Metachromism.

The Galaxy 7, 1876

An English chemist has been investigating the interesting subject of the changes of color which bodies undergo when heated. These bodies are called metachromes, and the action does not include the variations in intensity which are due to very high temperatures, but only the assumption of a positively different color at comparatively low temperatures. He arranges metachromes in two groups, those of the zinc-oxide class, which being colorless acquire a yellow tint in being heated, and those of the borate of copper class, which change from one spectrum color to the contiguous one. The law is that color change takes place in the order of the spectrum colors: when the metachrome is expanding, in the violet to red order; when contracting, red to violet order. For instance, the red iodide of mercury becomes darker as it is heated, until at 140 deg. C. it is converted into the yellow modification; and at higher temperatures this becomes darker until at 220 deg. C. it is deep orange. The order of change is as follows: white, colorless, violet, indigo, blue (metallic appearance), green, yellow, orange, red, brown, black. At the more refrangible end of the spectrum the colors may be replaced by a metallic appearance. It is evident that metachromism is nearly related to allotropism, a body expanding through the influence of heat presenting a continuous series of allotropes. Metachromism is due to the storage of potential energy, the author holding that molecular vibrations have nothing to do with this manifestation of potential energy. He also discusses the curious question of where the change would end by lowering the temperature of the contracting metachromes which change from less to more refrangible colors, and thinks the change would continue until the absolute zero was reached, when all would be white or metallic. On the other hand, starting from the absolute zero of color, any expanding metachrome would give all the hues as it successively passed through the appropriate temperatures.