By picturing this process in our mind we become aware of a certain
kinship of electricity with fire, since for ages the only known way of
kindling fire was through friction. We notice that in both cases man
had to resort to the will-power invested in his limbs for setting in
motion two pieces of matter, so that, by overcoming their resistance to
this motion, he released from them a certain force which he could
utilize as a supplement to his own will. The similarity of the two
processes may be taken as a sign that heat and electricity are related
to each other in a certain way, the one being in some sense a
metamorphosis of the other. Our first task, therefore, will be to try
to understand how it is that friction causes heat to appear in manifest
form.

There is no friction unless the surfaces of the rubbed bodies have a
structure that is in some way interfered with by the rubbing, while at
the same time they offer a certain resistance to the disturbance. This
resistance is due to a characteristic of matter, commonly called
cohesion. Now we know that the inner coherence of a physical body is
due to its point-relationship, that is to the gravitational force bound
up with it. Indeed, cohesion increases as we pass from the gaseous,
through the liquid, to the solid state of matter.

Whilst a body's cohesion is due to gravity, its spatial extendedness
is, as we have seen, due to levity. If we reduce the volume of a piece
of physical matter by means of pressure, we therefore release
levity-forces previously bound up in it, and these, as always happens
in such cases, appear in the form of free heat. Figuratively speaking,
we may say that by applying pressure to matter, latent levity is
pressed out of it, somewhat like water out of a wet sponge.