1!)0 



Liiyet and Hodap]) (193S1)) iiivi'slioatcd the dcslruclioii 

 ot" crystallization centers l)y tenii)eratiii'es above the I'reez- 

 inu' ])oint in potato tissue. They determined tiie pro])Oi-- 

 tion of sul)coolin«>'s and of spontaneous freezings in 

 material ])reviously heated for two minutes at various 

 temperatures. After exposure to 95°, 50° or 20°-24°, 

 the percentage of subcoolings was resj)ectively 100, 95 and 

 85; after a previous freezing followed by exposure to 20°- 

 22°, 9°-10° or 1.8°-3° the percentage of subcoolings 

 dropped to 75, 35 and respectively (about 20 experiments 

 at each temperature). 



2. luoculatioti. Crystallization of a supercooled liquid 

 can be initiated by bringing the liquid in contact with a 

 crystal of the same substance. This is called seeding or 

 inoculatiug. The procedure is used as a routine laboratory 

 method. 



The fact that congelation starts when a crystal is pres- 

 ent shows that the type of modification that a subcooled 

 liquid undergoes to produce the first crystallite is an or- 

 ganization and orientation of molecules. When the first 

 crystallite is already formed, no such modification is re- 

 quired, the molecules are simply attracted to the faces of 

 the crystal on which they deposit. 



Inoculation can be performed not only with crystals of 

 the same substance as the liquid but also with isomorphous 

 crystals, that is, with crystals in which the directions of 

 growth are the same. This confirms the view that the 

 mechanism of construction of a crystal, and evidently also 

 of a crystallization center, consists essentially in control- 

 ling the directions taken by the molecules when they enter 

 the crystalline structure. 



Besides isomorphous crystals, some substances are said 

 to be specific in starting the crystallization of some partic- 

 ular melts ; for instance, traces of potassium hydroxide or 

 of nitric acid are used for inducing crystallization of 

 phosphorus. Assuming that this is an observed fact, one 

 might attempt to explain it by the theory that certain sub- 

 stances contribute toward orienting the molecules of other 



