A HISTORY OF SCIENCE 



same law of gravitation which holds the stars and plan- 

 ets in their course. What, then, is this all-compassing 

 power of gravitation which occupies so central a posi- 

 tion in the scheme of mechanical things ? 



The simple answer is that no man knows. The 

 wisest physicist of to-day will assure you that he knows 

 absolutely nothing of the why of gravitation that he 

 can no more explain why a stone tossed into the air 

 falls back to earth than can the boy who tosses the 

 stone. But while this statement puts in a nutshell the 

 scientific status of explanations of gravitation, yet it 

 is not in human nature that speculative scientists 

 should refrain from the effort to explain it. Such 

 efforts have been made; yet, on the whole, they are 

 surprisingly few in number; indeed, there are but two 

 that need claim our attention here, and one of these 

 has hardly more than historical interest. One of these 

 is the so-called ultramundane-corpuscle hypothesis of 

 Le Sage; the other is based on the vortex theory of 

 matter. 



The theory of Le Sage assumes that the entire uni- 

 verse is filled with infinitely minute particles flying in 

 right lines in every direction with inconceivable rapid- 

 ity. Every mass of tangible matter in the universe is 

 incessantly bombarded by these particles, but any two 

 non-contiguous masses (whether separated by an in- 

 finitesimal space or by the limits of the universe) are 

 mutually shielded by one another from a certain num- 

 ber of the particles, and thus impelled towards one 

 another by the excess of bombardment on their oppo- 

 site sides. What applies to two masses applies also, 

 of course, to any number of masses in short, to all the 



214 



