188 



111 li\iiig- tissues one has to consider tlie fact that water 

 is \villi(li';nvii from the cells before it freezes, that crystal 

 ••rowlli is llicii i-etarded by this slow osmotic ])rocess and 

 that, tlicrcroi-c, less heat of crystallizniioii is lilx'i'ated 

 duriiii!,' the time B"B'. 



In the application of the method of the freezin<i;-cnrve- 

 areas to cases in which there is some snbcooling-, one 

 should remember that this method is based on the assump- 

 tion that the heat withdrawn by the bath is equal to the 

 heat produced by freezing. When there is a snbcooling, 

 the heat produced is (Hjual to the sum of the heat with- 

 drawn and of the heat necessary to bring the temperature 

 from the snbcooling to the freezing point. The heat with- 

 drawn is proportional to the area limited by the curve 

 CB'D, (Fig. 28) the time axis and two abscissas, one at 

 the beginning and the other at the end of the freezing- 

 period considered. To obtain the heat produced one 

 should add to the number of calories found in the determi- 

 nation of this area the number of calories necessary to 

 warm the material up to the freezing point. 



II. FACTORS AFFECTING CRYSTALLIZATION 

 IN A SUBCOOLED LIQUID 



1. Temperature. By bringing the temperature far 

 enough below the freezing point it is almost always pos- 

 sible to induce crystallization. The lowest temperature 

 required for that pui'pose, that is, the maximal degree of 

 snbcooling, varies greatly with the nature of the substance 

 studied. Some substances are classically known to pre- 

 sent a very high degree of snbcooling. Phosphorus and 

 sulfur, which are often used in demonstration experiments, 

 can be maintained liquid, respectively, at about 35 and 100 

 degrees below the freezing point ; iron and nickel have also 

 been observed sui)ercooled by 100 degrees. According to 

 Tammann (1925, p. 231), a supercooling of 20 degrees is 

 quite common in organic compounds. Water is often still 

 liquid at -3° or -4°, more rarely at -6° or -7°, and 

 exceptionally, for short times, at -10°. 



