122 



DISSERTATION SECOND. 



[fart k, 



which he thought that it was perhaps to be ascribed. " Is 

 not this medium (the ether) much rarer within the dense 

 bodies of the sun, stars, and planets, than in the empty 

 celestial spaces between them ? And, in passing from them 

 to great distances, does it not grow denser and denser 

 perpetually, and thereby cause the gravity of those great 

 bodies to one another, every body endeavouring to go from 

 the denser parts of the medium to the rarer ?" ' 



Notwithstanding the highest respect for the author of 

 these conjectures, I cannot find any thing like a satisfac- 

 tory explanation of gravity in the existence of this elastic 

 ether. It is very true that an elastic fluid, of which the 

 density followed the inverse ratio of the distance from a 

 given point, would urge the bodies immersed in it, and 

 impervious to it, toward that point, with forces inversely as 

 the squares of the distances from it ; but what could main- 

 tain an elastic fluid in this condition, or with its density 

 varying according to this law, is a thing as inexplicable 

 as the gravity which it was meant to explain. The nature 

 of an elastic fluid must be, in the absence of all inequality 

 of pressure, to become everywhere of the same density. 

 If the causes that produce so marked and so general a 

 deviation from this rule be not assigned, we can only be 

 said to have substituted one difficulty for another. 



A different view of the matter was taken by some of 

 the disciples and friends of Newton, but which certainly 

 did not lead to any thing more satisfactory. That philo- 

 sopher himself had always expressed his decided opinion 2 



1 Optics, Query 21, at the end of the Third Book. 



2 The passages quoted sufficiently prove that Newton did 

 not consider gravity as a property inherent in matter. The fol- 

 lowing passage in one of his Letters to Dr. Bentley is still more 

 explicit : " It is inconceivable that inanimate brute matter 

 should, without the mediation of something else, which is nofc 



