2 A Century of Science 



in which oxygen was unknown, and no man could 

 tell what takes place when a log of wood is burned 

 on the hearth. The language employed by Dr. 

 Priestley carries us back to the time when chem- 

 istry was beginning to emerge from alchemy. It 

 was Newton's contemporary, Stahl, who invented 

 the doctrine of plilogiston in order to account for 

 combustion. Stahl supposed that all combustible 

 substances contain a common element, or fire prin- 

 ciple, which he called phlogiston, and which es- 

 capes in the process of combustion. Indeed, the 

 act of combustion was supposed to consist in the 

 escape of phlogiston. Whither this mysterious fire 

 principle betook itself, after severing its connection 

 with visible matter, was not too clearly indicated, 

 but of course it was to that limbo far larger than 

 purgatory, the oubliette wherein have perished 

 men's unsuccessful guesses at truth. Stahl's the- 

 ory, however, marked a great advance upon what 

 had gone before, inasmuch as it stated the case in 

 such a way as to admit of direct refutation. Little 

 use was made of the balance in those days, but 

 when it was observed that zinc and lead and sun- 

 dry other substances grow heavier in burning, it 

 seemed hardly correct to suppose that anything 

 had escaped from these substances. To this objec- 

 tion the friends of the fire principle replied that 



