424 
MR. O. HEAVISIDE ON THE FORCES, STRESSES, AND 
optical bearing, it is convenient to assume that the matter and the ether in contact 
with it move together. This is the working hypothesis made by H. Hertz in his 
recent treatment of the electrodynamics of moving bodies; it is, in fact, what we 
tacitly assume in a straightforward and consistent working out of Maxwell’s 
principles without any plainly-expressed statement on the question of the relative 
motion of matter and ether; for the part played in Maxwell’s theory by matter is 
merely (and, of course, roughly) formularised by supposing that it causes the etherial 
constants to take different values, whilst introducing new properties, that of dissipating 
energy being the most prominent and important. We may, therefore, think of merely 
one medium, the most of which is uniform (the ether), whilst certain portions (matter 
as well) have different powers of supporting electric displacement and magnetic induc¬ 
tion from the rest, as well as a host of additional properties; and of these we can 
include the power of supporting conduction current with dissipation of energy according 
to Joule’s law, the change from isotropy to eolotropy in respect to the distribution 
of the several fluxes, the presence of intrinsic sources of energy, &c. # 
§ 3. We do not in any way form the equations of motion of such a medium, even 
as regards the uniform simple ether, away from gross matter ; we have only to discuss 
it as regards the electric and magnetic fluxes it supports, and the stresses and fluxes 
of energy thereby necessitated. First, we suppose the medium to be stationary, and 
examine the flux of electromagnetic energy. This is the Poynting flux of energy. 
Next we set the medium into motion of an unrestricted kind. We have now neces¬ 
sarily a convection of the electric and magnetic energy, as well as the Poynting 
flux. Thirdly, there must be a similar convection of the kinetic energy, &c., of the 
translational motion; and fourthly, since the motion of the medium involves the 
working of ordinary (Newtonian) force, there is associated with the previous a flux of 
energy due to the activity of the corresponding stress. The question is therefore a 
complex one, for we have to properly fit together these various fluxes of energy in 
harmony with the electromagnetic equations. A side issue is the determination 
of the proper measure of the activity of intrinsic forces, when the medium moves ; in 
another form, it is the determination of the proper meaning of t£ true current ” in 
Maxwell’s sense. 
§ 4. The only general principle that we can bring to our assistance in interpreting 
electromagnetic results relating to activity and flux of energy, is that of the per- 
* Perhaps it is best to say as little as possible at present about tbe connection between matter and 
ether, but to take tbe electromagnetic equations in an abstract manner. Tbis will leave us greater 
freedom for future modifications without contradiction. There are, also, cases in which it is obviously 
impossible to suppose that matter in bulk carries on with it the ether in bulk which permeates it. 
Either, then, the mathematical machinery must work between the molecules ; or else, we must make such 
alterations in the equations referring to bulk as will be practically equivalent in effect. For example, 
the motional magnetic force VDq of equations (88), (92), (93) may be modified either in q or in D, by 
use of a smaller effective velocity q, or by the substitution in D or cE of a modified reckoning of c for 
the effective permittivity. 
