82 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



rapidly, so that the water in the receiver " rushed violently 

 into the bottle and filled it almost entirely full " (Phil. Trans. 

 1766, 56, 157-158). 



Priestley (1772) isolates "marine acid air. "By making 

 use of mercury instead of water, Priestley had no difficulty 

 in obtaining this ACID AIR in a permanent form. He 

 showed that it could be driven off by heating spirit of salt 

 alone, the presence of copper being unnecessary. Its 

 peculiar behaviour depended entirely on its extreme solubility 



in water, which he 

 found to absorb 5 76 

 times its volume of 

 the gas. 1 Spirit of 

 salt was, indeed, 

 merely a solution of 

 acid air in perhaps 

 twice its weight of 

 water. Priestley also 

 prepared the gas by 

 the action of oil of 

 vitriol on salt, a more 

 efficient method 



FIG. 18. APPARATUS USED BY CAVENDISH TO PREPARE which had the ad- 



A GAS WHICH " LOST ITS ELASTICITY BY CONTACT r i 



WITH WATER." vantage of produc- 



ing a dry gas. He 



found that "acid air, extinguishes flame, and is much 

 heavier than common air" (Experiments on Air, 1774, I. 



I43-I47)- 



Priestley isolates "alkaline air." Encouraged by his 

 success in separating an elastic air from spirit of salt, 

 Priestley next endeavoured to collect an ALKALINE AIR from 

 the "volatile spirit of sal-ammoniac." In this he was 

 successful : the gas which was set free by heating the 



1 Two and a-half grains of water dissolved three ounce-measures 

 of the gas. 



