596 SPECIAL METHODS OF DISCOVERY. 



drawn from known facts. Lavoisier had found by experi- 

 ment similar facts to fchose discovered by Greber and 

 Boyle; and from these, and especially from Priestley's 

 experiment of obtaining oxygen gas from oxide of mer- 

 cury, he inferred the erroneous character of the doctrine 

 of ' phlogiston,' and the true explanation and theory of 

 oxidation, combustion, and general chemical union. He 

 devised and made the ingenious experiment of heating- 

 mercury in a closed vessel of air to enable it to take up 

 oxygen ; then by heating the oxide to a much higher 

 temperature, he re-obtained the metal and gas ; and by 

 weighing each of the substances before and after each of 

 these changes, he found that the metal increased just as 

 much in weight in forming the oxide as was lost by the air 

 in which it was placed, and that by subsequently heating 

 the oxide the same weight of oxygen was obtained as was 

 lost by the air. By burning charcoal in oxygen, and 

 analysing the gaseous product, he also proved that 6 fixed 

 air ' was a compound of carbon and oxygen. And from 

 these and other facts he inferred and proved the great 

 discovery that in oxidation and combustion gas is taken 

 from the air, and unites with the rusting or burning 

 body. He also inferred that common air consisted of 

 6 pure or vital air ' (i.e. oxygen), and of an unvital air 

 i.e. nitrogen), which he called ' azote ' ; that pure air was 

 the necessary agent in calcination, combustion, acidifi- 

 cation, and respiration ; and that all these processes were 

 analogous, and consisted in a decomposition of common 

 air, and of fixation of the pure air contained in it. His 

 oxygen theory rendered explicit many other facts which 

 had previously been misunderstood. Hooke, in 1665, had 

 already inferred, because red-hot charcoal was not con- 

 sumed if air was absent, that air acted upon heated sub- 

 stances. About the same time, Boyle enclosed a bird in a 



