I JOSEPH PRIESTLEY 15 



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 subsequently formulated by Lavoisier, that, 

 in chemical processes, matter is neither created 

 nor destroyed, and indicated the path along which 

 all future explorers must travel. Nor did he him- 

 self halt until this path led him, in 1784, to the 

 brilliant and fundamental discovery that water is 

 composed of two gases united in fixed and con- 

 stant proportions. 



It is a trying ordeal for any man to be compared 

 with Black and Cavendish, and Priestley cannot be 

 said to stand on their level. Nevertheless his 

 achievements are not only great in themselves, but 

 truly wonderful, if we consider the disadvantages 

 under which he laboured. Without the careful 

 scientific training of Black, without the leisure and 

 appliances secured by the wealth of Cavendish, he 

 scaled the walls of science as so many Englishmen 

 have done before and since his day ; and trusting 

 to mother wit to supply the place of training, and 

 to ingenuity to create apparatus out of washing 



