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



racter of motion, it may be observed, is the first important 

 condition required by Le Sage's theory — which condition 

 therefore follows as a rigid dynamical fact, not as an arbitrary 

 postulate, as he made it. This motion of the particles uni- 

 formly or eijually towards all directions is not a mere chance 

 fact, but a rigid adjustment, of such a character that when 

 by any artificial means this mode of motion of the particles is 

 disturbed they will automatically, of themselves, return back 

 to this regular form of motion {i. e. so that an equal number 

 of particles are moving in all directions'). The other condi- 

 tions put forward as postulates by Le Sage, viz. that the den- 

 sity of the streams of particles should be the sayne in all parts, 

 and the mean velocity the same in all parts, are equally neces- 

 sary results following from the kinetic theory of gases — not, 

 therefore, postulates at all. 



G. The only further condition required is that the mean 

 length of path of the particles, before being intercepted by 

 collision with each other, should be great enough to produce 

 the effects of gravity — i. e. so that the particles of the medium 

 may act as streams upon masses immersed in the medium, or 

 may stream past two opposed masses, which by their mutual 

 screening or sheltering action produce the observed effects of 

 gravity. The mean length of path of a particle depends (as 

 is known) upon its size. One size of particle is not a j^riori 

 more likely than another. By simply, therefore, making the 

 particles small enough, any mean path, however great, may 

 be attained. We thus observe that all the arbitrary postulates 

 of Le Sage's theory, together with all the eftects of gravity, 

 naturally and inevitably follow from the simple admission of 

 the existence of matter in space whose normal state is a state 

 of motion, or the existence of a medium in space constituted 

 according to the kinetic theory of gases. In this way the 

 mode in which the motion of the streams of particles through 

 each other is naturally kept up in a state of dynamical equili- 

 brium, is quite easily explained. 



7. Here we do not want an indefinite waste of matter, or 

 an indefinite supply of matter from ultramundane space (as 

 Le Sage imagined) to produce gravity, but gravity is produced 

 by matter or a medium which as a ichole is stationary, and 

 whose internal motion is kept up and perfectly naturally main- 

 tained by the rigid laws of dynamics. The difficulty of the 

 collisions is here completely got over ; for the collisions of the 

 particles among each other, so far (as Le Sage supposed) 



Kiuetic Theory," publislied in the 'Philosophical Magazine' for June 

 1877, without bein^- aware at that time of Professor Maxwell's result. 



