ON CRYSTALLOGRAPHY. 143 



is liable to considerable variations according to the circumstances of 

 the case : but there are a certain number of forms peculiar to 

 every substance, and the crystals of tliat substance, in every case, 

 adopts one or other of these forms, and no other ; and thus com- 

 mon salt, when crystallized, has always either the figure of a 

 cube or octahedron, or some figure reducible to these. 



2. As the particles of bodu s must be at liberty to move before 

 they crystallize, it is obvious that we cannot reduce any bodies to 

 the state of crystals, except those which we are able to make fluid. 

 Now there are two ways of rendering the bodies fluid, namely, 

 sol itioti in a liquid, and fusion by heat. These of course are the 

 only methods of forming crystals in our power. 



Solution is the common method of crystallizing salts. They are 

 dissolved in the water : the water is slowly evaporated, the saline 

 particles gradually approach each other, combine together, and 

 form small crystals ; which become constantly larger by the addi- 

 tion of other particles, till at last they fall by their gravity to the 

 bottom of the vessel. It ought to be remarked, however, that 

 there are two kinds of solution, each of which presents different 

 phenomena of crystallization. Some salts dissolve in very small pro- 

 portions in cold water, but are very solable in hot water ; that 

 is to say, water at the common temperature has little effect upon 

 them, but water combined with caloric dissolves them readily. 

 When hot water saturated with any of these salts cools, it be- 

 comes incapable of holding them in solution : the consequence of 

 which is, that the saline particles gradually approach each other 

 and crystallize. Sulphate of soda is a salt of this kind. To crys- 

 tallize such salts, nothing more is necessary than to saturate hot 

 water with them, and set it by to cool. But were we to attempt 

 to crystallize them by evaporating the hot water, we should not 

 succeed ; nothing could be procured but a shapeless mass. Many 

 of the salts which follow this law of crystallization combine with a 

 great deal of water; or, which is the same thing, many crystals 

 formed in this manner contain a great power of crystallization. 



There are other salts again which are nearly equally soluble in 

 hot and cold water ; common salt for instance. It is evident that 

 such salts cannot be crystallized by cooling; but they crystallize 

 v-ry well by evaporating their solution while hot. These salts 

 generally contain but little water of crystallization. 



There are many substances, however, neither soluble in water 



