2 MINERALOGY 



volatile and dissolved substances. The number of hydrogen atoms 

 combining with one atom of oxygen to form one molecule of water, 

 is two; yielding the chemical expression H 2 for water. The 

 chemical molecule is the smallest particle of a compound which 



FIG. 1. Octahedral Crystals of Arsenious Oxide, 



can exist and still retain the physical properties of the compound. 

 In the attempt to divide the chemical molecule of water, the 

 atoms being indivisible, the one atom of oxygen must be sepa- 

 rated from the two atoms of hydrogen, resulting in two substances, 

 oxygen and hydrogen, neither of which possesses the properties of 

 water. 



There is no method of demonstrating whether the solid or crys- 

 talline molecule and the chemical molecule are identical. Since a 

 number of atoms combine to form a chemical unit, it is also probable 

 that a number of chemical units combine to form a crystalline unit 

 or molecule. The study of the structure of crystals indicates that 

 there is such a combination, but as to the exact relation of the 

 one to the other, all that can be said is that the crystalline unit 

 may be expressed by a multiple of the chemical molecule ; in case 

 of ice, n(H 2 O). The light of recent research would indicate that 

 n is a small number, and the crystal unit is not the complex group- 

 ing of a large number of chemical molecules it was formerly thought 

 to be. 



