applicable to Le Sage's Theovij of Gravitation. 209 



of bodies or particles left to themselves, with nothing to guide 

 them but their own collisions (which might well be regarded 

 as fulfilling all the essentials of a chaos), produces and main- 

 tains the most rigid system of order, such that the number of 

 particles contained in unit volume of the system (taken any- 

 where) is equal, the mean velocity equal in all parts, the mean 

 distance of the particles the same in all parts, and the particles 

 are moving uniformly towards all directions in all parts. Such 

 is the result produced by pure dynamics. In fact it may be 

 said that leavinij the bodies to themselves constitutes the most 

 perfect system of control, for any interference whatever would 

 disturb the regularity of the motions. This regularity of 

 movement is not only naturally continued, but forcibly and 

 automatically maintained against any disturbance, — such that 

 if it were imagined that a system of bodies were purposely 

 put in motion in the most chaotic manner possible, the motion 

 would of itself in a short time become regular, or the whole 

 would become a system of order and uniformity. 



7. Clausius, as is known, has investigated a relation between 

 the mean length of path of the particles of a gas and the dia- 

 meter of the particles. From this investigation it follows that 

 the mean length of path of the particle of a gas {i. e. the ave- 

 rage distance which the particle moves before encountering 

 another particle) increases in proportion as the square of the 

 diameter of the particle diminishes. Thus by making the 

 particle small enough, its mean length of path may be in- 

 creased to any extent. No objection, evidently, can be made 

 to this, for a prioi^i one size of particle is just as likely as an- 

 other. This minute size would render it possible for the par- 

 ticle to possess a high velocity without producing thereby 

 disturbance or displacement among the molecules of ordinary 

 matter ; and this high velocity is necessary to accord with the 

 observed facts of gravity. One velocity cannot be said a 

 prio7'i to be more likely than another. We must just be 

 guided by the teaching of facts as to what the velocity is*. 



* I have shown (Phil. Mag. June 1877) that a physical relation exists 

 between the velocity of the particles of a medium constituted according 

 to the kinetic theory and the velocity of propagation of a wave in the 

 medium. Professor Maxwell has calculated (as given in postscript to 



the paper) the numerical value of this relation at -o". Thus it appears 



that if the velocity of propagation of a wave in any medium constituted 

 according to the kinetic theory can be measured, then the velocity of the 



particles of the medium is given by dividing this velocity by -q- . So, 



for example, the velocity of the molecules of air is given by dividing the 



Fhll. Mag. S. 5. Vol 4. No. 24. Sept. 1877. P 



