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 



23. attraction alone would result in an accumulation of all 



Attraction 



sion'^*'^"^' iw^tter 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, maintained 

 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.^ The phenomena of attrac- 



^ Newton himself seems to have 

 looked for a mechanical explanation 

 of gravitation. Long before the 

 publication of the ' Principia ' he 

 laid before the Royal Society a 

 paper containing " a hypothesis 

 explaining the properties of light " 

 by the assumption of an "jetherial 

 medium, much of the same consti- 

 tution with air, but far rarer, 

 subtiler, and more strongly elastic " 

 (Letter to Oldenburg, January 25, 

 1675-76, given in Brewster's ' Me- 

 moirs of Sir I. Newton,' vol. i. 

 p. 390 sqq.), which miglit explain 

 magnetic and electric phenomena, 

 as well a.s those of gravitation, and 

 especially light. And in a letter 

 to Robert Boyle, of 28th February 

 1678-79 (Brewster, vol. i. p. 409), 

 he reverts to this subject. Having, 



however, in the course of the next 

 decade found it more useful to work 

 out the mathematical conclusions 

 to be drawn from the phenomenon 

 of gravitation, which was a fact and 

 not a hypothesis, lie abandoned the 

 metaphysical part of the subject, 

 the question how gravitation was 

 to be explained, "finding" (as Mac- 

 laurin says in his account of New- 

 ton's discoveries) " that he was not 

 able, from experiment and obser- 

 vation, to give a satisfactory ac- 

 count of this medium and the 

 manner of its operation in produc- 

 ing the chief phenomena of nature." 

 And in his letter to Boyle, as well 

 as in a later one to Halley (20th 

 June 1886, Brewster, vol. i. p. 439), 

 he carefully distinguishes between 

 the results of the ' Principia ' and 



