188
VIII
EVAPORATION OF LIQUIDS
justifies the view adopted and establishes limits for the quan-
tities under consideration.
I. I started the experiments on the assumption that the
rate of evaporation of a liquid is at all events determined by
the temperature of the surface and the pressure exerted upon
it by the vapour which arises. In the course of the investiga-
tion I began to doubt, not whether these magnitudes were
necessary conditions, but whether they were sufficient condi-
tions for determining the amount of liquid which evaporates:
in the second section it will be shown that there was no reason
for this doubt. Hence I first set to work at the following
problem :-To find for any fluid simultaneous values of the
temperature t of the surface, the pressure P upon it, and the
height h of the layer of liquid which evaporates from it
in unit time. The difficulty experienced in solving this
apparently simple problem arises in the determination of t
and P. Even when the evaporation only goes on at a moderate
rate very considerable quantities of heat are required to keep
it up; the result of which is that the temperature increases
very rapidly from the surface towards the interior. Hence if
we dip a thermometer the least bit into the liquid it does not
show the true surface temperature. The experiments further
showed that at moderate rates of evaporation there was only
a slight difference between the pressure and the pressure of
saturated vapour.
As it is just this difference that we wish
to examine, it follows that both pressures must be very
accurately measured. Lastly, the interior of the liquids in
these experiments is necessarily in the superheated state; and
since boiling with bumping would render the experiments
impracticable, one is restricted to a very narrow range of
temperature and pressure.
I pass over certain experiments made with water, for I
soon observed that water, on account of its high latent heat
and low conductivity, was ill suited for my purpose. Mercury
appeared to be the most suitable liquid, for it has a relatively
small latent heat and a conductivity similar to that of metals;
and on account of its cohesion and the low pressure of its
vapour, it can be superheated strongly without boiling. The
first experiments were carried out with the apparatus shown