224 KINETIC THEORIES OF GRAVITATION. 



ones, particularly than gases, the dense bodies of the planets being 

 heated by tlje solar rays as well as by the medium about them, ought 

 it aj)peared to me, to be hotter than this medium, and consequently 

 ought to produce the same effects on the medium as the sun, though 

 cot in so great a degree. Therefore if as Newton imagines, the parti- 

 cles of the planets be impelled toward the sun by the inequality of the 

 pressure on their further and nearer sides, the denser parts of the me- 

 dium pressing more forcibly than the rarer, the same reason will like- 

 wise hokl good why bodies should be impelled toward the planets an<l 

 other material parts of the system."* 



After speaking of the discouragement resulting from his unsuccessful 

 attempts at arriving at the mathematical laws of heat, he proceeds: 

 " Yet sometimes when my thoughts were involuntarily turned this way, 

 the idea that two inanimate bodies could act on each other at a distance 

 without some other means than that of a mere tendency or inclination 

 in them to approach, would appear so strongly unphilosophical, and the 

 apparent coincidence of several j)henomena with conclusions I had 

 drawn from my notions of gravitation so very seductive, that I could 

 not avoid thinking the views I had taken were tolerably correct ; and 

 that there was only wanting the direction of some happy idea, which 

 patient perseverence might possibly attain, to set the whole in a clear 

 and irrefragable light. Thus between hope and despair, between un- 

 ceasing attempts and mortifying failures, I continued until May, 181-1, 

 at which time my ideas of heat underwent a complete revolution. Pre- 

 vious to this time I had conceived heat to be the eflFect of an elastic 

 fluid, and on this supposition, had repeatedly attempted to reduce its 

 laws to mathematical calculation ; but uniform disappointment at length 

 induced me to give this hypothesis a careful investigation, by compar- 

 ing it with general and particular phenomena. The result of this in- 

 vestigation convinced me that heat could not be the consequence of an 

 elastic fluid. . . . After I had revolved the subject a few times in 

 my mind, it struck me that if gases instead of having their particles 

 endued with repulsive forces, subject to so curious a limitation as New- 

 ton proposed, were made up of particles or atoms mutually impinging 

 on one another and the sides of the vessel containing them, such a con- 

 stitution of a?riform bodies would not only be more simple than repulsive 

 powers, but as far as I could perceive, would be consistent with phe- 

 nomena in other respects, and would admit of an easy application of 

 the theory of heat by intestine motion. Such bodies I easily saw pos- 

 sessed several of the properties of gases; for instance, they would ex- 

 pand, and if the particles be vastly small, contract almost indefinitely; 

 their elastic force would increase by an increase of motion or tempera- 

 ture, and diminish by a diminution ; they would conceive heat rapidly, 

 and conduct it slowly ; would generate heat by sudden compression, 



* Annals of Philosophy, 1821, vol. xvii, cr of new series, vol. i, p. 276. 



