Calomel is well known to be practically insoluble in pure water at the common temperature. It is decomposed to a certain extent by the action of light, or by a moderate heat in the presence of water, and especially by the aid of acids of various kinds, and by certain salts such as alkaline and other soluble chlorides--especially potassium, sodium, and ammonium chlorides. In all these cases of partial decomposition some of the mercurous chloride--the calomel--is changed into soluble mercuric chloride and metallic mercury. This decomposition is supposed to result from the action of the alkaline chlorides and the chloro-hydric and other acids of the gastric juice when calomel is taken into the stomach under ordinary circumstances. It is believed that the activity of the calomel depends mainly on the amount of this decomposition which takes place in the body. Especially does this partial decomposition of calomel into corrosive sublimate occur, to a great extent, when it is mixed in water with sal-ammoniac (ammonium chloride), as has been experienced in cases of poisoning by the administration of even moderate doses of calomel which had been mixed with this salt. In an experiment by the present writer in which three tenths of a gram of calomel and one and two tenths of a gram of sal-ammoniac with ten grams of water were allowed to react at the common temperature for twenty-four hours, as much as 0.019 of a gram of corrosive sublimate was found. No doubt these facts throw much light on the very irregular action of calomel in different persons and under various conditions, in doses which may be very small or very large. We can easily understand how, when the stomach secretes no gastric juice and when the salts of the blood have been greatly reduced in quantity by watery purging as in cholera, the calomel may pass through the alimentary canal unchanged, insoluble and inactive, or exert a doubtful topical action only. The present writer's own experience--when he was a medical student, and when fully impressed by the sincere and logical teachings of Doctor Cooke, who, however halting and hesitating may have been his manner or unadorned his style of lecturing, always commanded the fixed attention and highest respect of his pupils--soon opened his eyes to the faults in the theory of the professor.