70 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



air on potash ; the two latter compounds thus contained 

 twice as much fixed air, relatively to their neutralising 

 power, as the two former. 



Cavendish measures the solubility of fixed air. In 

 the case of fixed air, its solubility in water was measured 

 and used to identify the gas. Cavendish states that : 



" Water when the thermometer is about 55 will absorb 

 rather more than an equal bulk." " Water heated to the 

 boiling point is so far from absorbing air, that it parts with 

 what it has already absorbed " (Phil. Trans., 1766, 56, 163). 



The gas was also lost by exposure to air. 



By measuring the solubility, as .well as the density, 

 Cavendish was able to prove that the gas set free during 

 fermentation was identical with fixed air prepared from 

 chalk and the alkalis. 



In his investigation of fixed air Cavendish introduced the 

 method of storing over mercury the gas which he had col- 

 lected by the displacement of water from an inverted bottle. 

 This use of mercury in place of water for manipulating 

 soluble gases proved to be of very great value in Priestley's 

 experiments a few years later. 



B. GASES DERIVED FROM NITRIC ACID. 



Priestley's work on gases. Whilst Cavendish was the 

 first to make accurate measurements of the physical pro- 

 perties of gases, the discovery of their chemical properties 

 was mainly the work of Priestley. The state of knowledge 

 when Priestley began his work upon gases is described in 

 the following paragraphs : 



" Van Helmont, and other chymists who succeeded him, 

 were acquainted with the property of some vapours to suffo- 

 cate, and extinguish flame, and of others to be ignited . . . 

 But they had no idea that the substances (if, indeed, they 

 knew that they were substances, and not merely properties, 

 and affections of bodies which produced those effects) were 



