HEAT.

134. _Q._--What is meant by latent heat?

_A._--By latent heat is meant the heat existing in bodies which is not
discoverable by the touch or by the thermometer, but which manifests its
existence by producing a change of state. Heat is absorbed in the
liquefaction of ice, and in the vaporization of water, yet the temperature
does not rise during either process, and the heat absorbed is therefore
said to become latent. The term is somewhat objectionable, as the effect
proper to the absorption of heat has in each case been made visible; and it
would be as reasonable to call hot water latent steam. Latent heat, in the
present acceptation of the term, means sensible liquefaction or
vaporization; but to produce these changes heat is as necessary as to
produce the expansion of mercury in a thermometer tube, which is taken as
the measure of temperature; and it is hard to see on what ground heat can
be said to be latent when its presence is made manifest by changes which
only heat can effect. It is the _temperature_ only that is latent, and
latent temperature means sensible vaporization or liquefaction.

135. _Q._--But when you talk of the latent heat of steam, what do you mean
to express?

_A._--I mean to express the heat consumed in accomplishing the vaporization
compared with that necessary for producing the temperature. The latent heat
of steam is usually reckoned at about 1000 degrees, by which it is meant
that there is as much heat in any given weight of steam as would raise its
constituent water 1000 degrees if the expansion of the water could be
prevented, or as would raise 1000 times that quantity of water one degree.
The boiling point of water, being 212 degrees, is 180 degrees above the