THE MOLECULAR COMPOSITION OF MATTER 75 



was supposed to add luster and to make the substance 

 volatile. Salt conferred fixity rather than volatility 

 in the presence of fire. Sulphur added combustibility. 

 Later the " principle of combustibility" was assumed 

 to be due to other causes than the addition of sulphur, 

 so that a more inclusive term came to be desirable and 

 hi 1690 the principle was named " phlogiston." 



According to the phlogiston theory, combustion 

 meant the liberation of phlogiston, and those substances 

 which burned most readily released the larger quan- 

 tities. Charcoal, wood, and coal were thus assumed 

 to be nearly pure phlogiston. Now, metals exposed to 

 fire tarnish and in time form earthy powders. These 

 we call oxides. The familiar red rust of iron is an 

 oxide formed by a reaction with water and air. Some 

 of the iron ores are oxides. The method of reducing 

 metallic oxides by heating them with charcoal had 

 been used for centuries. The reaction which takes 

 place is one of combustion of the charcoal and the 

 oxygen of the metallic oxide. Carbon oxides, the 

 usual products of combustion, with which we are all 

 familiar, are thus formed and a residue of pure metal 

 and charcoal ash is left. The advocates of the phlogis- 

 ton theory curiously hi verted this idea of combustion. 

 They assumed that phlogiston left the charcoal and 

 entered the oxide, the earthy powder which they called 

 "calx." The calx thus became a metal by the addition 

 of phlogiston. Conversely, when a metal was burned 

 it was dephlogisticated and a calx resulted. 



The opponents of the phlogiston theory pointed out 

 that the calx was heavier before it became a metal by 

 the addition of phlogiston. Any such objection was, 



