208 Mr. S. T. Preston on some Dynamical Conditions 



mity of the motion of the particles takes place due to their 

 collisions with mundane matter, the particles themselves read- 

 just the uniformity of motion. 



4. Le Sage imagined that the collisions of the particles dis- 

 turbed permanently the uniformity of their motions, and there- 

 fore supposed these collisions to take place only at intervals 

 of time very remote from each other. Thus he assumes 

 " . . . . that not more than one out of every hundred of the 

 particles meets another during several thousands of years ; so 

 that the uniformity of their motions is scarcely ever disturbed 

 sensibly." We now know that, so far from the collisions of 

 the particles among themselves disturbing the uniformity of 

 their motions, this is the very cause which corrects and main- 

 tains the uniformity of motion, or preserves the uniformity 

 of motion in opposition to external disturbing causes. The 

 assumption, therefore, of the above enormous interval of time 

 between the collisions of the particles, though admissible, is 

 by no means necessary. The only necessary condition is that 

 the path of the particle should be a certain length, not that a 

 certain time should be occupied in traversing it. The time 

 taken by the particle in traversing its path depends on its 

 velocity ; and this time might therefore be small, provided, 

 under the conditions of the case, the velocity of the particle 

 were high. Le Sage imagined that the collisions were detri- 

 mental, not only in destroying the uniformity of the motion 

 of the particles, but also in destroying vis viva ; and he there- 

 fore supposed the collisions to take place as seldom as possible. 

 This belief in the destruction of vis viva at collision was uni- 

 versal at the time of Le Sage ; and he therefore assumed that 

 the gravific particles would finally come to rest, and gravity 

 cease to exist. We now know that this is an error, and that 

 motion is as naturally maintained among a system of particles 

 as rest. Thus the one thing requiring to be admitted to ac- 

 count for all the effects of gravity is, that the universe is im- 

 mersed in a gas the mean length of path of whose particles is 

 great. 



5. The other assumptions or postulates of Le Sage in con- 

 nexion with his theory. Adz. equal density in all parts of the 

 streams of moving particles, equal mean velocity in all parts, 

 follow no less as automatic consequences from the recent dy- 

 namical investigations connected with the kinetic theory of 

 gases. Thus the conditions of Le Sage's theory become con- 

 verted from a series of arbitrary assumptions or postulates, to 

 a series of deductions following from the rigid principles of 

 dynamics. 



6. It forms a truly wonderful fact to consider, that a system 



