��THE POPULAR SCIENCE MONTHLY
��extent by melted pliosphorus, carbolic acid, and many organic sub- stances like tbymol and bctol that are seldom encountered outside a chemical laboratory.
Although many undercooled liquids are similar to water in that they may be changed to the stable state by agitation, this property is by no means general. An infallible test for undercooling is the addition of a frarniicnt of tlie stable substanco. Thus a bit of ice causes water to
���Fig. 1. CiiY.ST.VLS Gi;ii\\i\<; i.\ rxDKitcooi.ED TiiYMni..
��solidify, similarly undercooled thymol becomes a crystalline mass upon the addition of a crystal of thymol, while undercooled sodium acetate solutions at once separate needle-like crystals of sodium acetate when a crystal of that substances is added. The addition of a solid fragment for the purpose of causing subsequent crystallization of the liquid i- called inoculation or vaccination. The name is particularly fortunate, for the growth and spread of the crystals resembles a bacterial growth. Undercooled liquids are as sensitive to the presence of a crystal of the solid material as milk is to the presence of certain bacteria, and Just as much care must be exercised in their preservation. In working with undeicooled sodium acetate under no circumstances may an open jflask of this substance be brought into a rtxuu in which sodium acetate has been ground in a mortar, for the dust in the air carries enough finely divided sodium acetate to inoculate the solution. Frequently the in-
tlie solution, without such a separation taking place. Sucli solutions are usually said to te supersaturated.