from fluids to
solids, though in general definite, are influenced by a few
circumstances such as motion and pressure.

When solids are converted into fluids, or fluids into gases, there is
always a loss of heat of temperature; and, _vice versa_, when gases are
converted into fluids, or fluids into solids, there is an increase of
heat of temperature, and in this case it is said that _latent_ heat is
absorbed or given out.

The expansion due to heat has been accounted for by supposing a subtile
fluid, or _caloric_, capable of combining with bodies and of separating
their parts from each other, and the absorption and liberation of latent
heat can be explained on this principle. But many other facts are
incompatible with the theory. For instance, metal may be kept hot for
any length of time by friction, so that if _caloric_ be pressed out it
must exist in an inexhaustible quantity. Delicate experiments have shown
that bodies, when heated, do not increase in weight.

It seems possible to account for all the phenomena of heat, if it be
supposed that in solids the particles are in a constant state of
vibratory motion, the particles of the hottest bodies moving with the
greatest velocity and through the greatest space; that in fluids and
gases the particles have not only vibratory motion, but also a motion
round their own axes with different velocities, and that in ethereal
substances the particles move round their own axes and separate from
each other, penetrating in right lines through space. Temperature may be
conceived to depend upon the velocity of the vibrations, increase of
capacity on the motion being performed in greater space; and the
diminution of temperature during the conversion of solids into fluids or
gases may be explained on the idea of the loss of vibratory motion in
consequence of the revolution of particles round their axes at the
moment when the body becomes fluid or aeriform, or from the loss of
rapidity of vibration in consequence of the motion of particles through
greater space.

4. _Chemical Attraction._ Oil and water will not _combine_; they are
said to have no chemical _attraction_ or _affinity_ for each other. But
if oil and solution of potassa in water be mixed, the oil and the
solution blend and form a soap; and they are said to attract each other
chemically or to have a _chemical affinity_ for each other. It is a
general character of chemical combination that it changes the qualities
of the