Kinetic Theory of Gases to Gravitation. 123 



which streams of particles are sweeping through uniformly in 

 all directions, the uniform motion of the particles equally in all 

 directions (necessary -to produce gravity) being automatically 

 kept up under the influence of the mutual collisions, in a way 

 demonstrated to take place in the case of a gas. It should be 

 observed that this self-adjustment of their motion by the par- 

 ticles is not a mere result of chance, but a rigid adjustment of 

 such a character that, if the uniformity of the motion were 

 artificially disturbed, the particles when left to themselves 

 would immediately correct the irregularity. The above length 

 of free path, though great in one sense, becomes small and 

 suitable for a gas pervading the vast range of the visible 

 universe. Unlike Le Sage, we do not object to the collisions 

 of the particles among themselves ; for these collisions (in the 

 case of a medium constituted as a gas) maintain the uni- 

 formity of motion. We require no supply of matter to produce 

 gravity, and no supply of energy. The energy is self-con- 

 tained. It is simply the case of the normal motion of the 

 particles of a gas. Motion is as natural as rest. Nothing 

 surely could be more simple than these conditions. 



12. It might be said that this theory implies a limited range 

 to gravity. It may be extended to any desired range simply 

 by making the particles small, and consequently the free path 

 great. We venture to think that rather than that a theory 

 should be required to explain that the stars gravitate, a theory 

 should be required to explain that they do not* gravitate. 

 For surely the idea of an indefinitely extended universe all of 

 whose parts gravitated towards each other, would represent 

 dynamical conditions of ^stability on the most gigantic scale. 

 Imagine the incongruity of the idea of the whole universe 

 tending to agglomerate in one (perhaps infinite) mass. To 

 our mind no theory of gravity would be satisfactory that did 

 not explain away this. The kinetic theory gets over this 

 difficulty in a most complete manner, by allowing gravity to 

 take place within a conformable range, without extending it 

 to indefinite distances and thereby involving conditions of 

 instability. 



13. As we have said, we do not shirk in the slightest degree 

 any criticism as regards this theory, but shall be glad to meet 

 it, knowing that, if true, it will stand a full examination ; and if 

 false, the sooner it is proved so the better. There is one other 

 point on which perhaps an objection might be raised. It 

 might be said, If a gas exists in space, how is it that we 

 do not detect its presence in experiments on the specific heat 

 of other gases, this gas being at the same time present ? or 



* Of course we do not refer to double stars, in close range. 



