222 CHEMICAL DISCOVERY AND INVENTION 



crystal is broken into fragments each portion has the same 

 external form and the same action on light. From such facts it 

 is obvious that a crystal consists of a number of exactly similar 

 parts which are repeated over and over indefinitely throughout 

 the mass. And if the subdivision is supposed to be carried so far 

 that the mass consists no longer of a number of molecules joined 

 together but of one molecule alone, the space required for the 

 accommodation of that one molecule within the crystal structure 

 would have the same proportions as the crystal itself. 



The atoms which enter into the composition of a molecule 

 must be assumed to act as centres under the operation of two 



FIG. 61. 



FIG. 62. 



opposing forces, namely, a repellent force due to the kinetic 

 energy of the atom and an attractive force which is the result 

 of " chemical affinity," both forces being governed by some un- 

 known law of inverse distance. 



If now a crystalline element be considered, the molecules of 

 which consist of one atom only, and every atom is like every 

 other atom, it is found that two homogeneous arrangements of 

 atoms are possible. These correspond respectively to the sym- 

 metry of the cubic and hexagonal crystalline systems. 



In the two figures above which show the appearance of models 

 representing these two arrangement) the atoms are supposed 

 to reside at the centres of the spheres which are seen to be in 

 contact. Each sphere represents the range of the influence or 



