A CENTURY OF CHEMISTRY. 77 



The Greeks and Romans who accepted the four 

 elements of Empedocles fire, water, earth, and air 

 regarded fire as a material substance, and combus- 

 tion as the separation or liberation of the fire-stuff 

 from other material. In the seventeenth century, 

 Becher and Stahl regarded combustion as the separa- 

 tion of " inflammable earth," or the escape of 

 "phlogiston," a compound substance; for "only 

 compound substances can burn." For a long time 

 this Phlogiston theory was generally accepted, and 

 proved a useful stimulus to research. But the re- 

 peated demonstration of increase of weight on com- 

 bustion, the evidence that part of the air is absorbed 

 during the burning, Newton's suggestion that fire 

 was not a special substance at all, and, especially, 

 the discovery of oxygen, hydrogen, carbon-dioxide, 

 and other gases, seriously affected the vitality of the 

 theory, and finally shattered its constitution. It be- 

 came the subject of most ingenious doctoring, and 

 died a lingering death in the end of the eighteenth 

 century. 



What John Mayow, with penetrating insight, had 

 almost discerned more than a century before, that 

 burning means a union of something in the air with 

 inflammable particles in the stuff that burns, became 

 clearer when Priestley discovered oxygen in 1771, 

 when Lavoisier interpreted combustion as oxidation 

 in 1775, and when Cavendish showed that water was 

 a combination of hydrogen and oxygen in 1784. 



It is interesting to notice that although Priestley 

 had discovered oxygen and supposed that air sup- 

 ports combustion in virtue of the oxygen which it 

 contains, he died a believer in phlogiston; and that 

 although Scheele " the ideal of a pure experimental 

 chemist, the discoverer of numberless substances, who 



