sense, or upon
other matter, and are not susceptible of being confined. It cannot be
doubted that there is such matter in motion in space. Ethereal matter
differs either in its nature, or in its affections by motion, for it
produces different effects; for instance, radiant heat, and different
kinds of light.

All these forms of matter are under the influence of active forces, such
as gravitation, cohesion, heat, chemical and electrical attraction, and
these we must now consider.

1. _Gravitation._ When a stone is thrown into the atmosphere, it rapidly
descends towards the earth. This is owing to gravitation. All the great
bodies in the universe are urged towards each other by a similar force.
Bodies mutually gravitate towards each other, but the smaller body
proportionately more than the larger one; hence the power of gravity is
said to vary directly as the mass. Gravitation also varies with
distance, and acts inversely as the square of the distance.

2. _Cohesion._ Cohesion is the force which preserves the forms of
solids, and gives globularity to fluids. It is usually said to act only
at the surface of bodies or by their immediate contact; but this does
not seem to be the case. It certainly acts with much greater energy at
small distances, but the spherical form of minute portions of fluid
matter can be produced only by the attractions of all the parts of which
they are composed, for each other; and most of these attractions must be
exerted at sensible distances, so that gravitation and cohesion may be
mere modifications of the same general power of attraction.

3. _Heat._ When a body which occasions the sensation of heat on our
organs is brought into contact with another body which has no such
effect, the hot body contracts and loses to a certain extent its power
of communicating heat; and the other body expands. Different solids and
fluids expand very differently when heated, and the expansive power of
liquids, in general, is greater than that of solids.

It is evident that the density of bodies must be diminished by
expansion; and in the case of fluids and gases, the parts of which are
mobile, many important phenomena depend upon this circumstance. For
instance, if heat be applied to fluids and gases, the heated parts
change their places and rise, and the currents in the ocean and
atmosphere are due principally to this movement. There are very few
exceptions to the law of the expansion of bodies at the time th