network of fine checks along
the medullary rays.

In the first place, it should be borne in mind that it is the heat
which produces evaporation and not the air nor any mysterious property
assigned to a "vacuum."

For every pound of water evaporated at ordinary temperatures
approximately 1,000 British thermal units of heat are used up, or
"become latent," as it is called. This is true whether the evaporation
takes place in a vacuum or under a moderate air pressure. If this heat
is not supplied from an outside source it must be supplied by the
water itself (or the material being dried), the temperature of which
will consequently fall until the surrounding space becomes saturated
with vapor at a pressure corresponding to the temperature which the
water has reached; evaporation will then cease. The pressure of the
vapor in a space saturated with water vapor increases rapidly with
increase of temperature. At a so-called vacuum of 28 inches, which is
about the limit in commercial operations, and in reality signifies an
actual pressure of 2 inches of mercury column, the space will be
saturated with vapor at 101 degrees Fahrenheit. Consequently, no
evaporation will take place in such a vacuum unless the water be
warmer than 101 degrees Fahrenheit, provided there is no air leakage.
The qualification in regard to air is necessary, for the sake of
exactness, for the following reason: In any given space the total
actual pressure is made up of the combined pressures of all the gases
present. If the total pressure ("vacuum") is 2 inches, and there is no
air present, it is all produced by the water vapor (which saturates
the space at 101 degrees Fahrenheit); but if some air is present and
the total pressure is still maintained at 2 inches, then there must be
less vapor present, since the air is producing part of the pressure
and the space is no longer saturated at the given temperature.
Consequently further evaporation may occur, with a corresponding
lowering of the temperature of the water, until a balance is again
reached. Without further explanation it is easy to see that but little
water can be evaporated by a vacuum alone without addition of heat,
and that the prevalent idea that a vacuum can of itself produce
evaporation is a fallacy. If heat be supplied to the water, however,
either by conduction or radiation, evaporation will take place in
direct proportion to the amount of heat supplied, so long as the
pressure is kept do