V2S 



involve a pormanont osmotic disequilibrium between the 

 cell content and the intercellular spaces. 



II. PROGRESS OF CRYSTALLIZATION 

 A. THE GROWTH OF CRYSTALS 



AVhen a crystallization center is formed, it <»rows with 

 a speed which depends jirimarily on the temperature. 

 Tnder the same conditions, the dit^'erent faces of a given 

 crystal grow with different velocities, each set of faces 

 having a velocity coefficient. To simplify the study of the 

 crystallization velocity, the physicists consider separately 

 the linear growth of each face of the crystal, that is, the 

 growth in a direction perpendicular to that face. The 

 same general laws of growth, with various coefficients, 

 apply to all faces. 



It is generally thought that the velocity of crystalliza- 

 tion increases when the temperature decreases (below the 

 freezing point), in other words, that the colder a body is, 

 the faster it freezes. Several experimenters who studied 

 this relation have obtained data which confirm the com- 

 monly accepted view and which follow a curve of the type 

 represented in CDEF, Figure 8. But Tammann, amongst 

 others, pointed out that an increase in velocity for a de- 

 crease in temperature would be contrary to the general 

 laws governing physical and chemical tranformations {cf. 

 Tammann, op. cit., 1925, p. 251). He then showed that the 

 direct experimental determination of the temperature of 

 crystallization always involved an error. The temperature 

 measured by the experimenter is that of the liquid in 

 which crystallization occurs and not that of the growing 

 surface of the crystal. When the thermometer or thermo- 

 couple is placed as near as practically possible to the 

 crystal, it is still relatively too far from the active surface 

 to give any adecpuite measure of the temperature of this 

 surface itself. The heat continuously produced by crys- 

 tallization on the growing surface maintains the local 

 temperature there higher than that of the surrounding 

 medium where the thermometer is placed. Even the finest 



