V.] JOSEPH PRIESTLEY. 105 



a philosopher, and that the duties of the two former 

 positions are at least as imperative as those of the 

 latter. Moreover, there are men (and I think Priest- 

 ley was one of them) to whom the satisfaction of 

 throwing down a triumphant fallacy is as great as 

 that which attends the discovery of a new truth ; who 

 feel better satisfied with the government of the world, 

 when they have been helping Providence by knocking 

 an imposture on the head ; and who care even more 

 for freedom of thought than for mere advance of 

 knowledge. These men are the Carnots who organise 

 victory for truth, and they are, at least, as important 

 as the generals who visibly fight her battles in the 

 field. 



Priestley's reputation as a man of science rests 

 upon his numerous and important contributions to 

 the chemistry of gaseous bodies ; and to form a just 

 estimate of the value of his work of the extent to 

 which it advanced the knowledge of fact and the 

 development of sound theoretical views we must 

 reflect what chemistry was in the first half of the 

 eighteenth century. 



The vast science which now passes under that 

 name had no existence. Air, water, and fire were 

 still counted among the elemental bodies ; and though 

 Van Helmont, a century before, had distinguished 

 different kinds of air as gas ventosum and gas syl- 

 vestre, and Boyle and Hales had experimentally 

 defined the physical properties of air, and discri- 

 minated some of the various kinds of aeriform bodies, 



