VIII
187
EVAPORATION OF LIQUIDS
is incomplete, since we might equally well regard the supply
of heat, conversely, as being determined by the evaporation.
For both depend upon the temperature of the outermost layer
of liquid; and this again is determined by the relation between
the possible supply of heat by conduction and the possible loss
of heat by evaporation. Now one of two things must happen.
Either (a) evaporation has no limit beyond that which is
involved in the supply of heat; so that if sufficient heat is
supplied, an unlimited amount of liquid can evaporate from a
given surface in unit time, and the temperature, density, and
pressure of the vapour produced will not differ perceptibly
from that of saturated vapour. In this case all liquid surfaces.
in the same space must assume the same temperature; and
this temperature as well as the amounts of liquid which
evaporate are determined by the relation between the possible
supply of heat and the different areas. Or (b) only a limited
quantity of liquid can evaporate from a liquid surface at a
given temperature. In this case there may be surfaces at
different temperatures in the same space, and the pressure and
density of the vapour arising must differ by a finite amount
from the pressure and density of the saturated vapour of at
least one of these surfaces: the rate of evaporation will depend
upon a number of circumstances, but chiefly upon the nature
of the liquid; so that there will be for every liquid a specific
evaporative power. It will be seen that the alternative (a)
can be regarded as a limiting case of (b). Hence in the
absence of any hypothesis or experimental information we
should have to assume the latter, which is the more general,
to be correct. But we shall presently show by a more detailed
discussion that the first-mentioned alternative is an extremely
improbable one.
I have made a number of experiments on evaporation in
vacuo in the hope of arriving at an experimental decision
between these two alternatives, if possible by exact measure-
ments of the evaporative power of any liquid under different
conditions. The experiments have only partly achieved their
aim: nevertheless I describe them here, because they throw
light upon the problem, and may clear up the way for better
methods. The experiments are described in the first section:
in the second section is given a theoretical discussion, which