COHESION AND CRYSTALLIZATION 85 



If, however, a crystal or other heterogeneous structure is present. 

 it forms a centre of 

 cohesion which will 

 attach any molecules 

 that are brought by 

 diffusion into its 

 sphere of attraction. 

 We have succeeded 

 in photographing 

 the arrangement of 

 the molecules of a 

 liquid around a crys- 

 tal in the act of 

 formation (Fig. ;2(>). 

 For tins purpose we 

 add to the solution 

 traces of some col- 

 loidal substance, such 

 as gelatine or gum, 



Fig. 26. — Field of crystallization of sodium 

 chloride (magnified 60 diameters). 



so as to delay the crystallization. 



It may thus be shown 

 that the molecules of 

 the surrounding liquid 

 are already arranged 

 in crystalline order 

 for some distance from 

 the crystal, forming 

 a sort of field of 

 crystallization. The 

 arrangement of this 

 j regular field varies in 

 i different cases, and 

 ! is more or less com- 

 plicated according to 

 circumstances. One 

 of the most frequent 

 forms is that shown 

 in Fig. 27, which is 

 the Held around a crystal of sodium chloride. In the centre 



s 







Wh 



Fig. 27. — Field of crystallization around a crystal 

 of sodium chloride in process of lormation. 



