150 OUR PHYSICAL WORLD 



the submergence of the old civilization by the hordes of 

 barbarians. Joseph Priestly, an English clergyman, who also 

 delighted to make chemical experiments, discovered oxygen 

 (1774), and described its properties with considerable accuracy. 

 He and the chemists of his time were beginning to realize that 

 these four so-called elements were not elements at all. Priestly 

 showed that one of the substances in the air was oxygen, or as he 

 called it "dephlogisticated air." He used the term air as we use 

 the term gas. Thus hydrogen he also knew as ( 'inflammable air/ 7 

 and carbon dioxide as "fixed air," because it was fixed or united 

 with other substances in limestone. He believed as did most of 

 the chemists of his day that fire was due to the escape of 

 an "inflammable principle" called phlogiston from substances 

 when they burned. Since oxygen would not burn as inflammable 

 air did, he thought the phlogiston had been in some way taken 

 out of it, so it was "dephlogisticated." 



Lavoisier, a French contemporary of Priestly, proved that 

 when a substance burned it gained weight rather than lost it, 

 and so must take up something instead of giving it off. He was 

 convinced that burning was the union of oxygen with the burning 

 substance. Priestly had visited Lavoisier and talked this matter 

 over with him and yet the old notion of phlogiston was so fixed 

 in his mind he could not see the truth. 



We really owe a very great deal to the scientists who devote 

 themselves to discovering truth for its own sake. It is only when 

 we understand the nature of things and of the forces that operate 

 about us that we can make rapid progress in the invention of 

 those devices that make life easier and more agreeable. 



When the nature of fire was understood there soon came 

 discoveries of new applications of it to the arts and industries 

 and improvements in the old ways of doing things. Primitive 

 man warmed himself beside a fire built in the open, and cooked 

 his food over it too. When he built a fire in his cave or shelter 

 the smoke made its escape as best it could. Cottager and 

 nobleman alike among our Anglo-Saxon forebears must choose 



