n is lost. Heat may be changed into
light, and light into heat; electricity into light, and light into
electricity; heat into electricity, and electricity into heat. Indeed,
starting from any one form, any of the other modes of motion may be
produced, either directly or indirectly, and mechanical effects or work
may be produced by each and all. Then, again, the order can be reversed,
as by doing work which is simply applied motion, any of the other modes
of motion can be produced. Thus heat can be produced by friction, and if
the friction which is the outcome of muscular energy be continued long
enough, a light will be the result, in the form of fire. When certain
forms of work are done, as the turning of the handle of an electrical
machine, frictional electricity will be produced. So that not only are
all the modes of motion convertible into work, but work itself can be
transformed into the modes of motion known as heat, light, electricity,
and magnetism.

Now, if Gravitation be due to motion of the Aether, and if it is true
that all modes of motion are convertible, then the application of this
principle should also hold good in relation to Gravitation. It has been
demonstrated by Joule and others that Gravitation can be converted into
heat, light, and electricity. It can be converted first into heat. Joule
made a number of experiments to ascertain what quantity of heat is
produced by falling bodies, that is bodies under the influence of
Gravitation. From experiments he has calculated that if one lb. of water
falls through a space of 772 feet, it would raise the temperature of
the water one degree Fahrenheit--that is, the water after its fall will
be one degree hotter than when it started to fall. Here, then, we have
the exact equivalence of a certain amount of gravitational motion
expressed in terms of heat. So that, whenever motion of a falling body
produced by gravity is arrested, heat is generated, and as heat is a
mode of motion, it follows that the motion of Gravitation has been
converted into the motion of heat. Again, the motion of gravity may be
converted into that of light. This may be demonstrated as follows: Lord
Kelvin has suggested that the light and heat of the sun are maintained
by the falling into the sun of meteorites. Now the cause of the falling
of these meteorites into the sun is the Attraction of Gravitation, and
therefore if the falling of these meteorites produces light and heat, it
necessarily