124 CRYSTALLISATION. [SECT. xiv. 



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 in- 

 clinations. 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 simi- 

 lar, 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 numerous, 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 crys- 

 tals, and, according to the experiments of MM. Haidinger 

 and Mitscherlich, seems in some cases to give the peculiar de- 

 termination to their constituent molecules. These gentlemen 

 have observed that the same substance crystallising at different 

 temperatures unites with different quantities of water and 

 assumes a corresponding variety of forms. Seleniate of zinc, 

 for example, unites with three different portions of 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 

 determination. Professor Mitscherlich found that prismatic 

 crystals of sulphate of nickel (N. 161), 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 



