The Romance of Modern Chemistry

FROM SOLUTIONS TO CRYSTALS soda. Indeed its behaviour in this respect has been turned to practical account in railway foot-warmers. When these are filled with a hot, strong solution of sodium acetate instead of hot water, the store of available heat is about four times as great Reluctance to pass into the crystallised state is exhibited not only by dissolved substances, but also by many fused compounds. It is possible in the latter case to cool the molten substance below its freezing-point without crystal- lisation setting in; the substance is said to be “ super- cooled.11 As with supersaturated solutions, mere contact with a crystal is sufficient to induce crystallisation. There is, however, a remarkable difference in the rates at which different supercooled substances respond to this stimulus. This is very well shown by filling a long, narrow tube with the supercooled liquid and touching one end of the column with a crystal of the solid material. Crystallisation starts immediately at the point of contact, and is propagated through the tube at a rate which is perfectly regular, but differs from one case to another. For instance, yellow phosphorus and benzophenone (a substance well known to the student of organic chemistry) can both be obtained in the supercooled condition, but the rates of solidification in narrow tubes are very different in the two cases. The crystallisation of the phosphorus proceeds at the very great speed of 39 inches a second; in the case of benzophenone, the rate is only yjth of an inch per second. The most effective way of inducing crystallisation in a supersaturated solution or in a supercooled substance is, as already indicated, the addition of a crystal of the solid. But suppose no solid is available! what then ? This is the position in which a chemist frequently finds himself 317