We have shown that calomel vapour contains no measurable amount of Hg2Cl2 or of HgCl, and consists wholly of the dissociation products, mercury and mercuric chloride. According to chemical theory, therefore, when, by the removal of all moisture, the dissociation is prevented, the vapour pressure of the dry substance should be equal only to the partial pressure of the undissociated molecules of the undried vapour; that is to say, it should be negligibly small. Brereton Baker, however, was able to measure the vapour density of such dry calomel, finding it to be almost double the ordinary density. Since the measurement was made by the V. Meyer method, at 445°, it follows that this dry calomel must have possessed a vapour pressure approximating, if not exceeding, one atmosphere.