier had formed a clear, consistent, and suggestive mental picture of chemical changes.