13 
of Ediiiburgh, Session 1882 - 83 . 
(1) So far as our modern knowledge goes there are but two 
objective things in the physical world — matter and energy. Energy 
cannot exist except as associated with matter, and it can be per- 
ceived and measured by us only when it is being transferred, by a 
“ dynamical transaction,” from one portion of matter to another. 
In such transferences it is often transformed ” ; but no process 
has ever been devised or observed by which the quantity, either of 
matter or energy, has been altered. 
(2) Hence the true bases of our subject, so far as we yet know, 
are — 
1. Conservation of matter. 
2. Conservation of energy. 
3. That property (those properties of matter, in virtue of 
which it is the necessary vehicle, or as the case may be, the store- 
house, of energy. 
(3) The third of these alone presents any difficulty. So long 
as energy is obviously kinetic, this property is merely our old friend 
inertia. But the mutual potential energy of two gravitating masses, 
two electrified bodies, two currents, or two magnets, is certainly 
associated (at least in part, and in some as yet unknown way) with 
matter, of a kind not yet subjected to chemical scrutiny, which 
occupies the region in which these masses, &c., are situated. And, 
even when the potential energy obviously depends on the strain of 
a portion of ordinary matter, as in compressed air, a bent spring, a 
deformed elastic solid, &c., we can, even now, only describe it as due 
to “ molecular action,” depending on mechanism of a kind as yet 
unknown to us, though, in some cases, at least partially guessed at. 
(4) The necessity for the explicit assumption of the third 
principle, and a hint at least of the limits within which it must be 
extended, appear when we consider the very simplest case of motion, 
viz., that of a lon,e particle moving in a region in which its potential 
energy is the same at every point. For the conservation of energy 
tells us merely that its speed is unaltered. We know, however, 
that this is only part of the truth : the velocity is constant. It will 
be seen later that this has most important dynamical consequences 
in various directions. 
(The remarkable discussion of this point by Clerk-Maxwell is then 
referred to, in which it is virtually shown that, were things other- 
