THE PATH OF EVOLUTION 



had properties entirely different from those of atmos- 

 pheric air, being what was known heretofore as 

 "fixed air." 



Joseph Priestley (1733-1804) pursued these investi- 

 gations into the nature of gases and into the composi- 

 tion of the atmosphere. He found that when charcoal 

 was burned in a closed vessel containing air, and the 

 fixed air thus produced was then absorbed by lime 

 water, the volume of the air was diminished by one- 

 fifth, and the remaining four-fifths were no longer 

 capable of supporting combustion or respiration. 

 This remainder he considered to be phlogisticated air, 

 since it had no longer an affinity for phlogiston. In 

 1774, by heating the red oxide of Mercury (the " Hy- 

 dragyrum precipitatum per se" of the Alchemists), 

 he obtained a gas which he thought was entirely free 

 from phlogiston; it eminently supported combustion. 

 He considered it to be dephlogisticated air. This was 

 the all-important discovery of oxygen. It is well 

 worthy of notice that neither Priestley, Cavendish 

 nor Scheele, each of whom occupied himself with 

 and extended his investigations into the properties 

 of oxygen, ever gave up the phlogiston doctrine, or 

 accepted the true philosophy of combustion. 



This shows how difficult it is to displace erroneous 

 views from the minds of men, who have found therein 

 a satisfactory explanation of the facts as they were 



164 



