342 SCIENTIFIC THOUGHT. 



and all through ancient and mediaeval philosophy, figured 



as one of the occult causes or forces which regulate the 



behaviour of living and dead matter. That the force of 



as. attraction alone would result in an accumulation of all 



Attraction 



matter in one body was of course recognised, and a second 

 arbitrary and occult force that of repulsion was intro- 

 duced as a counteracting or balancing agent. 



In Newton's system of the universe the balancing force 

 was found to be that of an inherent initial motion which 

 matter, in consequence of its mass or inertia, maiutained 

 in addition to the motion due to gravitation. If motion 

 and inertia were able to account for the apparent repul- 

 sion of bodies at a distance, it might be that they could 

 also account for their apparent attraction. This idea, 

 though expressed about the time when the Newtonian 

 gravitation formula was established, did not meet with 

 serious attention till far on in our century other lines of 

 thought led to similar views. 1 The phenomena of attrac- 



1 Newton himself seems to have however, in the course of the next 

 looked for a mechanical explanation decade found it more useful to work 

 of gravitation. Long before the out the mathematical conclusions 



publication of the 'Principia' he 

 laid before the Royal Society a 



to be drawn from the phenomenon 

 of gravitation, which was a fact and 



paper containing " a hypothesis ! not a hypothesis, he abandoned the 



explaining the properties of light" ' metaphysical part of the subject, 



by the assumption of an " setherial \ the question how gravitation was 



medium, much of the same consti- j to be explained, "finding" (as Mac- 



tution with air, but far rarer, laurin says in his account of Xew- 



subtiler, and more strongly elastic " ton's discoveries) " that he was not 



(Letter to Oldenburg, January 25, able, from experiment and obser- 



1675-76, given in Brewster's ' Me- vation, to give a satisfactory ac- 



raoirs of Sir I. Xewton,' vol. L ' count of this medium and the 



p. 390 sqq.), which might explain | manner of its operation in produc- 



magnetic and electric phenomena, ing the chief phenomena of nature/' 



as well as those of gravitation, and I And in his letter to Boyle, as well 



especially light. And in a letter as in a later one to Halley (20th 



to Robert Boyle, of 28th February j June 1886, Brewster, vol. L p. 439), 



1678-79 (Brewster, vol. i. p. 409), he carefully distinguishes between 



he reverts to this subject. Having, the results of the ' Principia ' and 



