In the process of making mercaptan by collecting the distillate from a mixture of ethyl-sulphate of calcium and sulphydrate of potassium, along with water and mercaptan, a considerable quantity of crystalline substance was observed to collect in the receiver, and also towards the end of the condenser. The receiver having been placed in a freezing mixture, to condense as much mercaptan as possible, it was thought that the crystalline substance was ice, and the freezing mixture was removed. The crystals, however, continued to be formed, and even stopped up the end of the condenser, so that it was necessary to pour in hot water to melt them. In a few minutes they were again formed, not only in the receiver, but half way up the condensing tubes, through which water at about 2–3° C. was running. As it was evident that these crystals could not be ice, the conditions under which they were formed, and their composition, were subjected to investigation.