452 THE WORLD MACHINE 



Elasticities and the Rarities of the Mediums taken to- 

 gether." 



When some of Newton's followers proposed flatly to accept 

 actio in distans Newton would have nothing whatever to do 

 with it. In his well-known letter to Bentley he evidences his 

 impatience. He says : 



" It is inconceivable that inanimate brute matter can, without 

 the mediation of something else which is not material, operate 

 upon and affect other matter, without mutual contact, as it 

 must do if gravitation, in the sense of Epicurus, be essential and 

 inherent to it. This is the reason why I desire that you would 

 not ascribe an innate gravity to me. That gravity should be 

 innate, inherent, and essential to matter, so that one body may 

 act upon another at a distance, through a vacuum, without the 

 mediation of anything else by and through which their action 

 may be conveyed from one to another is to me so great an 

 absurdity that I believe no man, who has in philosophical matters 

 competent faculty of thinking, can ever fall into. Gravity must 

 be caused by an agent acting constantly according to certain 

 laws ; but whether this agent be material or immaterial, I 

 have left to the consideration of my readers." 



The riddle of Newton's time remains a riddle to our own. 

 In some regards the mystery has deepened rather than cleared. 

 One of the greatest works of the last century was undoubtedly 

 the establishment of the idea of the- conservation of energy ; 

 but gravitation is a standing negation of such a concept. So 

 far as we can see now, it represents an inexhaustible supply of 

 energy with no corresponding dissipation. It is true that so 

 far as our earthly concerns go we are not able to utilise gravita- 

 tion for the establishment of perpetual motion ; but possibly 

 the universe does. According to our present ideas it is the im- 

 pact of suns, 1 doubtless for the most part cold suns, combined 

 with the heat of attraction, which gives rise to the incandescence 

 of the stars. When this initial store of energy has been dis- 

 sipated and the suns become cold again, they have merely to 

 come into a second collision that the cycle may be resumed. 

 So far as we can now see this might go on for ever. The 

 universe itself might realise that perpetuity of utilisable force 

 or energy which is denied to man. 



1 James Croll, Stellar Evolution. 



