9 6 THE POPULAR SCIENCE MONTHLY. 



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bishop. While thus engaged, the kindly, cheerful doctor felt no more 

 wrath or uncharitableness toward his opponents than a smith does 

 toward his iron. But if the iron could only speak ! and the priests 

 and bishops took the point of view of the iron. 



No doubt what Priestley's friends repeatedly urged upon him 

 that he would have escaped the heavier trials of his life and done 

 more for the advancement of knowledge, if he had confined himself 

 to his scientific pursuits and let his fellow-men go their way was 

 true. But it seems to have been Priestley's feeling that he was a man 

 and a citizen before he was 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 Priestley 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 organize victory for 

 truth, and they are, at least, as important as the generals who visibly 

 fight their 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 exist- 

 ence. 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 sylvestre, and Boyle 

 and Hales had experimentally defined the physical properties 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, 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 neutralizing the strongest alkalies ; and it took 

 the world some time to become accustomed to the notion. 



