60 BIOGEAPHIES OF SCIENTIFIC MEN 



the phlogistic doctrine according to which metals lose 

 something (phlogiston) when calcinated (dephlogisticated). 

 The true explanation of combustion (oxidation) and 

 deoxidation was left for Lavoisier to interpret. Priestley 

 was the workman, Lavoisier the philosopher. The dis- 

 covery placed in the latter's hand became perfect and 

 in harmony with the universal laws of nature ; and a 

 revolution in chemistry was accomplished exactly corre- 

 sponding to that effected by Copernicus in astronomy 

 nearly four hundred years ago. 



Priestley seemed " to look on all c airs ' (gases) as easily 

 changeable one into the other," a kind of transmutation. 

 During his lifetime he examined the burning of candles, 

 the respiration of animals upon air, the action of living 

 plants on the air, and invented the pneumatic trough 

 (and used both water and mercury therein). He pre- 

 pared nitrous air (nitric oxide) dephlogisticated nitrous 

 air (nitrous oxide of the dentist), and carbon monoxide. 

 He used mercury in many of his pneumatic researches, 

 collected alkaline air (ammonia gas), vitriolic air (S0 2 gas), 

 marine acid air (HC1 gas), and fluor acid air (SiF 4 ). In 

 1772 he proved that common air was a mixture. By 

 burning charcoal in it he obtained fixed air (C0 2 ), which 

 on absorption left a residual (phlogisticated) air incapable 

 of supporting either respiration or combustion. Again, 

 Priestley could not see the importance of his discovery, 

 namely, that air is a mixture of gases. 



