faculties permit us to
perceive." The phrase "Correlation of Forces" is employed rather to
express their mutual convertibility, or change from one to the others.
Thus, heat excites electricity, and, through that force, magnetism,
chemical action, and light. Or, if we start with magnetism, this may
give rise to electricity, and this again to heat, chemical action, and
light. Or we can begin with chemical action, and obtain the same train
of effects.

It has long been known that machines do not create force, but only
communicate, distribute, and apply that which has been imparted to them,
and also that a definite amount of fuel corresponds to a definite amount
of work performed by the steam-engine. This means simply that a fixed
quantity of the chemical force of combustion gives rise to a
corresponding quantity of heat, and this again to a determinate amount
of mechanical effect. Now this principle of equivalency is found to
govern the transmutations of all forms of energy. The doctrine of the
conservation and correlation of forces has been illustrated in various
ways, but nothing has so powerfully contributed to its establishment as
the investigation of the relations of heat to mechanical force.
Percussion and friction produce heat. A cold bullet, struck upon an
anvil by a cold sledge-hammer, is heated. Iron plates, ground against
each other by water-power, have yielded a large and constant supply of
heat for warming the air of a factory in winter; while water inclosed in
a box, which was made to revolve rapidly, rose to the boiling-point.
What, now, is the source of heat in these cases? The old caloric
hypothesis utterly fails to explain it; for to suppose that there is an
indefinite and inexhaustible store of latent heat in the rubbing iron
plates is purely gratuitous. It is now established, that the heat of
collision, and of friction depends, not upon the nature of the bodies in
motion, but upon the force spent in producing it.

When a moving body is stopped, its force is not annihilated, but simply
takes another form. When the sledge-hammer strikes the leaden bullet and
comes to rest, the mechanical force is not destroyed, but is simply
converted into heat; and if all the heat produced could be collected, it
would be exactly sufficient, when reconverted into mechanical force, to
raise the hammer again to the height from which it fell. So, when bodies
are rubbed together, their surface-particles are brought into coll