applicahle to Le Sage^s Theory of Gravitation. 371 



saiy that not less than a certain total of energy should be con- 

 tained in a given A^olume of the gravific medium, not that 

 thereby the energy of each particle should necessarily be great. 

 The energy of each particle (whose sum produces a given total 

 of energy) would evidently depend on the number of particles 

 in unit volume. Professor Maxwell assumes that it is ^' tole- 

 rably certain that N, the number of (gravific) corpuscles which 

 are at any one time w^ithin unit of volume, is small compared 

 with the value of N for the molecules of ordinary bodies." 

 Now we may ask. Is this certain or necessary ? for the whole 

 hinges upon this. If, on the contrary, the number of gravific 

 particles in unit volume were not restricted, then by adding 

 to the number of particles, and thus subdividing the total 

 energy among them, the energy of each particle might be 

 made indefinitely small. It might possibly be thought that 

 such a number of particles would be inconsistent with a long 

 free path. But if the subject be considered, it will be observed 

 that a free path of given adequate length may be obtained 

 with an indefinite number of particles, provided the particles 

 be minute — or that, no consequence how numerous the par- 

 ticles (and therefore how small the energy of each), an ad- 

 equate mean path can be got by reducing their size, their 

 velocity being augmented so as to keep the energy in unit of 

 volume constant. This high velocity of the particles may be 

 show^n on other grounds to be a likely condition ; for by this 

 means the whole medium, is rendered completely impalpable, 

 or its presence vanishes from the senses — the medium opposing 

 no measurable resistance to the passage of bodies through it. 

 Accordingly, as by a given amount of energy in unit volume 

 the energy of each particle is inversely as their number, so by 

 multiplying the particles the energy of each may be made 

 indefinitely small; and therefore the energy transferred to 

 the molecules of matter would be made indefinitely small, or 

 there would be no measurable rise of temperature at all. This, 

 I submit, removes the difficulty in question. 



14. It was pointed out by Le Sage that, in order to explain 

 gravity, it is necessary to assume that the gravific particles 

 rebound from the molecules of matter at a less velocity than 

 they strike. Since, after the average kinetic energy of a mo- 

 lecule of matter has become at least equal to that of a gravific 

 particle, no further transference of energy can take place from 

 the gravific medium to matter (i. e, of course in the case of 

 matter at rest), it is necessary therefore to explai]! the di- 

 minished velocity of rebound of the gravific particles. Sir 

 "William Thomson (Phil. Mag. May 1873) has pointed out that 

 this may be a natural consequence of a difference of elastic rigi- 



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