Phototropy in Inorganic Compounds

Abstract

So far no coherent or consistent explanation of the phenomenon of phototropy has been given, although numerous compounds which manifest this property have been studied. A study of the phototropy of dithiotrimercuric salts of the general formula 2HgS.HgX₂ or 2HgS.HgX, according as X is the negative ion of a monobasic or a dibasic acid, has revealed the interesting result that the phototropy in these compounds is due to the decomposition of the compound by the absorption of actinic energy, forming mercuric sulphide and the other mercuric compound, and hence the compounds blacken in sunlight. When preserved in the dark, they again recombine with the emission of radiant energy, giving the original compound.