6\6 ~ LECTUIiE XLIX. 



the empty celestial spaces between them, and to grow more and more dense 

 at greater distances from them, so that all these bodies are naturally forced 

 towards each other by the excess of pressure. 



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 every where such as to produce tlie appearance of an attraction vary- 

 ing like that of gravitation. Such an ethereal medium would therefore have 

 the advantage of simplicity, in the original law of its action, since the re- 

 pulsive 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 modifi- 

 cations 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, there- 

 fore, be no occasion to apprehend any difficulty from a retardation of the 

 celestial motions ; the ultimate impenetrable particles of matter being perhaps 

 scattered a? 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 con- 

 tinuous surface : and there seems no reason to doubt the possibility of the 

 propagation of an undulation through the Newtonian medium with the actual 

 velocity of light. It must be remembered that the difference of its pressure 

 is not to be estimated from the actual bulk of the earth or any other planet 

 alone, but from the effect of the sphere of repulsion of Avhich that planet is 

 the centre; and we may then deduce the force of gravitation from a medium 

 of no very enormous elasticity. 



We shall hereafter find that a similar (pombination of a simple pressure with 

 a variable repulsion is also observable in the force of cohesion ; and suppos- 

 ing two particles of matter, floating in such an elastic medium, capable of pro- 

 ducing gravitation, to approach each other, their mutual attraction would at 

 once be changed from gravitation to cohesion, upon the exclusion of the 

 portion of the medium intervening between them. This supposition is. 



