794 THE POPULAR SCIENCE MONTHLY. 



Thirdly, the ether has been conceived to be the ordinary elastic 

 gases or atmospheres freely expanded into space. But these have no 

 co-efficient of elasticity sufficient to give them such expansion, and 

 they would be liable to condensation and compression by their own 

 gravity about the planets, which would cause a rise of temperature 

 and dissipation of energy which would rob the ether of its permanent 

 character. Besides, Maxwell has shown that our atmosphere expanded 

 into space would be far too rare in the interplanetary spaces to satisfy 

 the required conditions ; nor is there any molecular velocity at all ade- 

 quate to the propagation of wave-energy with the velocities observed, 

 as will be shown further on. 



This brings us to the fourth concept, which is that of a pure pri- 

 mordial gaseous plenum, of sufficiently high tension, and in the condi- 

 tion assumed by gases in a rarefied receiver, where the mean path is so 

 long in proportion to the mean distance that a symmetrical movement 

 arranges itself, according to the law first pointed out by Maxwell as a 

 corollary from the equilibrium of pressure observed in confined gases, 

 and the performance of gases in a rarefied space first observed by 

 Crookes, that particles in free collision in sj)ace tend constantly to re- 

 arrange their motions automatically so as to move uniformly in all 

 directions in radial lines from every point. With the gases experi- 

 mented with the mean path at normal pressure and density is very 

 short (only about -g^oWo f an i ncn > when the molecules have a mean 

 distance from each other of -fo^irFFo f an inch), but, at the extreme of 

 rarefaction which we are able to effect (about -j ooooo ir ^ an atmos- 

 phere, when the distance of the molecules is still only t, ^ 6o of an 

 inch), the mean path rises to about four inches ; and C. T. Preston has 

 shown ("Nature," vol. xxiii, p. 463), that could we carry the exhaus- 

 tion to the third power of that obtainable, so that the distance of the 

 molecules apart should be so much as one seventh of an inch, the mean 

 path would be raised to 60,000,000 miles, since it increases in the trip- 

 licate ratio with the distance. But with the ether no such rarity need 

 be postulated, since mean free path is but a question of size of mole- 

 cule, and in comparison with the hydrogen-molecule the size of the 

 particle can only be infinitesimal. It is clear, however, that, with the 

 enormous velocity due to the particle, all the effect of continuity would 

 be produced, so far as vision is concerned, by a mean distance apart, 

 not merely of one seventh of an inch, but of many miles. 



We may therefore assign to the ether any required free path, and 

 any necessary density, tension, and velocity, all of these latter being 

 imperceptible to molecular structures which float in and are permeated 

 by it. The motions also of molecules in a gas so constituted would be 

 practically as unaffected as in free space, since it is demonstrable that 

 the resistance to motion offered by a medium such as the hypothesis 

 calls for would be in the ratio of the motion of the moving mass to 

 that of the particle of the medium, which in the case supposed would 



