CHEMICAL APPARATUS AND PRODUCTS. 199 



alkalies was, that the latter contained fixed air a kind of air 

 identical with that obtained from fermenting liquids. Black's 

 pneumatic trough and balance are exhibited. Cavendish clearly 

 pointed out the differences between inflammable air, which we now 

 call hydrogen, and fixed air, now known as carbonic acid gas. He 

 determined, although not quite accurately, the relative weights oi 

 these gases compared with common air, and described several 

 processes by which they may be obtained, and the mode of collect- 

 ing and experimenting with gases in general. The balance with 

 which these determinations were made has been sent by the 

 Managers of the Royal Institution. In 1774, Priestley (and almost 

 simultaneously Scheele) discovered oxygen gas, which he found 

 supported combustion and life more powerfully than common air. 

 It had been known previously that air in which combustibles had 

 been burnt became incapable of supporting combustion and life, 

 and was hence said to be phlogisticated. Priestley, therefore, called 

 his new gas dephlogisticated air ; common air being intermediate 

 between these two extremes. In consequence of the inflammability 

 and lightness of inflammable air, Cavendish imagined that it was 

 phlogiston itself, and expected that when inflammable air was burnt 

 in dephlogisticated air that the result would be phlogisticated air. 

 On the experiment being tried the gases disappeared, and the sole 

 product was water. Besides oxygen, Priestley discovered and de- 

 scribed several other important gases, and improved the apparatus 

 employed for such experiments. The experiments of Priestley 

 on the preparation of oxygen were carefully examined by Lavoi- 

 sier; and they, together with many of his own discoveries, enabled 

 him to work a complete revolution in the theory of chemistry. 

 Lavoisier showed that when bodies burn, and metals are calcined, 

 the combustibles unite with a portion of the air, and that the metal 

 increases in weight exactly as much as the air diminishes; so that 

 instead of a combustible losing anything when burning, as the 

 phlogistians believed, it actually combines with a gaseous matter 

 from the atmosphere. He showed also that when calces are 



