e phases may occur, is called the _triple point_. Let
us represent it by T3; if the term "condensation of gases" is taken in
the sense of "liquefaction of gases"--which is usually done--the
condition for condensation is Tc > T > T3. The opinion sometimes held
that for all substances T3 is the same fraction of Tc (the value being
about 1/2) has decidedly not been rigorously confirmed. Nor is this to
be expected on account of the very different form of crystallization
which the solid state presents. Thus for carbon dioxide, CO2, for which
Tc = 304 deg. on the absolute scale, and for which we may put T3 = 216
deg., this fraction is about 0.7; for water it descends down to 0.42,
and for other substances it may be still lower.

If we confine ourselves to temperatures between Tc and T3, the gas will
pass into a liquid if the pressure is sufficiently increased. When the
formation of liquid sets in we call the gas a _saturated vapour_. If the
decrease of volume is continued, the gas pressure remains constant till
all the vapour has passed into liquid. The invariability of the
properties of the phases is in close connexion with the invariability of
the pressure (called _maximum tension_). Throughout the course of the
process of condensation these properties remain unchanged, provided the
temperature remain constant; only the relative quantity of the two
phases changes. Until all the gas has passed into liquid a further
decrease of volume will not require increase of pressure. But as soon as
the liquefaction is complete a slight decrease of volume will require a
great increase of pressure, liquids being but slightly compressible.

Critical pressure.

The pressure required to condense a gas varies with the temperature,
becoming higher as the temperature rises. The highest pressure will
therefore be found at Tc and the lowest at T3. We shall represent the
pressure at Tc by pc. It is called the _critical pressure_. The pressure
at T3 we shall represent by p3. It is called the _pressure of the triple
point_. The values of Tc and pc for different substances will be found
at the end of this article. The values of T3 and p3 are accurately known
only for a few substances. As a rule p3 is small, though occasionally it
is greater than 1 atmosphere. This is the case with CO2, and we may in
general expect it if the value of T3/Tc is large. In this case there can
only be a question of a real boiling-point (under the normal pressure)
if