KINETIC THEORIES OF GRAVITATION. 261 



a single essence tor matter, and the molecules of ordinary matter must 

 appear to ns as aggregates of setberial atoms."* 

 In his concluding essay, M. Saigey thus expounds the aetherial action: 

 " Let us imagine the aether uniformly diffused throughout space. Its 

 atoms endowed with motions of progression and rotation, strike each 

 other in the manner mentioned. Let us now suppose that at some point 

 within the medium there is a special and disturbing cause, as for ex- 

 ample, a molecule having weight [inertia ?] and itself endowed with a 

 vibratory motion. The disturbance goes on extending throughout the 

 aetherial mass, and by reason of the nature of this medium is propa- 

 gated in all directions. The atoms nearest to the heavy molecule will 

 receive violent shocks ; they will be powerfully urged and their ranks 

 will grow thin in the neighborhood of the center of disturbance, and the 

 layer contiguous to the molecule will become less dense than the rest 

 of the medium. The motor action continuing, this same effect becomes 

 propagated from layer to layer throughout space. As a final result, 

 the aether becomes arranged around the center of disturbance in concen- 

 tric layers, the first of which and nearest to the molecule will be least 

 dense, and they will go on indefinitely increasing in density. This con- 

 dition of things might be easily represented and tiie figure traced : the 

 molecule at the center, around it spheres of atoms, wide apart at first, 

 then nearer and nearer to each other. Let us remark, in passing, that 

 the difference in density of contiguous layers, like all effects which are 

 propagated by concentric spheres, is inversely proportional to the sur- 

 face of these spheres, that is to the square of their radii. [?] This es- 

 tablished, suppose a second molecule to be situated at any point of this 

 system. It will encounter on the side toward the first molecule, layers 

 of aether less dense than upon the opposite side : pressed upon by the 

 aether in all directions, it will however receive fewer shocks on the side 

 toward the first molecule, and it will consequently tend to approach it. 

 Such would seem to be the cause of gravity." t The law of inverse 

 squares does not appear to be well made out, although this is a point 

 to which all kinetic theorists make ostentatious reference. The density 

 of the aether, instead of following this ratio, should ex hypothesis 

 follow directly the opposite ratio ; or rather the ratio of increase directly 

 as the distance ; — which is perhaps what was intended. Of the other 

 five conditions it is quite unnecessary to speak. 



Ceoll. 1867. 



In a communication to the Philosophical Magazine, in 1867, by James 

 Croll, of Edinburgh, " On certain Hypothetical Elements in the Theory 

 of Gravitation," the author revived the difficulties which had been felt 

 and proclaimed by Faraday. He says : " It was demonstrated by New- 

 ton, and has been proved by general observation and experience, that 



* lit nit de Deux Maudes, 1st November, 1866, vol. lxvi, pp. 148, 149. 

 t Revue dc Deux Maudes, 15th December, 1866, vol. lxvi, pp. 922, 923. 



