.1821.] Phcenotnena of lieatj Gases, Gravitation, S^c, 411 



expression for the law of gravitation towards any spherical body, 

 the gravitating forces, or the weights of any two spherical 

 bodies towards any other body at equal distances, are as the 

 quantities of matter in the attracted bodies. This law has been 

 proved experimentally by Sir Isaac Newton .; but though this be 

 true, the converse case does not, according to our principles,, 

 hold good ; namely, that the attractive forces of bodies are 

 directly proportional to their quantities of matter. Our princi- 

 ples do not, therefore, corroborate Newton's third law of motion, 

 respecting the equality of action and reaction in attracting 

 forces ; for by our theory a body might, by the agency of the 

 fluid medium, be impelled towards another without any recipro- 

 cal action, which is by no means surprising, if we consider 

 attraction not to be an inherent or essential property of matter^ 

 but merely the action of a third body. When, however, we take 

 homogeneous^ bodies of the same temperature, the equality of 

 action and reaction seems to hold true ; but in different bodies, 

 differently composed, or the same body at different temperatures, 

 a difference should be observed in their attractive forces propor- 

 tional to the greatness of the dissimilarity. Other things being 

 alike, the attractive forces should be proportional to the temper- 

 atures ; so that the attraction of all bodies at the extreme tem- 

 peratures of the fluidity of water, should have a ratio of about 

 6 to 7. Unfortunately the difficulties attending the corroborat- 

 ing by experiment of this part of our formula, much exceed 

 those of the other ; nor have I as yet devised any method of 

 satisfying myself respecting it in the manner that I could wish.^l* 

 But though great difficulties lie in the way of a numerical proof, 

 there are some pheenomena which seem pretty decisively to 

 sanction the general result of our formula ; namely, that an 

 increase of temperature produces an augmentation of attraction. 

 Thus Euler has found that the focus of a convex lens is con- 

 tracted by a greater and extended by a less temperature ; from 

 which it plainly appears, that the refracting force is increased in 

 the one and diminished in the other case. M.Laplace in the Meca- 

 nique Celeste has calculated the annual equation of the moon at 

 IV 22"^ while, by the best tables, it is only about 11' 84.'^ 

 The difference between these quantities appears too much for 

 the error of either observation or calculation, and therefore indi- 

 cates an increased attraction in the perihelion, and a decreased 

 in the aphehon earth. By all the calculations that have hitherto 

 been made, a regular diminution in the attractive forces of the 

 planets, reckoning from the sun, has been observed, which 

 shows that the colder planets have less attraction than the 



• By homogeneous solid or fluid bodies is here meant those all of whose parts lure 

 ^milar, both with respect to their constituent elements and the association of them. 



-|- Since writing this, I have contrived a method, by a modification of Mr. Cavendish's 

 apparatus, but have not yet met with a proper jdace and opportunity for putting it fai 

 practice. . 



