ixed, and
_vice versa_.

Saturated steam is water vapor in the condition in which it is generated
from water with which it is in contact. Or it is steam which is at the
maximum pressure and density possible at its temperature. If any change
be made in the temperature or pressure of steam, there will be a
corresponding change in its condition. If the pressure be increased or
the temperature decreased, a portion of the steam will be condensed. If
the temperature be increased or the pressure decreased, a portion of the
water with which the steam is in contact will be evaporated into steam.
Steam will remain saturated just so long as it is of the same pressure
and temperature as the water with which it can remain in contact without
a gain or loss of heat. Moreover, saturated steam cannot have its
temperature lowered without a lowering of its pressure, any loss of heat
being made up by the latent heat of such portion as will be condensed.
Nor can the temperature of saturated steam be increased except when
accompanied by a corresponding increase in pressure, any added heat
being expended in the evaporation into steam of a portion of the water
with which it is in contact.

Dry saturated steam contains no water. In some cases, saturated steam is
accompanied by water which is carried along with it, either in the form
of a spray or is blown along the surface of the piping, and the steam is
then said to be wet. The percentage weight of the steam in a mixture of
steam and water is called the quality of the steam. Thus, if in a
mixture of 100 pounds of steam and water there is three-quarters of a
pound of water, the quality of the steam will be 99.25.

Heat may be added to steam not in contact with water, such an addition
of heat resulting in an increase of temperature and pressure if the
volume be kept constant, or an increase in temperature and volume if the
pressure remain constant. Steam whose temperature thus exceeds that of
saturated steam at a corresponding pressure is said to be superheated
and its properties approximate those of a perfect gas.

As pointed out in the chapter on heat, the heat necessary to raise one
pound of water from 32 degrees Fahrenheit to the point of ebullition is
called the _heat of the liquid_. The heat absorbed during ebullition
consists of that necessary to dissociate the molecules, or the _inner
latent heat_, and that necessary to overcome the resistance to the
increase in volume, or the _outer