On account of the identity of the physiological action of nitroglycerine with that of simple alkaline nitrites, it occurred to the author either that nitroglycerine was not a nitrate of glyceryl as commonly represented, but contained nitrous acid in its constitution, or that it was readily decomposed in the body with the formation of nitrous acid. This suggestion led to an extended investigation of the constitution of nitroglycerine and of the decomposing action of various substances on nitroglycerine. The conclusion arrived at as to the constitution of nitroglycerine was confirmatory of the common belief that it is a tri-nitrate of glyceryl. For it was found that although nitroglycerine yielded a large quantity of nitrous acid when decomposed by alkalies, yet nitroglycerine could not be obtained by acting on glycerine with nitrous anhydride gas, nor did the presence of urea prevent its formation when it was prepared in the usual way by allowing nitric and sulphuric acids to act on glycerine. The yield of nitroglycerine from a given weight of glycerine also closely agreed with its being a tri-nitrate, and was considerably in excess of the yield obtainable had nitrous acid entered into its constitution. An analysis of the elementary composition of nitroglycerine, which was carried out in conjunction with Mr Masson, also led to the same opinion.