IN NATURE of June 9, p. 775, Messrs. Venkataramaiah and Rao describe “A New Phototropic Compound of Mercury” of the composition which they regard as “the most phototropic compound as yet known”; or that this compound shows appreciable change in colour on exposure to light in less time than that required by any other known phototropic compound. In 1917, while working in the College of Science, Calcutta, in an attempt to prepare (SHgI)2, described by Ray (Trans. Chem. Soc., III, 109), without using any organic compound, I obtained 2HgS.HgI2, which showed phototropy to a remarkable degree. The orange yellow powder turned black very quickly on exposure to sunlight, but only gradually in diffused daylight. On keeping the black powder in the dark, the reverse change took place. At room temperature, it took several hours to recover, but at higher temperatures the change of colour was quicker; at about 85° C., for example, only a few seconds. Both varieties had the same chemical composition. This substance was exhibited before the Indian Science Convention of that year, and a preliminary note was published in the Report of the Indian Association for the Cultivation of Science, 1917. Since then I have found that phototropy is exhibited more or less by all the complex sulphides of mercury of the general formula HgS. HgX2 or 2HgS.HgX2, where X is a halogen or a monovalent acid radicle, including CNS, of which 2HgS.HgI2 is the most sensitive.