[33] In a letter to me, dated Oct. 28, 1799, after giving an account of some experiments on the phlogistication of nitric acid by heat and light, he says, “It was from an attentive examination of the manner in which the nitric acid was phlogisticated in these experiments, that I was confirmed in the suspicion I had long before entertained, of the real difference between the _nitrous_ and _nitric_ acids. It is not enough to shew that in the _nitrous_ acid, (that is, the nitric holding nitrous gas in solution), the proportion of oxygene in the whole compound is less than that entering into the composition of the nitric acid, and that it is therefore less oxygenated. By the same mode of reasoning we might prove that water, by absorbing carbonic acid gas, became less oxygenated, which is absurd. Should any one attempt to prove (which will be necessary to substantiate the generally received doctrine) that the oxygene of the nitrous gas combines with the oxygene of the acid, and the nitrogene, in like manner, so that the resulting acid, when nitrous gas is absorbed by nitric acid, is a binary combination of oxygene and nitrogene, he would find it somewhat more difficult than he at first imagined; it appears to me impossible. It is much more consonant with experiment to suppose that nitrous acid is nothing more than nitric acid holding nitrous gas in solution, which might in conformity to the principles of the French nomenclature, be called nitrate of nitrogene. The difficulty, and in some cases the impossibility, of forming nitrites, arises from the weak affinity which nitrous gas has for nitric acid, compared with that of other substances; and the decomposition of nitrous acid (that is, nitrate of nitrogene) by an alkaline or metallic substance, is perfectly analogous to the decomposition of any other nitrate, the nitrous gas being displaced by the superior affinity of the alkali for the acid. “Agreeable to this theory, the salts denominated _nitrites_ are in fact triple salts, or ternary combinations of nitric acid, nitrous gas, and salifiable bases.” This theory is perfectly new to me. Other Chemists to whom I have mentioned it, have likewise considered it as new. Yet in a subsequent letter Mr. Thomson mentions that he had been told of the belief of a similar opinion among the French Chemists.