CH. xxvn. OXYGEN. 231 



expect you to know. Indeed, his merit was so great that 

 Bergmann, his friend and patron, once said, * The greatest 

 discovery he ever made was when he discovered Scheele.' 



Joseph Priestley, the discoverer of oxygen, was born in 

 1733. The greater part of his life was spent in writing upon 

 religious subjects, and it was only in his leisure hours that 

 he studied chemistry. He tells us in his autobiography that 

 he first began to take an interest in such things in conse- 

 quence of visiting a brewery next door to his house and 

 watching the fixed air which rose from the beer-vats. His 

 first chemical experiment of any value was to force this 'fixed 

 air ' into pure water, thus making an effervescing drink, much 

 the same as the soda-water we drink now. He next tried 

 what effect growing plants have upon air, and by keeping a 

 pot of mint under a bell-jar in which the air had been spoilt 

 by burning or breathing, he proved that plants take up the 

 bad air and render the remainder fit again to support a 

 flame or life. He did not, however, yet know why this took 

 place. He also invented a number of troughs and other 

 apparatus for collecting and washing gases, and amused 

 himself as Hales had done in driving gas out of different 

 substances. 



And thus it happened that one day, August r, 1774, he 

 made an experiment which led to a great discovery. He 

 took a red powder called mercuric oxide, which he knew 

 contained mercury and something else besides, and he put 

 it into the bulb, a, Fig. 41 ; the rest of the tube he filled 

 with mercury, and passed it into the basin , and up 

 into the jar <:, both b and c being also filled with mercury. 

 He next took a powerful burning-glass, d, and brought the 

 rays of the sun to a focus upon the red powder. As soon 

 as the powder became very hot a gas rose out of it and 

 passed along the tube into the jar, c, driving out the mer- 



