185 



other hand, the destruction, by previous heating, of nuclei 

 in formation, should render crystallization more difficult. 

 The hindrance of molecular motion by capillary forces, 

 in thin tubes or in small droplets, should delay the for- 

 mation of centers. Finally, if the formation of nuclei is 

 a random process subject to failure according to the laws 

 of probability, one should expect some irregularity in the 

 success of experiments designed to induce crystallization, 

 under apparently identical conditions. We shall discuss 

 below, after an introductory analysis of a typical sub- 

 cooling curve, the most important of the factors just men- 

 tioned, which either cause or prevent crystallization in a 

 subcooled liquid. 



Subcooling is generally considered an exception to the 

 rule that a substance crystallizes at a definite temperature. 

 It seems more logical, however, to consider subcooling as 

 the general condition, and crystallization at a fixed tem- 

 perature as a particular case, since the fixed freezing tem- 

 perature is only one, the highest, of the several tempera- 

 tures at which a fluid can start crystallizing, and since 

 congelation will not take place at the freezing point except 

 in the particular case w^here a crystallization center, that 

 is, an infinitesimal crystal, is present. Accordingly, it 

 would be preferable, from this point of view, to define the 

 freezing point not as the temperature of solidification but 

 as the highest temperature of solidification. 



The problems which concern the biologist in the study 

 of subcooling are : 1. Whether, to what extent and under 

 what conditions the fluids present in living animals and 

 plants, in tissues, in cells and in protoplasm, are brought 

 into, or barred from the subcooled state. 2. How life is 

 affected by this state and, in particular, whether injury 

 and death result from it or from its cessation? Only the 

 first of these two problems belongs to the program of the 

 present chapter, the other will be investigated elsewhere. 



I. THE SUBCOOLING CURATE 

 When a liquid is subcooled, the curve representing the 

 drop of temperature in terms of time, AB, (Fig. 28) con- 



