106 JOSEPH PRIESTLEY. [LEGT- 



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 back- 

 wood 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, attended Black's lectures when he was a 

 student in Edinburgh. Black's researches gave the 

 world the novel and startling conception of a gas that 

 was a permanently elastic fluid like air, but that 

 differed from common air in being much heavier, very 

 poisonous, and in having 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 sub- 

 sequently formulated by Lavoisier, that, in chemical 

 processes, matter is neither created nor destroyed, and 

 indicated the path along which all future explorers 



