ILLUSTRATIVE EXAMPLE OF CRYSTALLISATION. 59 



Heaven, and the cavern-secreted gem, are equally the 

 result of forces which are known to us in only a few of 

 their modifications. 



Every substance, when placed under circumstances 

 which allow of the free movement of its molecules, lias 

 a tendency to crystallise. All the metals may, by slowly 

 cooling from the melting state, be exhibited with a crys- 

 talline structure. Of the metallic and earthy minerals, 

 nature furnishes us with an almost infinite variety of 

 crystals, and, by a reduction of temperature, yet more 

 simple bodies assume the most symmetric forms. Water, 

 in the conditions of ice and snow, is a familiar and beau- 

 tiful example ; and, by such extreme degrees of cold as 

 are artificially produced, many of the gases exhibit a 

 tendency to a crystalline condition. 



May not the solid elementary atoms be susceptible of 

 change of form under different influences? May not 

 the different states under which the same bodies are 

 found as, for example, silica, carbon, and iron be due 

 entirely to a change in the form of the primitive atom ? 



Admitting the probability of this, we then easily sec 

 that the central molecule, formed of an aggregation of 

 such atoms, uniting by particular faces, would present a 

 determinate form ; and that the resulting crystal, a mass 

 of such molecules, cohering according to a given law, at 

 certain angles, would present such geometric figures as 

 we find in nature, or produce in our laboratories, when 

 we avail ourselves of processes which nature has taught 

 us. 



If we take a particle of marble, and place it in a large 

 quantity of water acidulated with sulphuric acid, it 

 dissolves, and a new compound results. The marble 

 disappears the eye cannot detect it by form or colour : 

 the acid also has been disguised the taste discovers 

 nothing sour in the fluid. We have, in combination 

 with the water, the lime and sulphuric acid ; but that 

 combinatiou appears to the eye in no respect different 



