MICROSCOPIC TECHNIQUE AND REAGENTS. 769 



individuals, its particles tend to arrange themselves in regular 

 order, so that a definite structure is produced. The external 

 form of the individuals is also regular, being bounded by planes 

 in definite relation to each other so that polyhedral solids are 

 produced which are called CRYSTALS. The regular arrangement 

 of the atoms among themselves, and of the molecules which 



FIG. 422. Strychnine sulphate: tetragonal crystals in polarized light, showing side aspect. 



they build up, is so characteristic of substances of definite com- 

 position that the crystalline condition of dead matter is the normal 

 condition. Differences in chemical constitution are accompanied 

 by differences of physical structure, and the crystallographic test 

 of differences of chemical constitution is recognized as the most 

 delicate test of such differences." 

 49 



