SUSPENDED CHANGES IN NATURE 23 



SUSPENDED CHANGES IN NATURE 



By JAMES H. WALTON, Jr., Ph.D. 



ASSOCIATE PROFESSOR OF CHEMISTRY, UNIVERSITY OF WISCONSIN 



:N 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 mix- 

 tures 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 1 and also to a marked 



1 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 



