238 EIGHTEENTH CENTURY. FT. III. 



and he obtained, of course, the gas which Priestley had 

 called ' dephlogisticated air.' He afterwards found by more 

 exact experiments that the amount of this gas contained in 

 the mercuric oxide exactly equalled the amount lost by the 

 air in which the mercury had been heated. 



Now see what Lavoisier had done : he had proved that 

 the reason why air shrinks when substances are burnt in it, 

 is because the substances take up a gas out of the air, and he 

 had also shown that this gas is the same as that which 

 Priestley discovered. Now, at last, the false theory was 

 destroyed, and the starting-point of a true theory was found. 

 The imaginary phlogiston, which had been supposed to load 

 the air when anything was burnt in it, was proved never to 

 have had any existence; for it was clear that just the 

 opposite effect takes place. All burning and breathing and 

 the change in metals is caused by a gas being taken up out 

 of the air and joined to other substances. Lavoisier called 

 this gas oxygen (from 6v, acid ; ye wau, I produce), because 

 he found that most substances were acid after they had 

 been united with it. This, too, led him to suspect that as 

 ' fixed air ' was an acid, and could be made by burning char- 

 coal, it must be composed of oxygen and carbon. So he 

 burnt small quantities of charcoal in pure oxygen, and by 

 analysing the ' fixed air ' produced proved that 100 parts by 

 weight of this gas contained 72 parts of oxygen and 28 of 

 carbon. For this reason he called it ' carbonic acid,' a 

 name which it still bears. By burning a diamond in oxygen 

 and producing carbonic acid, he also proved that a diamond 

 is pure carbon. 



Lavoisier had very great difficulty in persuading the 

 other leading chemists that they had been labouring under a 

 false idea, and that substances when burning do not put 



