794 
UU. J. LARMOR ON" A DYNAMICAL THEORY OF 
wliidi contain inside them the same atoms. If we want to make this integral 
constant througiiont time, we may imagine that the medium was originally in equi- 
lilDi’inm without compression, and was then strained by altering the volume of each 
electrically charged atom by a definite amount. The state of strain thus represented 
w. the epther has a pressure at each point equal to A.multiplied into the gravitation 
potential of a mass ecjual to this constant, supposed placed at the atom. Its energy 
is however 
(It, instead of —| 
which it ought to be"^ if it were gravitational energy; so that there is no means of 
explaining gravitation here. 
102. If we could imagine for a moment that the electric charges of the two ions 
in a molecule do not exactly compensate each other, but that there is a slight excess 
always of the same sign, we should have a repulsive force of gravitational type, trans¬ 
mitted hy a stress in a rotational aether. A term of this form in the energy, if it 
v'ere kinetic instead of potential, would account for gravity. The question thus sug¬ 
gested is, whether the kinetic energy of the primordial medium has been sufficiently 
expressed, in view of the inherent rotational quality in its elements. It was proved 
by Laplace that the velocity of gravitation must be enormously great compared with 
that of light ; so that the gravitational energy, whatever its origin, must preserve a 
purely statical aspect with respect to all the other phenomena that have l)een here 
under discussion. 
The objection has been raised, by Clerk Maxwell and others, to the vortex-atom 
theory of matter, that it can give no account of mass for the case of sensible bodies. 
But it may be urged that mass is a djmamical conception, which in complicated cases 
it would be hard to define exactly or give an account of. The clearest view of 
dyjiamics would appear to be tlie one maintained by various writers, notably by 
L. N. M. Carnot and by Kirchhofe, that the function of that science is to correlate, 
or give a general formula for, the sequence of physical phenomena. The ultimate 
formula which is, it is hoped, to embrace the physical universe is the law of Least 
Action ; and the ultimate definition of mass is to make it a coefficient in the kinetic 
part of the energy-function of the matter in that formula. As the theories here dis¬ 
cussed are referred to the single basis of this law of Least Action, the objection that 
they do not take account of mass can hardly be prohibitive ; though they may not be 
able to explain how the idea of mass is originated by aggregation of terms in that 
equation. 
L03. It is conceivable that the rotational elasticity of the fundamental medium is 
really due to a rotatory motional distribution in it, which resists disturbance from 
* Cf. Maxwell, “ A dynamical theory of the Electroniao-netic Field,” § 82, ‘ Pliil. Trans.,’ 1864. 
