applicable to Le Sage's Theory of Gravitation, 373 



lation is again restored to its normal value by the collisions of 

 tlie particles among themselves. This, it may be observed, 

 is therefore a rigid dynamical fact, not an hypothesis. 



16. There is therefore no expenditure of energy or work 

 whatever in the maintenance of gravity, since the total amount 

 of energy in the particle is unaltered by collision. Also no 

 supply of energy to the gravitic particles is required, since a 

 state of motion is as natural as a state of rest. Further, no 

 supply or waste of matter is required for the maintenance of 

 gravity. Le Sage imagined that a continual supply of matter 

 from ultramundane space was necessary. He endeavours to 

 got over this incongruous idea by making the excuse '' that 

 nature makes frequently such Avaste " (page 108). This is 

 evidently no satisfactory excuse at all ; in fact Le Sage, with 

 the limited knowledge of his day, naturally could not get over 

 the difficulty of the collisions, or could not form an idea of 

 the conditions of equilibrium of streams of particles of matter 

 moving in the way he assigned. With our modern knowledge 

 we may deduce that the conditions of equilibrium of such 

 streams of particles are of a perfectly definite character, so as 

 to produce gravity as an inevitable fact. The gravilic medium, 

 therefore, within the bounds of the visible universe is as a whole 

 at rest ; and no supply of matter whatever is required. The 

 medium producing gravity is simply a medium constituted as 

 a gas according to the kinetic theory — but quite exceptional in 

 character as regards the extreme minuteness of its particles, their 

 extremely high velocity, and long mean path, the high velocity 

 rendering the medium completely impalpable, or its presence 

 imperceptible to the senses. It is evident that the presence 

 of such a medium could be only rendered directly palpable to 

 the senses by the i^esistance attendant on the motion of bodies 

 in it. Now it is a known dynamical fact that this resistance 

 diminishes as the velocity of the particles of the medium in- 

 creases ; hence with a given velocity no resistance whatever 

 will be felt, and therefore the presence of the medium must 

 elude detection. These deductions are therefore in perfect 

 harmony with the facts. The mean length of path of the par- 

 ticles of the medium, though great compared with that in the 

 case of ordinary gases, may be considered small in proportion 

 for a gaseous medium that pervades the area of the visible 

 universe. 



17. It would be a wrong idea to imagine that, because the 

 particles of the gravitic medium are relatively very close 

 compared with the molecules of ordinary substances, there- 

 fore the quantity of matter forming the gravific medium 

 must be relatively great. It is a mathematical fact that the 



