IN the Physical world we are familiar with the fact that changes of all kinds are continually taking place, Prominent among these are changes of state, such as the evaporation of water, the melting of ice and the condensation of steam. These familiar transformations seem to have a common property; as usually observed they take place at very definite temperatures. To be sure, in the laboratory it is possible to heat water to 105-106° without boiling it, and to cool vapors below the temperature at which they ought to condense, but such experiments have usually been regarded as quite exceptional. Within the last few years, however, it has been found that such a reluctance to enter a new state is by no means unusual, that many cases similar to the above actually exist; moreover, that they are not restricted to ultra-refined laboratory experiments, but are a matter of common experience.
If ice is heated it melts, and the temperature at which this process takes place is sharp and definite, as is evidenced by the fact that mixtures of ice and water are used to calibrate and test the accuracy of the most delicate thermometers. No one has ever succeeded in heating ice above the temperature of its melting point, zero degrees, which is a property that ice shares with other solids such as lead, gold, saltpeter and ordinary table salt. But the reverse is not true, for many liquids can be cooled below their freezing points without solidifying. Water, for example, can be cooled below zero without changing to ice. When water freezes in nature, as in ponds and lakes, the transition of water to ice takes place at this temperature and is fairly sharp, but if a clean flask is filled with water the surface of which is protected from dust by means of a layer of oil, it is easily possible to cool the water ten degrees below its freezing point. True, it becomes solid on shaking the flask or upon the introduction of a fragment of ice, but it can be kept for hours in the liquid state, offering a passive resistance to the forces of nature which are operating to-change it to ice, the stable form of water at this temperature. Water that has been cooled below its freezing point in this way is said to be in the metastable state; it is also called undercooled water, that is, water that has been cooled under the temperature at which it ought to solidify. The phenomenon of undercooling is not restricted to water, but is shown by aqueous solutions of salts and also to a marked
- Solutions may be said to be undercooled when they have been cooled to a temperature at which crystals of the dissolved substance ought to separate from