latent heat_. These two make up the
_latent heat of evaporation_ and the sum of this latent heat of
evaporation and the heat of the liquid make the _total heat_ of the
steam. These values for various pressures are given in the steam tables,
pages 122 to 127.

The specific volume of saturated steam at any pressure is the volume in
cubic feet of one pound of steam at that pressure.

The density of saturated steam, that is, its weight per cubic foot, is
obviously the reciprocal of the specific volume. This density varies as
the 16/17 power over the ordinary range of pressures used in steam
boiler work and may be found by the formula, D = .003027p^{.941}, which
is correct within 0.15 per cent up to 250 pounds pressure.

The relative volume of steam is the ratio of the volume of a given
weight to the volume of the same weight of water at 39.2 degrees
Fahrenheit and is equal to the specific volume times 62.427.

As vapors are liquids in their gaseous form and the boiling point is the
point of change in this condition, it is clear that this point is
dependent upon the pressure under which the liquid exists. This fact is
of great practical importance in steam condenser work and in many
operations involving boiling in an open vessel, since in the latter case
its altitude will have considerable influence. The relation between
altitude and boiling point of water is shown in Table 12.

The conditions of feed temperature and steam pressure in boiler tests,
fuel performances and the like, will be found to vary widely in
different trials. In order to secure a means for comparison of different
trials, it is necessary to reduce all results to some common basis. The
method which has been adopted for the reduction to a comparable basis is
to transform the evaporation under actual conditions of steam pressure
and feed temperature which exist in the trial to an equivalent
evaporation under a set of standard conditions. These standard
conditions presuppose a feed water temperature of 212 degrees Fahrenheit
and a steam pressure equal to the normal atmospheric pressure at sea
level, 14.7 pounds absolute. Under such conditions steam would be
generated _at_ a temperature of 212 degrees, the temperature
corresponding to atmospheric pressure at sea level, _from_ water at 212
degrees. The weight of water which _would_ be evaporated under the
assumed standard conditions by exactly the amount of heat absorbed by
the boiler under actual conditio