484 Prof. Oliver Lodge on the 
piece of “ matter’? cannot act immediately on another piece. 
A and B therefore are not two pieces of ‘‘ matter ’”’ in the 
ordinary sense. A may be a molecule of matter, M may be 
the nearest molecule to it, and energy may be transferred 
from A to M, but not directly ; A cannot act on M, cannot 
do work on it, because of the intervening gap. A can act on 
B, transferring its energy to B, B can act on C, C on D, and 
so on, handing on the energy to L, which is in contact with, 
and can act on, M, doing work on it and giving up to it the 
energy lust by A. 
What B, C, D,....L are, I do not presume to say ; 
but of course one supposes them to be successive portions of 
the perfectly continuous space-filling medium Aither, 
Relation between Potential and Kinetic Energy, from the contact 
point of view. Reason of the two forms; and Transforma- 
tion into one another. 
In the older and more hazy view of conservation of energy 
the idea of “ potential energy” has always been felt to be a 
difficulty. It was easy enough to take account of it in the 
formule, but it was not easy or possible always to form a 
clear and consistent mental image of what was physically 
meant by it. | 
A stone is raised, it gains potential energy ; but how does 
the stone “‘up”’ differ from the stone “ down”’ ?, and how can 
an inert and quiet stone be said to possess energy ? Well, then, 
the stone hasn’t the energy but the earth has, or rather “the 
system of earth and stone possesses energy in virtue of its con- 
figuration.” True, but foggy. The usual ideas and language 
current about potential energy are proper to notions of action 
at a distance. When universal contact action is admitted, 
the haze disappears*; the energy is seen to be possessed, not 
by stone or by earth or by both of them, but by the medium 
which surrounds both and presses them together; and the 
foilowing statement may be made. 
Knergy has two fundamental forms because work has two 
factors, force and motion, F, s. 
Work cannot be done except by a body exerting force 
and in motion. Force without motion is no good. Motion 
without force is no good. Wither factor separately may be 
energy, but it is not work. 
* It is by no means intended that the natures of gravitation, elasticity, 
cohesion, &c, become clear. What is meant is, that the seat of the energy 
is Clearly recognized: the season of the stress recognizable in the medium 
is a much higher and more difficult problem, 
