THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
221 
transfer by ordinary radiation at small distances, as Fourier imagined ; and it would 
not appear why convection by a g’as, even when highly rarefied, is so much more 
efficient in the transfer of heat than radiation. 
12. The result obtained by Ramsay and Young, and others, that all over the gas- 
liquid range the characteristic equations of the substances on which they experimented 
proved to be very approximately of the form jp = aT -1- 6, where cl and h are functions 
of the density alone, also supplies corroboration to this view. Expressing the 
increment of energy per unit mass dE = Mdv + /cdT, we have for the increment 
of heat supplied clK = dE + pdv ; and the fact that dE and dH/T are perfect 
differentials shows immediately that M is equal to — 6 and k is independent of v, so 
that the total (non-constitutive) energy per unit mass consists of two independent 
parts, an energy of expansion and an energy of heating.^' The latter part is the 
thermal energy of the individual molecules ; it is a function of their mean states and 
velocities alone, and constitutes almost all the energy in the gaseous state. The 
former part is the energy of mutual actions between the molecules ; it is neg’ative 
and bears a considerable ratio to the whole thermal energy in the liquid state, in the 
case of substances with high latent heats of evaporation ; for all gases except 
hydrogen, inasmuch as they are cooled by transpiration through a porous plug, h is 
negative at ordinary densities. § 62, infra. 
There would be no warrant for a view that the forces of chemical affinity fall off and 
finally vanish as the ultimate zero of temperature is aiiproached. The translatory 
motions of the molecules would diminish without limit, and therefore also the 
opportunities for reaction between them, so that many chemical changes would cease 
to take place for the same reason that a fire ceases to burn when the supply of air is 
insufficient, or coal gas ceases to explode when too much diluted with air: but the 
energies of affinity exist all the time in probably undiminished strength, while the 
forces of cohesion are modified by the fall of temperature but not necessarily in the 
direction of extinction. 
The Equations of the TEtheveal Field, with j\Ioving Matter: various applications: 
rnfluence of Motion through the FEther on the Dimensions of Bodies. 
13. Let {u, V, iv) represent the total circuital current, and {u', v', w') the conducted 
part of it, which will be taken to include the current {uq, Vq, Wq) of migration of the 
free electric charge as this is in all cases very small in comparison ; let {f', g', h') 
denote the electric polarization of the material, and [f g, li) the aethereal elastic 
displacement, so that the total circuital displacement of Maxwell’s theory is their 
sum {f , g , h') let the space of reference be fixed with respect to the stagnant 
aether, and [p, q, r) be the velocity with which the matter situated at the jioint 
{x, y, z) is moving, and let hjdt represent djdtpdflxqd/dyrdflz let 
Of. G. F. IitzGeeald, ‘Roy. Soc. Pi’oc.’ 42, 1887 : cf, also Clausius’ early ideas on ‘ disgregation.’ 
