THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
219 
previous ones over the part of the surface that is common to the two elements ; and 
thus the uncompensated traction is passed on from element to element until finally 
the boundaiy of the material system is reached where it remains uncompensated and 
must be balanced extraneously. The outstanding irregular part of the aggregate 
mutual potential energy of the individual molecules, which cannot be included in a 
function of strain of the element of volume, cannot on that account take part in the 
transmission of mechanical forces, and is evidenced only in local changes of tlie 
physical properties and temperature of the material. Cf. § 48 infra. 
The other main division of the energy is the kinetic part, which is specified in 
teims of the rate of change of configuration of the material system with respect to 
an extraneous spacial framework to which its position is referred. Whatever notions 
may commend themselves a 'priori as to the impossibility of absolute space and 
absolute time, the fact remains that it has not been found possible to construct a 
system of dynamics which has resj^ect only to the relative positions of moving 
bodies; and the reason suggests itself, that there is an underlying part of the 
phenomena, which does not usually explicitly appear in abstract material dynamics, 
namely, the gethereal medium, and that the spacial framework in absolute rest, 
which was introduced by Newton and was probably a main source of the great 
advance in abstract dynamics originated by the Principia, is in fact the quiescent 
underlying lether. In this way the purely a priori standpoint is pushed away a 
stage, and we may find justification against the reproach that a philosophical 
formulation of dynamics should be concerned, only with relative motions. 
Relcotion to Gas-Theory : Internal Molecular Energy. 
II. The kinetic theory of gases is considerably affected by the view here taken of 
the constitution of a molecule. In those simple and satisfactory features which are 
concerned only with the translatory motion of the molecules, it stands intact; but it 
is diffeient with problems, like that of the ratio of the specific heats, which involve tlie 
mternal energy. According to the usual hypothesis of the theory of gases, all the 
internal^ kinetic energy of the molecule is taken to be thermal and in statistical 
equilibrium, through encounters, witli the translatory energy. But on the present 
view, the energy of the steady orbital motions in the molecule (including therein slow 
free piecessions) makes up both the energy of chemical constitution and the internal 
thermal energy ; while it is only when these steady motions are disturbed that the 
lesulting vibration gives rise to radiation by which some of the internal energy is 
lost. The amount of internal energy can however never fall below the minimum that 
coiiesponds to the actual conserved rotational momenta of the molecule; this 
minimum is the energy of chemical combination of its ultimate constituents, while 
the excess above it actually existing is the internal thermal energy.'^'" The present 
* As a concrete illustration, we can imagine two ideal atoms, each consisting of a single gyrostat 
2 P 2 
