SPECIFIC FORM. • , 283 



trillion cells ought each to find its own predetermined 

 place. 



In crystallography also we understand by form the 

 organization which crystals present. The grouping 

 of the elements of crystals is, perhaps, more simple. 

 They are none the less organized, in the same sense 

 that living bodies are. 



Their organization, while more uniform than that 

 of living bodies, still shows a considerable amount of 

 variation. It should not be assumed that the area of 

 a crystal is completely filled, with contiguous parts 

 applied one to the other by plane faces, as might be 

 supposed from the phenomenon of cleavage which 

 dissociates the parts of the crystalline body into 

 solids of this kind. In reality, the constituent parts 

 are separated from each other by spaces. They are 

 arranged in a quincunx, aS Hauy put it, or along the 

 lines of a network, to use the terms of Delafosse and 

 Bravais. The intervals left between them are incom- 

 parably larger than their diameters. So that in the 

 organization of a crystal it is necessary to take into 

 account two quite different things : — An element, the 

 crystalline particle, which is a certain aggregate of 

 chemical molecules having a determinate geometrical 

 form ; and a more or less regular, parallelopipedic net- 

 work, along the edges of which are arranged in a con- 

 stant and definite manner the aforesaid particles. The 

 external form of the crystal indicates the existence 

 of the network. Its optical properties depend upon 

 the action of the particles, as Wallerant has shown : 

 Thus we must distinguish in a crystal between two 

 kinds of geometrical figures — that of the network 

 and that of the particle — and their characters of 

 symmetry may be either concordant or discordant 



