It results from the experiments contained in this memoir that when mercury is dissolved in nitric acid, this metallic substance acquires the pure air contained in the nitric acid and constituting it an acid. On the one hand this metal, when combined with the purest air, is reduced to a calx; on the other the acid deprived of this same air expands and forms nitrous air, and the proof that such are the facts in this experiment is that if after having thus separated the two airs which enter into the composition of the acid of nitre, you combine them anew, you make pure acid of nitre such as you had before, with the single difference that it fumes.
The acid of nitre, drawn from saltpetre by clay, is consequently nothing but nitrous air combined with nearly an equal volume of the purest part of the air and with a fairly large amount of water; nitrous air, on the contrary, is the acid of nitre deprived of air and of water. People will no doubt ask here if the phlogiston of the metal does not play some part in this process. Without daring to decide a question of so great importance, I will reply that since the mercury comes out of this experiment just as it went in, there are no signs that it has lost or gained any phlogiston, unless we claim that the phlogiston which brought about the reduction of the metal passed through the vessels. But that is to admit of a particular sort of phlogiston, different from that of Stahl and his school; it is to return to the theory of fire as a principle, to fire as an element of bodies, a theory much older than Stahl's and very different from it.
I will end this memoir as I began it, by thanking M. Priestley, to whom the greater part of whatever interest it possesses is due; but the love of truth and the progress of knowledge, towards which all our efforts should be directed, oblige me at the same time to correct a mistake which he has made, which it would be dangerous to leave unchallenged. This rightly famous physicist, who had discovered that when he combined the acid of nitre with any earth, he invariably obtained ordinary air or air better than ordinary air, believed that he could thence draw the conclusion that the air of the atmosphere is a compound of acid of nitre and of earth. This bold conception is quite overthrown by the experiments contained in this memoir. It is clear that it is not air that is composed of acid of nitre, as M. Priestley claims; but, on the contrary, it is the acid of nitre that is composed of air; and this single remark gives the key to a large number of experiments contained in Sections III., IV. and V. of M. Priestley's second volume.
