The way in which light is absorbed or reflected by a crystalline material is determined by the structure of the crystal. This crystal structure can be changed by the application of heat. In some cases this change is reversible and the original structure, and therefore the original color, returns upon cooling. Two compounds are formed when metal salts are reacted with the [HgI4]2- Ion. Each compound is heated on a hot plate and a change in color is noted. The initial color of the compound returns upon cooling.