PHYSICS OF NINETEENTH CENTURY 163 



heavens, fulfilling the required condition of vapour- 

 like diffusion, is a well-ascertained fact, and the nebular 

 hypothesis gains rather than loses support from recent 

 astronomical research. The French Encyclopaedists, 

 earlier in the eighteenth century, had begun to think 

 that they could see their way to explain all nature on 

 mechanical principles, and, as we have said, Laplace 

 imagined " a mind competent to foretell the progress 

 of nature to all eternity, if but the masses, their posi- 

 tions and initial velocities were given." This may 

 be regarded as the complete view of thorough-going 

 mechanical philosophy, which gained increasing hold 

 over the scientific world in the period now under con- 

 sideration. Its confident completeness is well illus- 

 trated by Laplace's reply to Napoleon's query about 

 the place of God in the scheme of nature, " that he 

 had no need of that hypothesis." 



But while mechanical science developed in paths 



already mapped out, chemistry underwent an entire 



transformation. Thus, at the beginning 



"Phlogiston." . A , . . ^ A .. 



of the period, the Aristotelian conception 

 of a body essentially light kept its hold in Stahl's 

 hypothetical substance "phlogiston." Misled by the 

 phenomena of flame, which naturally suggest an 

 escape of something from the burning body, Stahl 

 framed a theory in which that something was accepted 

 and named " phlogiston." Since, when all the 

 products are collected, the balance shows a gain, 

 phlogiston must possess a negative weight. In terms 

 of this hypothesis chemical science had learnt to 

 express its facts, and, owing to its influence, isolated 

 investigations, which pointed to more modern views, 



