189 



It is generally tlioiight that biological material and, in 

 particular, living matter can be snbcooled to a larger ex- 

 tent than water and aqueous solution, and several biolo- 

 gists attribute to that property the high resistance offered 

 by some plants to death by congelation. But systematic 

 investigations on that subject are lacking. 



AVhile lowering the temperature induces crystallization 

 in a subcooled liquid, warming the liquid is supposed to 

 prevent freezing in a subsequent undercooling. The 

 crystallization centers, it is assumed, are not quite com- 

 pletely destroyed when the temperature is raised above 

 the freezing point, some molecular orientation would be 

 maintained; but, if the material is warmed to a tempera- 

 ture far above the freezing point, the nuclei would be en- 

 tirely destroyed and a high degree of subcooling or a 

 longer duration of the subcooled state should be possible 

 at a given temperature. 



De Coppet (1907), in experiments on salol heated at 

 temperatures up to 40 degrees above the freezing point 

 (which is 42.6°), showed that the higher the temperature 

 of the previous warming the longer the material stayed in 

 the subcooled state at a given temperature. However, 

 there were a number of exceptions. 



Othmer (Z. anorg. n. aUg. Chem., 91, 235, 1915) studied, 

 in a series of experiments on piperonal the number of 

 crystallization nuclei formed at 2 degrees below the freez- 

 ing point w^hen the material had previously been heated to 

 3 and to 33 degrees above the freezing point. He found 228 

 and 33 nuclei respectively. 



Concerning the time that the material is left at a tem- 

 perature above the freezing point for destroying the crys- 

 tallization centers, some physicists have expressed the 

 opinion that it plays no significant role and that the tem- 

 perature reached is the only factor to be considered. De 

 Coppet (1907) conducted a few experiments on this ques- 

 tion and he pointed out, in particular, the results of one 

 experiment in which, after having maintained salol for 

 one hour at 68 to 78 degrees above the freezing point, he 

 could keep it in the subcooled condition for 6 years. 



