PHYSICS OF NINETEENTH CENTURY 165 



cognition. A discovery or a theory that is too much 

 before its age stands but a poor chance in the ordeal 

 of selection. Of many men's work it may be said, 

 as Kant said of his own writings, that they had come 

 a century too soon. 



The beginning of the change appears in the work 

 of Dr Joseph Black of Edinburgh, who about 1755 

 discovered that a new ponderable gas, distinct from 

 atmospheric air, was combined in the alkalies. This 

 gas he named " fixed air " ; it is what we now call 

 carbon dioxide or carbonic acid. Nevertheless, for 

 yet a time phlogiston lived. 



In Joseph Priestley (1733-1804), theologian and 

 experimenter, we find the man who stood at the 

 threshold of the new era. He prepared oxygen by 

 heating mercuric oxide, and discovered its marked 

 power of supporting combustion. But he described 

 it as dephlogisticated air, and failed to perceive that 

 his discovery turned a new page. As Cuvier said, 

 he was the father of modern chemistry, but never 

 acknowledged his daughter. Priestley had the distinc- 

 tion of having his critical history of the corruptions 

 of Christianity burned by the common hangman at 

 Dort, and his papers and experimental laboratory 

 destroyed by a Birmingham mob. 



The theory of phlogiston maintained its hold also 

 on Henry Cavendish (1731-1810), a grandson of the 

 second Duke of Devonshire, who inherited alike his 

 scientific ability and the means wherewith to develop 

 it from a family where both attributes are not uncom- 

 mon. Cavendish made many discoveries, especially 

 in electricity, which have been credited to other men. 

 His retiring and secretive disposition led him often to 



