388 



SCIENTIFIC THOUGHT. 



Phlogistic 

 theory. 



scientific point of view, the principal defect in this theory 

 was, that its explanations could not be subjected to any 

 strict and exact numerical verification. Whenever an 

 element enters into our operations which has either no 

 weight or a negative weight, and thus evades exact de- 

 termination and control, explanations and observations 

 become vag-ue and uncertain. 



In the time of Lavoisier, and pre-eminently through his 

 exertions, this vague and unmeasurable principle phlo- 

 giston was eliminated from the laboratory and the text- 

 books : quantities took the place of indefinable qualities, 

 and numerical determinations increased in frequency and 

 accuracy. The vague phlogistic theory, which contained 

 a germ of truth, l)ut one which at that time could not be 

 put into definite terms, had helped to gather up many 

 valuable facts and observations : these were collected and 

 restated in a new and precise language. It has been said 

 that every science must pass through three periods of 

 development. The first is that of presentiment, or of 

 faith ; the second is that of sophistry ; and the third is 

 that of sober research. Liebig states the case somewhat 



substances enter into chemical re- 

 action ; and the mobility or inert- 

 ness of chemical substances was to 

 be measured by the presence or ab- 

 sence of a definite something. A 

 hundred and fifty years after Stahl, 

 science had so far advanced, that 

 besides the change of weight or 

 mass, the change of the power 

 of entering into chemical com- 

 bination could also be measuj-ed, 

 and the term " potential energy " 

 was introduced to describe many of 

 those properties and processes which 

 Stahl had fastened upon, when he, 

 as the pioneer, undertook to co- 



ordinate chemical phenomena. If 

 Stahl considered phlogiston to be 

 a substance, though he did not in- 

 quire into its mass or ponderable 

 property, the question might be 

 put again, whether " energy " is not 

 to be considered after all as a sub- 

 stance. Cf. Tait, ' Properties of 

 Matter' (2nd ed., introduction, es- 

 pecially p. 5 sqq.) ; 'Recent Ad- 

 vances of Science,' introduction ; 

 also Clerk Maxwell, ' Electricity and 

 Magnetism ' (last chapter) ; Ost- 

 wald, ' Chemische Energie ' (Leip- 

 zig, 1893, p. 41). 



