14 JOSEPH PRIESTLEY i 



just estimate of the value of his work — of the 

 extent to whieli 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 Ilelmont, a century before, had dis- 

 tinguished different kinds of air as gas ventosum 

 and gas syhestre, and Boyle and Hales had ex- 

 perimentally 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 175-i, 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 diftercd from common air 

 in being much heavier, very poisonous, and in 

 having the properties of an acid, capable of neutral- 



