the latter is set in motion, and we have what are commonly known as
winds. It is unnecessary for me to prove that the motion of winds can be
transmitted to other matter, as we have numerous examples from our
observation and experience, in the case of windmills driven by the
motive power of the winds, and also balloons urged along by the same
cause; apart from the devastating effect produced in towns and country
by a hurricane or storm.

The point which I wish to emphasize is, that Matter, strictly defined,
is that which can be acted upon by motion, such as heat or electricity,
both being forms of motion, and which can exert the motion so derived
upon some other body.

Wherever, therefore, in the universe we find any body, whether it be
solid, liquid or gaseous, or any medium which can be acted upon by
motion, and after being so acted upon, can exert motion, that body or
medium may legitimately be included in the term Matter, although it may
be absolutely invisible to the eyes, or insensible to the sense of
touch, or taste, or smell. In the same work,[2] Tait states that in the
physical universe there are but two classes of things, "Matter and
Energy," and then goes on to give examples of both. He adds that a
stone, piece of brass, water, air, _aether_, are particles of matter,
while springs, water-power, wind, waves, heat and electric currents are
examples of energy associated with Matter.

Now I may add here, that within these two statements is to be found the
germ of the physical cause of Gravitation, together with the
satisfactory explanation of all phenomena that the universe reveals to
us, either by observation or by experiments. I purpose therefore, before
giving any detailed accounts of that medium which is to form the
physical basis for the cause of Gravitation, to look at the term Matter
in all its aspects, in order that we may get a right conception of the
universe, and of the part that Matter plays in the same.

[Footnote 2: Tait, _Natural Philosophy_.]

ART. 30. _Conservation of Matter._--The Theory of the Indestructibility
of Matter was first introduced by Lavoisier in 1789. This theory may be
thus summed up; that Matter which fills the universe is unchangeable in
quantity, so that the total quantity ever remains the same. Changes may
take place in regard to the state of the Matter, but the sum-total of
Matter throughout all the changes remains unaltered. Thus when we burn
coal, it is changed into