Introductory.—In the course of an inquiry into the physiological and therapeutical action of alkaline nitrites, and allied substances, I was struck with the strong resemblance which the action of nitroglycerine bears to that of the nitrites. The resemblance is, indeed, so well marked, that the action of the one may be held to be identical with that of the other, unless in respect of intensity. The suggestion, therefore, naturally occurred to me, that nitroglycerine is not a nitrate of glyceryl, as it is always represented, but a nitrite. For no ordinary nitrate, as an alkaline nitrate or nitrate of ethyl, nor any compound of glyceryl with another acid, as sulphuric acid, produces an action on the body at all resembling that of nitroglycerine. On referring to the various investigations which had been made for the purpose of ascertaining the chemical constitution of nitroglycerine, I found that none of them was sufficiently extended and exact to place beyond doubt its precise nature. The danger in manipulating so explosive a body had evidently prevented the various chemists from making a thorough examination of its composition. I at first thought that nitroglycerine might be a nitrite of glyceryl, having its nitrous acid so intimately combined with the glyceryl, that the acid did not exhibit its reactions when tested for in the usual way ; just as the acids of other ethereal compounds will not yield their usual reactions, unless special means are taken to forcibly dissociate the acid from the base ; for example, the acid of acetate of ethyl, or of chloride of ethyl. Certainly nitroglycerine gives no blue colour with a solution of starch and iodide of potassium and sulphuric acid, a very delicate test for the presence of nitrous acid. In order, however, to apply the test to the separated acid of nitroglycerine, I mixed an alcoholic solution of nitroglycerine with an alcoholic solution of pure caustic potash. The potash was ascertained to be free from nitrite, which I have frequently found present in small quantity in various specimens of ordinary potash. Decomposition of the nitroglycerine quickly occurred, and the fluid, when now tested for nitrous acid, was found to contain the acid in abundance, and so much of it, that for the moment I believed that nitroglycerine was, in reality, a nitrite of glyceryl; and hence the nature of its physiological action. Some estimations, however, of the quantity of the nitrous acid proved to me that whilst the larger portion of the nitrogen of the nitroglycerine appeared as nitrous acid in the decomposed products, yet a considerable portion was present in some other form.