motion of any kind or sort, there we get
energy, or what used to be termed force. The consideration of this phase
of the question will be more fully dealt with in the chapter on Energy
and Motion. Suffice to say, that all experience teaches us that force is
the outcome of motion.

Accepting this definition therefore of force, Tait's definition of
matter will read thus, if brought up to date: "Matter is that which can
be perceived by the senses, or is that which can be acted upon by
motion, or which can exert motion."

The common idea that matter can only be that which can be seen or
actually felt, is not large enough for a definition of Matter. There are
numbers of things in Nature which cannot either be seen or felt, yet
which are included in the term Matter. Let us take one or two examples.
Every one admits that nitrogen and oxygen are matter, yet I venture to
say that no one has actually seen or felt either of these gases. Both of
these gases are colourless and invisible, and are both tasteless. You
may open your mouth and inspire both gases, and yet if they are pure,
you cannot taste either of them. They are only matter, in the sense that
they appeal to our sense of force through the motion which they may
acquire.

Or again, take air, which is a mechanical mixture of several gases. Can
you see air? If it be free from vapour and smoke, air is invisible, and
on a clear day you may look for miles across the sea, or from the top of
a mountain, and yet not have your sight impeded in any way by the
atmosphere. Neither can it be felt by the sense of touch. Open and shut
your hand, and see if you can feel the air while you do so. In similar
ways it may be demonstrated that the air is tasteless. So that it is not
necessary for us to see, or feel, or taste, or even smell that which we
term Matter, in order for it to be included in that term. So long as
that which we term Matter is able to accept motion in any manner from
any body that is either moving, or in a state of vibration, and not only
accepts, but also transmits the vibratory, or the kinetic motion so
called of the moving body, then that which accepts the motion is
legitimately termed Matter.

It becomes perfectly clear, therefore, why air, aether, oxygen, and
hydrogen are termed Matter. Because they can be all acted upon by
motion, and after being so acted upon, they can exert motion upon some
other body. Heat is a form of motion, and when heat acts upon the air,