122 Mr. S. T. Preston on the Application of the 



directions and coming from indefinite distances (or "ultra- 

 mundane " particles, as lie termed them). We do not assume 

 that the particles producing gravity in -the visible universe 

 converge towards it in streams from ultramundane space. On 

 the contrary, we assume that the matter producing gravity 

 within the confines of the visible universe is as a whole at 

 rest ; or we regard the medium producing gravity simply as 

 a gas. This gas differs from an ordinary gas only in the mul- 

 tiplicity of its particles, their excessive minuteness, and (con- 

 sequently) extremely long free path. It is a direct consequence 

 of the kinetic theory of gases that, within the range of free 

 path of the particles of this gas, the particles move in precisely 

 the right way to produce gravity ; i. e. all the assumptions that 

 Le Sage made arhitrarily as regards the motion of his streams, 

 take place as inevitable necessities within the range of free path 

 of the particles of a gas. The motion of the particles (in such 

 a way as to produce gravity) is automatically kept up by a 

 process of self-adjustment ; i. e. gravity is the inevitable result 

 of the existence of a medium in space constituted according 

 to the kinetic theory of gases. It has been mathematically 

 proved that the particles of a gas, within the range of free path, 

 move uniformly or equally towards all directions. This special 

 character of motion is automatically kept up under the influ- 

 ence of the collisions ; or, however each particle (by itself) 

 may change its course, this general character of motion is 

 rigidly kept up, and is required to satisfy the condition of 

 equal pressure in all directions. But this motion of particles 

 uniformly or equally in all directions is precisely what is re- 

 quired for gravity. 



11. The only further condition necessary is, that the range 

 of free path of the particles should be great enough, so that 

 (approximately) uninterrupted streams of particles move 

 through the full range through which gravity has been ob- 

 served to act. This length of free path (by any given number 

 of particles in unit of volume) may be increased to any extent, 

 simply by reducing the size of the particles. Taking, there- 

 fore, the visible universe as a whole, we have no streams of 

 particles, but simply a gas at rest. The streams only exist 

 within the range of free path of the particles, or within the 

 range of gravity. We may compare the medium filling the 

 visible universe to the air of a room, in which there are no 

 streams, but the air is as a whole at rest. Contract the room 

 (in imagination) up to the range of free path of the molecules 

 of air, and we have streams of molecules sweeping in all direc- 

 tions through the room. The space in which we observe 

 gravity may be compared to this contracted room, within 



