COMBINING WEIGHTS OF THE ELEMENTS 6i 



XIV. COMBINING WEIGHTS OF THE ELEMENTS 



In the foregoing studies evidence has been accumulated that 

 chemical combination takes place in definite proportions by 

 weight, and that each element has its own combining weight. 

 It was found that i part by weight of hydrogen, 3 parts by 

 weight of carbon, and 100 parts by weight of mercury combine 

 with 8 parts by weight of oxygen, forming respectively water, 

 carbonic acid gas, and red oxide of mercury. The question must 

 now be asked — Do these elements unite in these proportions 

 only? 



When lead is roasted in the air, the oxide of lead — litharge 

 — is formed. On further roasting, at a gentle heat, the 

 reddish yellow colour of the litharge darkens, and ultimately 

 red lead is obtained. It has been already found that red lead 

 at a higher temperature breaks up into litharge and oxygen. 

 Red lead must therefore be an oxide of lead, containing a 

 larger proportion of oxygen than litharge contains. Introduce 

 about 5 grams of red lead, previously dried by heating on a 

 water bath, into a weighed porcelain crucible, and weigh. 

 Heat very gently, taking care not to melt the contents of the 

 crucible, till oxygen ceases to be evolved, and the red lead is 

 entirely reduced to yellow litharge. Allow to cool, and 

 weigh. Calculate the proportions of litharge and oxygen per 

 cent, of red lead. 



Not only lead, but hydrogen, carbon, mercury, and, in fact, 

 most of the elements, combine with oxygen in more than one 

 proportion. The relation between the composition of the 

 oxides, however, if there be more than one, is always a very 

 simple relation. Thus in the two compounds of hydrogen and 

 oxygen, the combining proportion in the one is i to 8, in the 

 other I to 16. It is usual, therefore, to take as the combining 

 weight of the element a simple number that represents the 

 proportion in which it enters into its various combinations, 



