210 KINETIC THEORIES OF GRAVITATION. 



surface; and there seems no reason to doubt the possibility of the propa- 

 gation of an undulation through the Newtonian medium with the actual 

 velocity of light. Jt must be remembered that the difference of its 

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

 planet alone, but from the effect of the sphere of repulsion of which 

 that planet is the center; and we may then deduce the force of gravi- 

 tation from a medium of no very enormous elasticity. 



" We shall hereafter find that a similar combination of a simple press- 

 ure with a variable repulsion is also observable in the force of cohesion ; 

 and supposing two particles of matter (floating in such an elastic me- 

 dium capable of producing gravitation) to approach each other, their 

 mutual attraction would at once be changed from gravitation to cohe- 

 sion upon the exclusion of the portion of the medium intervening be- 

 tween them. This supposition is however, directly opposite to that 

 which assigns to the elastic medium the power of passing freely through 

 all the interstices of the ultimate atoms of matter, since it could never 

 pass between two atoms cohering in this manner. We cannot therefore 

 at present attempt to assert the identity of the forces of gravitation and 

 cohesion so strongly as this theory would allow us to do if it could be 

 established." * 



In his succeeding lecture " On Cohesion," Dr. Young adds at its con- 

 clusion: " With respect to the ultimate agent by which the effects of 

 cohesion are produced, if it is allowable to seek for any other agent than 

 a fundamental property of matter, it has already been observed that 

 appearances extremely similar might be derived from the pressure of a 

 universal medium of great elasticity ; and we see some effects so nearly 

 resembling them, which are unquestionably produced by the pressure 

 of the atmosphere, that one can scarcely avoid suspecting that there 

 must be some analogy in the causes. Two plates of metal which cohere 

 enough to support each other in the open air will often separate in 



vacuum But all suppositions founded on these analogies 



must be considered as merely conjectural ; and our knowledge of every 

 thing which relates to the intimate constitution of matter, partly from 

 the intricacy of the subject, and partly for want of sufficient experi- 

 ments, is at present in a state of great uncertainty and imperfection."! 



Very curiously, this ingenious scheme of universal repulsion leaves no 

 room for that self-repulsion of matter exhibited in the phenomena o 

 elasticity. That Young did not regard these speculations as reposing 

 on a very firm basis is shown by his memoir "On the Theory of Light 

 and Colors," in which the fourth "hypothesis" assumes the aether to be 

 denser within transparent bodies, and for a small distance around them, 

 than in the spaces beyond such bodies. J 



"Lectures on Natural Philosophy. 1807, 2 vols, quarto. Leet. xlix, vol. i, pp. (516, 

 (JIT. 



t Loco citat. Lecture 50, p. 630. 



: Philosophical Transactions of the Royal Society, 1802, vol. xcii, p. 21, and Young's 

 Lectures on Natural Philosophy, vol. ii, p. 618. 



