CHAPTER 1 



FREEZING, THE FROZEN STATE AND MELTING 



T. TXITIATIOX OF CRYSTALLIZATION 

 A. THE FORMATION OF CRYSTALLINE NUCLEI 



Pliysics has acquired a wealtli of information during the 

 last 30 years on the sti-ncture of crystals, but very little has 

 been learned yet on the mechanism of initiation of crystal- 

 lization. 



The condition s'nic qua von for the molecules of a liquid 

 to be hooked up in a crystalline pattern is the reduction of 

 their mean distance to a certain minimum, that is, the low- 

 ering of their temperature to the freezing point. 



When the liquid to be solidified is at a convenient tem- 

 perature and when a crystal of the substance is present, 

 crystallization proceeds at once, but if a crystal is not there 

 to initiate the process, that is, to give a direction to the 

 molecules, the latter have first to orient themselves and 

 build up the original nuclei. Crystallization nuclei are 

 understood to originate from the collision of molecules 

 coming in contact in certain detinite conditions of orienta- 

 tion. After a first successful collision, a pair of molecules 

 would be constituted, but this would not yet be the crystal 

 unit wanted; other successive collisions under definite 

 conditions would be required to complete the original 

 crystallite. 



Though our knowledge of the mechanism of formation 

 of the first crystallization nuclei is to a large extent theo- 

 retical, their coming into existence as individual centers 

 is a well observed fact ; the centers can be photographed 

 and counted. Figure 4 represents the formation of such 

 centers in a solution of gelatine spread between two glass 

 slides and immersed in a bath at -25°. Tammann (1925, 

 p. 228) has shown that during a gradual decrease of tem- 

 perature (below the freezing point), the number of crystal- 

 lization centers formed per unit of time increases to a 

 maximum and then decreases, following a curve of the 



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