addition, one recalls the
fact that the ether is homogeneous, that is all of one kind, and also
that it is not composed of atoms and molecules, then degree of
compactness and number of particles per cubic inch have no meaning, and
the term density, if used, can have no such meaning as it has when
applied to matter. There is no physical conception gained from the study
of matter that can be useful in thinking of it. As with elasticity, so
density is inappropriately applied to the ether, but there is no
substitute yet offered.

15. MATTER IS HEATABLE.

So long as heat was thought to be some kind of an imponderable thing,
which might retain its identity whether it were in or out of matter,
its real nature was obscured by the name given to it. An imponderable
was a mysterious something like a spirit, which was the cause of certain
phenomena in matter. Heat, light, electricity, magnetism, gravitation,
were due to such various agencies, and no one concerned himself with the
nature of one or the other. Bacon thought that heat was a brisk
agitation of the particles of substances, and Count Rumford and Sir
Humphrey Davy thought they proved that it could be nothing else, but
they convinced nobody. Mayer in Germany and Joule in England showed that
quantitative relations existed between work done and heat developed, but
not until the publication of the book called _Heat as a Mode of Motion_,
was there a change of opinion and terminology as to the nature of heat.
For twenty years after that it was common to hear the expressions heat,
and radiant heat, to distinguish between phenomena in matter and what is
now called radiant energy radiations, or simply ether-waves. Not until
the necessity arose for distinguishing between different forms of
energy, and the conditions for developing them, did it become clear to
all that a change in the form of energy implied a change in the form of
motion that embodied it. The energy called heat energy was proved to be
a vibratory motion of molecules, and what happened in the ether as a
result of such vibrations is no longer spoken of as heat, but as ether
waves. When it is remembered that the ultimate atoms are elastic bodies,
and that they will, if free, vibrate in a periodic manner when struck or
shaken in any way, just as a ball will vibrate after it is struck, it is
easy to keep in mind the distinction between the mechanical form of
motion spent in striking and the vibratory form of the motion