120 THE PATH OF SCIENCE 



Stone or to transmute metals. They were, in fact, anxious 

 to work on applied chemistry, and their efforts to apply chem- 

 istry instead of observing and studying the facts delayed the 

 discovery of the nature of the reactions that constitute the 

 science of chemistry. Then when experimental chemistry got 

 under way, in the seventeenth century, its progress was gi^eatly 

 delayed by an entirely incorrect hypothesis that was adopted. 



George Ernst Stahl, physician to the King of Prussia, 

 studied the phenomena of combustion and accepted the idea 

 suggested by J. J. Becker, one of the last alchemists, that they 

 were due to the loss by the burning substance of the prin- 

 ciple of combustibility, to which he gave the name phlogiston. 

 When flame is observed escaping from a piece of burning 

 wood, what is more reasonable than to assume that the prin- 

 ciple that renders the material combustible is escaping in the 

 flame? And this was the more reasonable because the alchem- 

 ists had laid great stress on the existence of various principles 

 in all things, the principle of combustibility being generally 

 termed sulfur by the alchemists. We now know, of course, 

 that combustion is the combination of the burning substance 

 with the oxygen of the air, but this idea was completely re- 

 versed by the followers of the phlogiston theory, even though 

 measurements of the change of weight during combustion 

 showed that the burning substance increased in weight. This 

 was explained by the ad hoc assumption that phlogiston had 

 a negative weight. Even Joseph Priestley, the English non- 

 conformist minister who discovered oxygen gas in 1774 simul- 

 taneously with Karl Scheele, insisted on calling it dephlo- 

 gisticated air, his idea being that this was the component of 

 the atmosphere with which the phlogiston united when it 

 escaped from a burning substance. 



The true nature of combustion was demonstrated by 

 Lavoisier in 1772 as a result of quantitative measurements, 

 in which he found that the burning of sulfur and phosphorus 

 and the oxidation of metals resulted in an increase of weight. 

 He then repeated Joseph Priestley's experiments on the heat- 

 ing of mercuric oxide to obtain oxygen and showed that com- 



