KINETIC THEORIES OF GRAVITATION. 209 



altogether with a medium which apparently must tend to retard the 

 planetary movements, and which he thought insufficient to account for 

 the ordinary behavior of the luminous ray. He concludes that as " there 

 is no evidence for its existence, therefore it ought to be rejected. And 

 if it be rejected, the hypotheses that light consists in pression, or motion, 

 propagated through such a medium, are rejected with it."* This appears 

 to have been the turning-point in the suspended balance of his judgment, 

 determining his choice betweeu the alternative conceptions of emission 

 and of undulation. 



Afterward, as if driven back from every assault to the only retreat, 

 which in earlier years he had stigmatized as "so great an absurdity" 

 that no competent thinker could " ever fall into it," he despairingly 

 asks: "Have not the small particles of bodies certain powers, virtues. 

 or forces, by which they act at a distance f .... What I call 

 'attraction' may be performed by impulse, or by some other means un- 

 known to me. I use that word here to signify only in general any force 

 by which bodies tend toward one another, whatsoever be the cause." t 

 And beyond this point, no human research has since been able to pene- 

 trate. 



This last and presumably deliberate judgment of Newton is a quarter 

 of a century later than the inconsiderate utterances of his third "Bent- 

 ley letter," which have been so eagerly seized upon by every speculative 

 writer intent on propounding new theories of the universe. 



The thoughtful philosopher Doctor Young, about a century later, 

 commenting on Newton's suggestion of an retherial medium — rarer 

 toward and within dense bodies, — with great ingenuity remarks: "The 

 effects of gravitation might be produced by a medium thus constituted, 

 if its particles were repelled by all material substances with a force 

 decreasing like other repulsive forces, simply as the distances increase. 

 Its density would then be everywhere such as to produce the appear- 

 ance of an attraction varying like that of gravitation. Such an aetherial 

 medium would therefore have the advantage of simplicity in the origi- 

 nal law of its action, since the repulsive force which is known to belong 

 to all matter would be sufficient, when thus modified, to account for the 

 principal phenomena of attraction. 



" It may be questioned whether a medium capable of producing the 

 effects of gravitation in this manner would also be equally susceptible of 

 those modifications which we have supposed to be necessary for the 

 transmission of light. In either case it must be supposed to pass 

 through the apparent substance of all material bodies with the most 

 perfect freedom, and there would therefore be no occasion to apprehend 

 any difficulty from a retardation of the celestial motions, the ultimate 

 impenetrable particles of matter being perhaps scattered as thinly 

 through its external form as the stars are scattered in a nebula, which 

 has still the distant appearance of a uniform light and of a continuous 



*Lococitat. Query 28. t Loco citat. Query 31. 



S 14 



