PRIESTLEY. 75 



these : and an examination of all the products of mix- 

 ture 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 published 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 Eoyal 

 Society his observations on different kinds of air, which 

 ascertained the important fact, that atmospheric air, 

 after having been corrupted by the respiration of ani- 

 mals or by the burning of inflammable bodies, is re- 

 stored 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 experiments, 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 experiments the Copley Medal 

 was, in 1773, justly awarded to him by the Royal 

 Society. 



The following year was destined to be the period 

 of a discovery most important for science, and truly 

 glorious for its author. Having exposed red-lead, or 

 minium, in a close vessel to the sun's rays concen- 

 trated by a burning-glass, he found that an aeriform 

 body, permanently elastic, was evolved, and that this 

 air had the peculiar property of increasing exceedingly 

 the intensity of flame. This gas he called ' dephlo- 

 gisticated air,' upon the principle that the matter of heat 

 and light, the phlogiston of Stahl, being abstracted from 

 it by the return of the calx to its metallic state, which 

 phlogiston was supposed by that theory to effect, the 

 air had great avidity for phlogiston, and seized it from 

 the inflammable bodies it came in contact with. This 



