IDEAS OF PHYSICAL CHEMISTRY 15 



remained fixed, but their ratio toward the third varied; in 

 such cases we might expect marked similarity in their proper- 

 ties, as compared with those cases where all the ratios varied. 

 Such reasoning, carried out more fully, might explain the 

 peculiarities of the Periodic Law, even though this does not 

 necessarily imply that no other explanation is possible. 



Even if we do not altogether understand the relations 

 between the elements, our chemical theory concerning the 

 nature of these elementary substances is very simple. Assum- 

 ing the elements to exist, each element is represented by one 

 set of extremely small indivisible atoms, the atoms of the 

 same element being exactly alike in all respects. They are 

 independent bodies exerting upon other atoms an attraction 

 which varies according to their nature. . . . 



These atoms are supposed to unite into small clusters that 

 we call molecules, and consider as the smallest independent 

 bodies that have the properties of ordinary tangible sub- 

 stances. They remind us strongly of planetary systems, like 

 our solar systems, because they execute their own movements 

 quite independently of the movements of their constituent 

 atoms. The molecules present comparatively easy problems 

 to the theorist; their own motions seem to be simple and their 

 mutual attractions are at least in part those of gravitation. 

 The so-called "kinetic theory of gases" explains all phenom- 

 ena of the gaseous state very readily by assuming that 

 molecules move in straight lines until they hit an obstacle and 

 are then reflected. If molecules come very close to one an- 

 other, their relative attraction becomes much greater than 

 that which corresponds to gravitation. I think it fair to as- 

 sume that then their more complex chemical attractions 

 come into play. 



Thus our chemical philosophy becomes an attempt to inter- 

 pret the actions of these imaginary atoms constituting matter 



