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 discovery 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 corresponding to that effected by Copernicus in astronomy nearly four hundred years ago.
Priestley seemed "to look on all '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 prepared 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 (SO2 gas), marine acid air (HCl gas), and fluor acid air (SiF4). In 1772 he proved that common air was a mixture. By burning charcoal in it he obtained fixed air (CO2), 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.
