. Popular science monthly. le form of water at thistemperature. Water that has been cooled below its freezing point in thisway is said to be in the metastable state; it is also called undercooledwater, that is, water that has been cooled under the temperature at whichit ought to solidify. The phenomenon of undercooling is not restrictedto water, but is shown by aqueous solutions of salts1 and also to a marked 1 Solutions may be said to be undercooled when they have been cooled to atemperature at which crystals of the dissolved substance ought to separate from 24 THE POPULAR SCIENCE MONTHLY ext
. Popular science monthly. le form of water at thistemperature. Water that has been cooled below its freezing point in thisway is said to be in the metastable state; it is also called undercooledwater, that is, water that has been cooled under the temperature at whichit ought to solidify. The phenomenon of undercooling is not restrictedto water, but is shown by aqueous solutions of salts1 and also to a marked 1 Solutions may be said to be undercooled when they have been cooled to atemperature at which crystals of the dissolved substance ought to separate from 24 THE POPULAR SCIENCE MONTHLY extent by melted phosphorus, carbolic acid, and many organic sub-stances like thymol and betol that are seldom encountered outside achemical laboratory. Although many undercooled liquids are similar to water in thatthey may be changed to the stable state by agitation, this property is byno means general. An infallible test for undercooling is the addition ofa fragment of the stable substance. Thus a bit of ice causes water to. Fig. 1. Crystals Growing in Undercooked Thymol. solidify, similarly undercooled thymol becomes a crystalline mass uponthe addition of a crystal of thymol, while undercooled sodium acetatesolutions at once separate needle-like crystals of sodium acetate whena crystal of that substances is added. The addition of a solid fragmentfor the purpose of causing subsequent crystallization of the liquid iscalled inoculation or vaccination. The name is particularly fortunate,for the growth and spread of the crystals resembles a bacterial liquids are as sensitive to the presence of a crystal ofthe solid material as milk is to the presence of certain bacteria, and justas much care must be exercised in their preservation. In working withundercooled sodium acetate under no circumstances may an open flaskof this substance be brought into a room in which sodium acetate hasbeen ground in a mortar, for the dust in the air carries enough finelydivided sodium a
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