Kinetic Theory of Gases to Gravitation. 121 



we have something to explain here. What, for example, 

 would be thought of any one saying that the intensity of light 

 varied as the square of the distance because it was its u pro- 

 perty " to do so. The worst of this want of appreciation of 

 the logical necessity for an explanation is, that the attention 

 is called away and the inquiring faculties deadened, and thus 

 these grand problems secure a share of attention which is 

 utterly insignificant compared with that devoted to those of 

 minor importance. 



9. To prevent any misconception, we would remark here 

 that the theory we have to suggest as an explanation of gravity 

 is different in several essential points from that of Le Sage. 

 The theory of Le Sage was dynamically defective in several 

 essential points (probably owing to the comparatively small 

 advance made in dynamics at his time). His assumption of 

 continuous streams of particles coming from a number of dif- 

 ferent directions equiangularly scattered in space, the particles 

 being supposed to come from indefinite distances (" ultramun- 

 dane " particles), must appear evidently somewhat fantastic ; 

 for it appears inconceivable how the motion of such a system 

 of streams of particles coming from ultramundane space 

 should be kept up without confusion ensuing, owing to the 

 mutual collisions of the particles of the streams which cross 

 each other in all directions, if (as he assumed) each separate 

 stream were to move continuously in one direction. For, how- 

 ever much the collisions might be reduced by reducing the 

 size of the particles, they must occur in a long course of time, 

 especially considering the high velocity at which it is neces- 

 sary to assume the streams to move. Moreover the great ob- 

 jection to this view is that it involves, for the maintenance of 

 gravity in the visible universe, a continual supply of matter 

 from ultramundane space. This objection Le Sage distinctly 

 recognized and could not surmount. The real merit of his 

 theory was his fundamental idea that " gravity/'' or the ten- 

 dency to approach of two masses, was due to the one mass 

 sheltering or screening the other from the action of the streams 

 of particles in which the two masses of matter were immersed — 

 so that the remote sides of the two masses (where there is no 

 shelter) are struck by a greater number of particles than the 

 near sides (where there is shelter), and thus the two masses are 

 urged together. The rest of his assumptions are in the nature 

 of postulates, some of them unrealizable. He had little know- 

 ledge to draw upon at his time. 



10. The points we have to bring forward are briefly as fol- 

 lows. We do not assume, as Le Sage did, the existence of 

 streams of particles flowing as continuous currents in assigned 



