“Dichlorethylsulphide is very slightly soluble in water and very freely soluble in organic solvents, or has a high lipoid solubility or partition coefficient. It would, therefore, be expected to penetrate cells very readily. Its rapid powers of penetration are practically proven by its effects upon the skin. Having penetrated within the living cell, it would undoubtedly hydrolyze. The liberation of free hydrochloric acid _within the cell_ would produce serious effects and might account for the actions of dichlorethylsulphide. To summarize, then, the mechanism of the action of dichlorethylsulphide appears to be as follows: “1. Rapid penetration of the substance into the cell by virtue of its high lipoid solubility. 2. Hydrolysis by the water within the cell, to form hydrochloric acid and dihydroxyethylsulphide. 3. The destructive effect of hydrochloric acid upon some part or mechanism of the cell. “Although hydrochloric acid does not penetrate cells readily and is easily neutralized by the buffer action of the fluids of the body, we might expect by flooding the body with large quantities of acid to produce some of the characteristic effects of mustard gas. Stimulation of the respiratory center is a well known effect of acid. Convulsions and salivation may be produced by injection of hydrochloric acid and we have been able to produce slowing of the heart by rapid injection of this acid. “The delayed action of mustard gas might be explained by the formation of some compound with some constituent of the blood. However, blood taken from dogs which had been poisoned with mustard gas and were exhibiting typical symptoms at the time, injected into normal dogs produced no effect. Serum treated in vitro with mustard gas and allowed to stand and then injected into a dog, produced no effect. The fluid which is formed in the vesicle and blebs produced by the application of mustard gas to the skin produces no mustard gas effects.” In studying the toxicity of mustard gas for dogs, it was observed that a concentration of 0.01 mg. per liter could be tolerated indefinitely. If this value is considered as a threshold value, and subtracted from the toxicity values for varying periods of time, it is found that there is a definite relation between the toxic concentration and the time of exposure. This is expressed by the formula (C - 0.01)_t_ = K where C is the concentration observed for a given time _t_. K has the approximate value of 1.7, where _t_ varies between 7.5 and 480 minutes. VESICANT ACTION