12 HISTORY OF CHEMISTRY. [LECTURE I. 



I shall deal more fully in the next lecture with the dis- 

 covery of oxygen and the theoretical revolution connected with 

 this discovery. I shall now say something with respect to 

 Cavendish's investigation of hydrogen, and shall refer particu- 

 larly to the modification in the prevailing phlogiston theory, 

 which Cavendish and some other chemists introduced as a 

 result of this investigation. 



Cavendish prepared his hydrogen from iron, tin, or zinc by 

 dissolving any of these in hydrochloric acid; he studied its 

 physical properties, called it inflammable air, and proved that 

 it was quite distinct from common air. Basing his opinion 

 on its mode of preparation, he regarded it, in the same way 

 -- that Lemery had already done, 13 as identical with phlogiston. 

 Priestley and Kirwan further developed this view, the. former 

 basing his opinion upon his own observation that the metallic 

 calces were reducible by means of hydrogen. 14 



The phlogiston theory in this new form is really based 

 upon the following views : When a metal is treated with a 

 diluted acid it decomposes into free phlogiston (hydrogen), 

 and a metallic calx which dissolves in the acid. If the acid is 

 concentrated (nitric acid or sulphuric acid) the phlogiston 

 unites with the acid and a phlogisticated sulphuric or nitric 

 acid is produced (sulphurous or nitrous acid). The explana- 

 tion of the reduction cf the calces by means of hydrogen 

 was very simple: what occurred was merely the taking up of 

 and the combination with phlogiston, whereby the metal was 

 regenerated. 



These ideas, in which we must recognise a touch of 

 genius, were pretty generally adopted by the phlogistians of 

 the period. They furnished the last glimpse of sunshine 

 accorded to the theory. The same person who discovered 

 the facts which rendered their advancement possible, soon 

 afterwards furnished the experiments which brought about the 

 downfall of the theory. 



The phlogiston theory, in the sense understood by Caven- 



13 Kopp, Geschichte. 3, 152. 14 Ibid. I, 242. 



