410 PRIESTLEY. 



hydrogen gases) to accurate experiments, showing 

 their relative specific gravities, and proving that they 

 were of the same nature from what bodies soever 

 they were obtained. The probability was, that other 

 gaseous fluids existed in nature as well as those two 

 and common air. The experimenter had, therefore, 

 thenceforth, his attention directed to meeting with 

 these : and an examination of all the products of 

 mixture and of heat, by precipitation or evaporation, 

 was now the natural course of experimental inquiry. 

 At. first, Priestley only tried in what way fixed air 

 could be most easily combined with water ; he pub- 

 lished in 1772 a pamphlet upon the means of 

 effecting this union, and the condensing process which 

 he employed is used to this day. He soon after gave 

 to the Royal Society his observations on different kinds 

 of air, which ascertained the important fact, that at- 

 mospheric air, after having been corrupted by the re- 

 spiration of animals or by the burning of inflammable 

 bodies, is restored to salubrity by the vegetation of 

 plants ; and that if the air is exposed to a mixture of 

 sulphur and iron filings, as in one of Hales's experi- 

 ments, its bulk is diminished between a fourth and 

 a fifth, and the residue is both lighter than common 

 air and unfit to support life. This residue he called 

 ' Phlogistic air;' afterwards it was called 'Azotic' or 

 ' Nitrogen gas ;' and Dr. Rutherford, of Edinburgh, 

 as well as Priestley, though unknown to each other, 

 discovered it about the same time. For these experi- 

 ments the Copley Medal was, in 1773, justly awarded 

 to him by the Royal Society. 



The following year was destined to be the period 



