PRIESTLEY. 411 



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 concentrated 

 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 ' dephlogisticated 

 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 most important discovery, which he thus con- 

 nected with an erroneous theory, was made on the 1st 

 of August, 1774. He afterwards discovered that its 

 absorption by the lungs in the process of respiration 

 gives its red colour to arterial blood, as it was proved 

 to act through the substance of thin bladder ; and he 

 found that when plants grow in close vessels, and 

 restore the purity of the air in which a candle has 

 burnt or an animal breathed, they do so by evolving 

 this pure air. The new nomenclature gave it the 

 name of ' oxygen gas,' from the belief then generally 

 entertained that it was the acidifying principle. Later 

 experiments have proved that there is at least one great 

 exception to this in chlorine, formerly called * oxy- 

 genated muriatic acid ;' but now found to be wholly 

 without oxygen, and yet to have all the properties 

 of an acid. But, indeed, water itself, and the atmo- 

 spheric air, having neither of them the nature of acids, 

 are both contrary to the theory ; and the fixed alkalis 



