.1821.] Phcenomena of Heat ^ Gases, Gravitation, S,'c, 409 



ing force of the whole central body, and x the distance between 

 the centres of the atom and compound body. And since this is 

 the law of gravitation of a single atom, it follows, by prop. 75, 

 lib. ] , Principia, if a number of them were equally distributed 

 throughout any spherical space, so that they could be individually 

 acted on by the fluid medium, the impelling force on the whole 



A. b 



body would be as —^ ; in which x is the central distance of the 



two bodies, and h the quantity of matter in the attracted body. 

 For the same reason if B be the mass of the central body, and 

 A' the whole agitating force of the other, its attraction on the 



central body will be as '—5-. But the whole agitating force of a 



homogeneous spherical body, other things being the same, is as 



its temperature T, its numeratom N, and its volume V conjointly; 



therefore, T being the temperature of the body B, its absolute 



T \ V 6 

 attractive force on b will be as — — — , and its accelerative force 



T N V 



as -—7—. And t, Uj v, representing the like things of the other 



body by its absolute attractive force on B will be as - — — , and 

 its accelerative as -4-. 



Now Newton has shown, p. 242, of his Optics, bodies to be 

 so rare that water has at least 40 times more pores than solid 

 parts ; and Biot, in his Traite de Physique, tom. iv. pp. 124, 

 125, has carried the idea so far, as even to suppose the solar 

 system itself to be but one great particle relatively to other 

 larger and rarer systems. Without inquiring into the merits and 

 probability of this bold idea, there is every reason to believe 

 with him, that " il se pourrait meme qui dans les corps qui nous 

 paraissent les plus denses, la capacite des interstices surpassat 

 plusieurs milliers des fois le volume des particules materielles ;" 

 or, at least, that the densest bodies we know of do indeed con- 

 tain vastly more pores than solid parts. Therefore, it follows, 

 that every one of the particles of any compound body might 

 agitate and produce those affections 1 have imagined on the 

 ethereal fluid, very nearly the same as if it was alone and unsur- 

 rounded by the other particles ; and conversely, the particles of 

 any compound body might be individually affected by the agita- 

 tions of the medium, almost the same as if they were alone. 



Before I proceed further, it is necessary to anticipate an 

 objection that at the outset might be made to this theory of gra- 

 vitation. The force of gravitation that has been here determined 

 is on the supposition that the impelled body is in a state of 

 , Cjuiescence ; and it might, therefore, be conceived, that the 

 attraction would be less on a body moving towards the central 

 body, and greater on one moving from it, which is contrary to 



