in Causal Research. 363 



planets for instance) through its substance. A molecular 

 medium with its constituent molecules at rest, or in fixed 

 positions (like a solid), could not conceivably be capable of 

 satisfying this condition of free passage. Hence we must 

 infer that the molecules (or atoms) of aether in their normal 

 state are perfectly free to move past each other without ob- 

 struction; for on no other condition could the aether yield 

 freely to a passing body. Next, we must infer that the atoms 

 of aether are in motion in their normal state. For, first, this 

 inference is necessary on the ground that a medium with atoms 

 at rest, and with no power of acting on each other at a distance, 

 would have no positive properties at all. Secondly, another 

 independent reason for this inference is that, if the atoms were 

 at rest, they would unavoidably be put in motion by a body 

 passing through the aether, which would entail some appreciable 

 resistance — which is contrary to observation. The only 

 means of avoiding the resistance to passage is for the atoms 

 of aether to be already in motion, whereby the transference of 

 motion by the moving body to the aether atoms in front may 

 be (sensibly) compensated for by the transference of motion 

 (conversely) from the aether atoms in rear, to the body. If 

 on the other hand, the aether atoms in rear of the body were 

 not already in motion, they could not of course transfer any 

 motion to the passing body. Indeed it has been proved 

 mathematically with perfect definiteness, that the greater the 

 velocity of motion of a system of atoms, the less is the resis- 

 tance opposed by the medium (formed of these atoms) to a 

 body passing through it ; so that by an adequate velocity for 

 the atoms, the resistance will be rendered inappreciable. This, 

 therefore, affords a complete means of explaining the observed 

 absence of appreciable resistance to bodies passing through 

 the aether. Next, considering therefore the aether as consisting 

 of atoms in free motion among each other, the question arises 

 as to what is the character of the motion. It will be evident 

 that atoms in free motion, and incapable of acting on each 

 other at a distance, can only move in straight lines. Hence 

 the inference that the aether is in principle a gas (constituted 

 according to the kinetic theory) would seem to suggest itself 

 at the very outset as an apparently inevitable deduction. No 

 postulates whatever need be resorted to as to the general 

 directions of the motions of the aether atoms among them- 

 selves, as that is all automatic, and dependent on principles 

 already investigated in connexion with the kinetic theory of 



tases. The store of enclosed energy, the pressure, the density 

 c, are then all connected by rigid dynamical relations, not 

 in any way permitting of arbitrary speculation. As regards 



2D2 



