SECT. XIV. CRYSTALLIZATION. 107 



aggregation of a few particles would, if continued, cause the 

 addition of more ; and the process would go on as long as any 

 particles remain free round the primitive nucleus, which would 

 increase in size, but would remain unchanged in form, the figure 

 of the particles being such as to maintain the regularity and 

 smoothness of the surfaces of the solid and their mutual incli- 

 nations. A broken crystal will by degrees resume its regular 

 figure when put back again into the solution of alum, which 

 shows that the internal and external particles are similar, and 

 have a similar attraction for the particles held in solution. The 

 original conditions of aggregation which make the molecules of 

 the same substance unite in different forms must be very nu- 

 merous, since of carbonate of lime alone there are many hundred 

 varieties ; and certain it is, from the motion of polarised light 

 through rock crystal, that a very different arrangement of 

 particles is requisite to produce an extremely small change in 

 external form. A variety of substances in crystallising combine 

 chemically with a certain portion of water which in a dry state 

 forms an essential part of their crystals, and, according to the 

 experiments of MM. Haidinger and Mitscherlich, seems in some 

 cases to give the peculiar determination to their constituent mole- 

 cules. These gentlemen have observed that the same substance 

 crystallising at different temperatures unites with different quan- 

 tities of water and assumes a corresponding variety of forms, 

 Seleniate of zinc, for example, unites with three different portions 

 pf water, and assumes three different forms, according as its 

 temperature in the act of crystallising is hot, lukewarm, or cold. 

 Sulphate of soda also, which crystallises at 90 of Fahrenheit 

 without water of crystallisation, combines with water at the 

 ordinary temperature, and takes a different form. Heat appears 

 to have a great influence on the phenomena of crystallisation, not 

 only when the particles of matter are free, but even when firmly 

 united, for it dissolves their union, and gives them another deter- 

 mination. Professor Mitscherlich found that prismatic crystals 

 of sulphate of nickel (N. 164), exposed to a summer's sun in a 

 close vessel, had their internal structure so completely altered 

 without any exterior change, that when broken open they were 

 composed internally of octahedrons with square bases. The 

 original aggregation of the internal particles had been dissolved, 

 and a disposition given to arrange themselves in a crystalline 



