applicable to Le Sage's Theorif of Gracitation. 207 



from the dynamical principles connected with the kinetic 

 theory of gases, or that Le Sage unconsciously enunciated the 

 inevitable principles of the kinetic theory — that, in short, all 

 the conditions laid down in Le Sage's theory are perfectly 

 satisfied by a gas whose particles are very minute, and conse- 

 quently the mean length of path of whose particles is very 

 great. In other words, it may be stated as a general propo- 

 sition, that when two bodies are immersed in a gas at a less 

 distance apart than the mean length of path of the particles of 

 the gas, the two bodies will tend to be urged together. Thus 

 all the arbitrary conditions of Le Sage's theory (and all the 

 facts of gravity) would follow as inevitable deductions from 

 the simple fundamental admission of the existence of matter 

 in space, whose normal state is a state of motion. 



3. The part of Le Sage's theory which most calls for expla- 

 nation, and which he makes no attempt to explain, is (even if 

 we allow as a purely arbitrary fact that the motion of his par- 

 ticles took place at one time uniformly or equally towards all 

 directions) how this uniformity of motion of the particles 

 could be kept up under the continual changes of direction 

 resulting from the collisionsof the particles against themselves 

 and mundane matter. IN^ow it has been proved mathemati- 

 cally by Professor Maxwell, in connexion with the kinetic 

 theory of gases, that a self-acting adjustment goes on among 

 a system of bodies or particles in free collision, such that the 

 particles are caused to move equally towards all directions, 

 this being the condition requisite to produce equilibrium of 

 pressure. The method of calculating the ^rate of the above 

 self-acting adjustment for any case is given in the Philoso- 

 phical Transactions for 1866. This adjustment is of such a 

 rigid character that, if by any artificial means the motions of 

 the particles Avere interfered with and made to take place irre- 

 gularly {i. e. unequally in different directions), the particles 

 when left to themselves would in a very short time automati- 

 cally return back to the above regula7' form of motion, i. e. so 

 that an equal number of particles are moving in any two op- 

 posite directions. Thus it follows that when a system of par- 

 ticles are left in space with nothing to guide them, they will, 

 by the rigid principles of dynamics adjust their motions in 

 such a way as to be competent to produce the effects of gravity. 

 In other words, the movement of streams of particles with 

 perfect uniformity at all angles (which Le Sage assumed as a 

 mere arbitrary postulate) is found to be the necessary conse- 

 quence of dynamical principles; or the particles themselves 

 adjust their motions so as to move in uniform streams in all 

 directions ; and, further, when any disturbance of the unifor- 



