JOSEPH PRIESTLEY. 



of air as gas ventosum and gas sylvestre, and Boyle and 

 Hales had experimentally defined the physical proper- 

 ties of air, and discriminated some of the various kinds 

 of aeriform bodies, no one suspected the existence of 

 the numerous totally distinct gaseous elements which are 

 now known, or dreamed that the air we breathe and the 

 water we drink are compounds of gaseous elements. 



But, in 1754, a young Scotch physician, Dr. Black, 

 made the first clearing in this . tangled backwood of 

 knowledge. And it gives one a wonderful impression 

 of the juvenility of scientific chemistry to think that 

 Lord Brougham, whom so many of us recollect, at- 

 tended Black's lectures when he was a student in Edin- 

 burgh. Black's researches gave the world the novel 

 and startling conception of a gas that was a permanent- 

 ly elastic fluid like air, but that differed from common 

 air in being much heavier, very poisonous, and in hav- 

 ing the properties of an acid, capable of neutralising 

 the strongest alkalies ; and it took the world some time 

 to become accustomed to the notion. 



A dozen years later, one of the most sagacious and 

 accurate investigators who has adorned this, or any 

 other, country, Henry Cavendish, published a memoir 

 in the " Philosophical Transactions," in which he deals 

 not only with the " fixed air " (now called carbonic acid 

 or carbonic anhydride) of Black, but with " inflammable 

 air," or what we now term hydrogen. 



By the rigorous application of weight and measure 

 to all his processes, Cavendish implied the belief subse- 



