ion of a
mass of matter and the equivalent heat have been experimentally
determined by Joule, so that the unit of heat may be expressed in the
motion of a mass of matter. This is deducible from a more general law,
known as the conservation of energy. The application in this place is,
that whenever heat appears through electric action, as in the
above-mentioned places, we know that it still is only _motion_ that is
the product, only that this motion is now among the molecules of the
body, instead of the motion of the whole body in space, as when a
pith-ball moves, or a galvanometer-needle turns.

3d, It can directly produce _light_. This is seen in every spark from an
electric machine, in the flash of lightning, and in the electric light.

It has been shown in numberless ways, that there is no essential
difference between light and heat, and that what we call light is only
the active relation which certain rays of radiant energy have to the
eyes. In order to make this plain, suppose that a beam of light, say
from the sun, be permitted to fall upon a triangular prism of glass: at
once it is seen that the beam is deflected, and instead of appearing a
spot of white light, as it did before it was deflected, it now appears
as a brilliant band of colors, which is called the solar spectrum. If
now this spectrum be examined as to the distribution of heat, by moving
a thermo-pile through it from the blue end towards the red end, it will
be noticed that the galvanometer-needle will be but slightly deflected
at the blue end; but, as the thermo-pile is moved, the deflections are
greater until it is past the red end, where the heat is greatest. On
this account it has been customary to say that the red end of the
spectrum was the heating end. With various pieces of mechanism the rays
may be separated from each other, and measured; and then it appears that
a red ray of light has a wave length of about 1/37000 in., and the
violet ray about 1/60000 in. The rays beyond the red have also been
measured, and found to be greater in length uniformly as one recedes
from the visible part of the spectrum.

In like manner, beyond the blue end the wave lengths become shorter and
shorter; and in each of these directions the spectrum that is invisible
is much longer than the visible one. Now, it has also been found that
where a prism of glass or other material is used to produce a spectrum,
it distributes the rays very unevenly; that is, towards th