ey become
capable of communicating the sensation of heat, and these exceptions
seem to depend upon some chemical change in the constitution of bodies,
or on their crystalline arrangements.

The power which bodies possess of communicating or receiving heat is
known as _temperature_, and the temparature of a body is said to be high
or low with respect to another in proportion as it occasions an
expansion or contraction of its parts.

When equal volumes of different bodies of different temperatures are
suffered to remain in contact till they acquire the same temperature, it
is found that this temperature is not a mean one, as it would be in the
case of equal volumes of the same body. Thus if a pint of quicksilver
at 100 deg. be mixed with a pint of water at 50 deg., the resulting temperature
is not 75 deg., but 70 deg.; the mercury has lost thirty degrees, whereas the
water has only gained twenty degrees. This difference is said to depend
on the different _capacities_ of bodies for heat.

Not only do different bodies vary in their capacity for heat, but they
likewise acquire heat with very different degrees of celerity. This last
difference depends on the different power of bodies for _conducting_
heat, and it will be found that as a rule the densest bodies, with the
least capacity for heat, are the best conductors.

Heat, or the power of repulsion, may be considered as the _antagonist_
power to the attraction of cohesion. Thus solids by a certain increase
of temperature become fluids, and fluids gases; and, _vice versa_, by a
diminution of temperature, gases become fluids, and fluids solids.

Proofs of the conversion of solids, fluids, or gases into ethereal
substances are not distinct. Heated bodies become luminous and give off
radiant heat, which affects the bodies at a distance, and it may
therefore be held that particles are thrown off from heated bodies with
great velocity, which, by acting on our organs, produce the sensations
of heat or light, and that their motion, communicated to the particles
of other bodies, has the power of expanding them. It may, however, be
said that the radiant matters emitted by bodies in ignition are specific
substances, and that common matter is not susceptible of assuming this
form; or it may be contended that the phenomena of radiation do in fact,
depend upon motions communicated to subtile matter everywhere existing
in space.

The temperatures at which bodies change their states