SECT. XIV. CRYSTALIZATION. 105 



lar forms a fact easily proved by dissolving a piece of 

 alum in pure water. The mutual attraction of the par- 

 ticles is destroyed by the water ; but if it be evaporated 

 they unite and form in uniting eight-sided figures called 

 octahedrons (N. 160). These, however, are not all the 

 same. Some have their angles cut off, others their 

 edges, and some both, while the remainder take the 

 regular form. It is quite clear that the same circum- 

 stances which cause the 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 inclinations. 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 ex- 

 ternal particles are similar and have a similar attraction 

 for the particles held in solution. The original condi- 

 tions 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 mo- 

 tion of polarized 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 crystalizing 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 molecules. These gentlemen have observed 

 that the same substance crystalizing at different tem- 

 peratures 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 crystalizing is hot, luke- 

 warm, or cold. Sulphate of soda, also, which crystal- 

 izes at 90 of Fahrenheit without water of crystaliza- 

 tion, combines with water at the ordinary temperature 



