CH. XXVII. 



LA VOISIEFS EXPERIMENTS. 



237 



could not succeed ; he could not make the metals give back 

 again what they had taken from the air, so that he might 

 examine it. At last, in 1778, it occurred to him that 

 Priestley had separated mercuric oxide into two substances ; 

 namely, the metal mercury and a gas. Here, then, was just 

 the step he wanted. If he could first make mercuric oxide 

 by heating mercury in the air, and then afterwards separate 

 it back again into mercury and a gas, he would thus prove 

 what it had taken out of the air. He therefore took some 

 mercury and put it into a tube A, Fig. 40, which was connected 



FIG. 40. 



Lavoisier's Apparatus for Heating Mercury and making it take up Oxygen. 



A, Bulb containing mercury. B, Vessel containing mercury, c, Bell-jar partly full 



of air. D, Stove. 



with a bell-jar c, containing air and standing over mercury. 

 Then he heated the bulb A over the stove D, and kept the 

 mercury boiling for twelve days. 



During the first five days little by little red specks began 

 to appear on the top of the mercury in c, that is, mercuric 

 oxide was formed ; but after that time, when about one-fifth 

 of the air in the bell-jar, c, had disappeared and mercury 

 risen in its place, no further change took place. He then 

 lifted off the bell-jar and took 45 grains of this red powder 

 and made Priestley's experiment with it (see p. 233), 



