98 On the Kinetic Theory of Gases. 



enormous numbers which compose the bodies for which the 

 laws of motion have been experimentally proved. 



But when one of the most important purposes for which 

 the existence of the aether is required is the communication 

 to, or abstraction of, heat-energy from material particles, it 

 seems absurd to try to derive temperature-equilibrium from 

 molecular action. We know that by means of the aether, 

 bodies at a distance and wholly prevented from acting on 

 each other molecularly, come to exactly the same temperature- 

 equilibrium as the particles of a gas are expected to do by 

 intermolecular collisions, although the aether between them 

 is perfectly competent to settle their temperature-equilibrium 

 without any assistance from their collisions. Hence there is 

 every reason to suppose that it is by the molecules interacting 

 through the aether that the temperature-equilibrium is de- 

 termined. Hence taken by themselves the molecules do not 

 form a complete dynamical system, for we cannot consider the 

 aether (or any medium having kinetic properties) as merely 

 limiting the number of degrees of freedom of the molecules, 

 and thus replace it by (geometrical) equations of condition. 

 Indeed it is no harm to observe here that equations of con- 

 dition should not be employed in atomic theories, though 

 perhaps they may have a place in molecular theories. For an 

 equation of condition either indicates that the force between 

 two particles P x and P 2 is not a function of the distance be- 

 tween them (and is therefore inadmissible) or else it is 

 resolvable into f(r l2 )=0, and then it should be discarded, 

 and a distinct expression of the law of force it implies should 

 be substituted for it. 



It would be very interesting if Sir W. Thomson would 

 give some outline of the way in which the initial conditions 

 could be introduced in a demonstration such as he believes 

 possible, for it appears to me that the fundamental error in all 

 assumptions about permanent states is that they persistently 

 ignore the initial condition on which obviously the whole 

 theory must depend, if the system be reversible. It is interest- 

 ing, too, to remark that in the case of vortex motion (in which 

 one attends simply to the dynamical equations and does not 

 reason from preconceived physical ideas), Sir W. Thomson 

 " has found no indication of the dissipation of translational 

 velocities, or of vibrations of slow period, into those of 

 shriller and shriller vibrations." Of course I do not mean to 

 say that things do not actually come to a permanent state, 

 but only to assert that permanence is a consequence of the 

 interaction of molecules and wther and not of the interaction of 

 molecules among themselves. No doubt the mode in which 



