t were abstracted from the
body at a uniform rate, a time-temperature curve would again be a
smooth falling curve. Such a curve is called a "cooling curve."

However, we find that when a body is melting, vaporizing, or otherwise
suffering an abrupt change in physical properties, a quantity of
heat is absorbed which disappears without changing the temperature
of the body. This heat absorbed during a change of state is called
"latent heat," because it is transformed into the work necessary to
change the configuration and disposition of the molecules in the
body; but it is again liberated in equal amount when the reverse
change takes place.

From these considerations it would seem that should the cooling
curve be continuous and smooth, following closely a regular course,
all the heat abstracted during cooling is furnished at the expense
of a fall in temperature of the body; that is to say, it disappears
as "sensible heat." These curves, however, frequently show horizontal
portions or "arrests" which denote that at that temperature all
of the heat constantly radiating is being supplied by internal
changes in the alloy itself; that is, it is being supplied by the
evolution of a certain amount of "latent heat."

In addition to the large amount of heat liberated when a metal
solidifies, there are other changes indicated by the thermal analysis
of many alloys which occur _after_ the body has become entirely
solidified. These so-called transformation points or ranges may
be caused by chemical reactions taking place within the solid,
substances being precipitated from a "solid solution," or a sudden
change in some physical property of the components, such as in
magnetism, hardness, or specific gravity.

It may be difficult to comprehend that such changes can occur in
a body after it has become entirely solidified, owing to the usual
conception that the particles are then rigidly fixed. However, this
rigidity is only comparative. The molecules in the solid state
have not the large mobility they possess as a liquid, but even so,
they are still moving in circumscribed orbits, and have the power,
under proper conditions, to rearrange their position or internal
configuration. In general, such rearrangement is accompanied by a
sudden change in some physical property and in the total energy
of the molecule, which is evidenced by a spontaneous evolution or
absorption of latent heat.

Cooling curves of the purest iron show at least