CH. XXVII. PRIESTLEY'S DISCOVERIES. 233 



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 1, 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, tf, Fig. 39 ; the rest of the tube he filled 



Fig. 39. 



Priestley's Apparatus for procuring Oxygen. 



a. Bulb conuiniiig red mercuric oxide, b. Vessel containing mercnry. c, TnvertcJ 

 jar for collecting the gas. d. Burning glass. 



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

 into the jar <r, 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, <:, driving out the mer- 

 cury ; while the red colour began to disappear in the bulb, 

 a^ and only pure shining mercury remained behind. So far 

 he had only proved that red mercuric oxide is made up of 

 mercury and a gas. 



When he had collected enough gas to experiment upon, 



