In comparing the experiments of the illustrious Cavendish on the synthesis of nitrous acid, with those of Lavoisier on the decomposition of nitre by charcoal, we find a much greater difference in the results than can be accounted for by supposing the acid formed, and that decomposed, of different degrees of oxygenation. In the most accurate experiment of Cavendish, when the nitrous acid appeared to be in a state of deoxygenation, 1 of nitrogene combined with about 2,346 of oxygene.[3] In an earlier experiment, when the acid was probably fully oxygenated, the nitrogene employed was to the oxygene nearly as 1 to 2,92.[4] Lavoisier, from his experiments on the decomposition of nitre, and combination of nitrous gas and oxygene, concludes, that the perfectly oxygenated, or what he calls nitric acid, is composed of nearly 1 nitrogene, with 3,9 of oxygene; and the acid in the last state of deoxygenation, or nitrous acid, of about 3 oxygene with 1 nitrogene.[5] [3] Phil. Trans. v. 78, p. 270. [4] Phil. Trans. v. 75 p. 381. [5] Elem. Kerr’s Trans. page 76, and 216, and Mem. des Sav. Etrang. tom. 7, page 629. Great as the difference is between the estimations of these philosophers, we find differences still greater in the accounts of the quantities of nitrous gas necessary to saturate a given quantity of oxygene, as laid down by very accurate experimentalists. On the one hand, Priestley found 1 of oxygene condensed by 2 of nitrous gas, and Lavoisier by 1⅞. On the other, Ingenhouz, Scherer, and De la Metherie, state the quantity necessary to be from 3 to 5.[6] Humbolt, who has lately investigated Eudiometry with great ingenuity, considers the mean quantity of nitrous gas necessary to saturate 1 of oxygene, as about 2,55.[7] [6] Ingenhouz sur les Vegetaux, pag. 205. De la Metherie. Essai sur differens Airs, pag. 252. [7] Annales de Chimie, tome 28, p. 168. II. To reconcile these different results is impossible, and the immediate connection of the subject with the production of nitrous oxide, as well as its general importance, obliged me to search for means of accurately determining the composition of nitrous acid in its different degrees of oxygenation. The first desideratum was to ascertain the nature and composition of a fluid acid, which by being deprived of, or combined with nitrous gas, might become a standard of comparison for all other acids. To obtain this acid I should have preferred the immediate combination of oxygene and nitrogene over water by the electric spark, had it been possible to obtain in this way by a common apparatus sufficient for extensive examination; but on carefully perusing the laborious experiments of Cavendish, I gave up all thoughts of attempting it.