liquid suddenly solidifying. The heat latent in that liquid, and
necessary to keep it a liquid, is no longer necessary and comes out, and
the substance appears to become hot. Quicklime is a cold, white, solid
substance, but there is a compound of water and lime--slaked lime--which
is also a solid powdery substance, called by the chemist, hydrate of
lime. The water used to slake the quicklime is a liquid, and it may be
ice-cold water, but to form hydrate of lime it must assume a solid form,
and hence can and does dispense with its heat of liquefaction in the
change of state. You all know how hot lime becomes on slaking with
water. Of course we have heat of chemical combination here as well as
evolution of latent heat. As another example, we may take a solution of
acetate of soda, so strong that it is just on the point of
crystallising. If it crystallises it solidifies, and the liquid
consequently gives up its latent heat of liquefaction. We will make it
crystallise, first connecting the tube containing it to another one
containing a coloured liquid and closed by a cork carrying a narrow tube
dipping into the coloured liquid. On crystallising, the solution gives
off heat, as is shown by the expansion of the air in the corked tube,
and the consequent forcing of the coloured liquid up the narrow tube.
Consequently in your works you never dissolve a salt or crystal in water
or other liquid without rendering heat latent, or consuming heat; you
never allow steam to condense in the steam pipes about the premises
without losing vastly more heat than possibly many are aware of. Let us
inquire as to the latent heat of water and of steam.

_Latent Heats of Water and Steam._--If we mix 1 kilogram (about 2 lb.)
of ice (of course at zero or 0° C.) with 1 kilogram of water at 79° C.,
and stir well till the ice is melted, _i.e._ has changed its state from
solid to liquid, we find, on putting a thermometer in, the temperature
is only 0° C. This simply means that 79° of heat (centigrade degrees)