VIII
189
EVAPORATION OF LIQUIDS
in Fig. 20. Into the retort A, placed inside a heating vessel,
was fused a glass tube open above and closed below; inside
this and just under the surface of the mercury was the ther-
mometer which indicated the temperature. To the neck of the
retort was attached the vertical tube B, which was immersed
in a fairly large cooling vessel, and could
thus be maintained at 0° or any other
temperature. By brisk boiling and simul-
taneous use of a mercury pump all per-
ceptible traces of air were removed from
the apparatus. The rate of evaporation
was now measured by the rate at which the
mercury rose in the tube B.
The pressure
FIG. 20.
P was not to be directly measured.
I sup-
posed, as is frequently done, that P could
not exceed the pressure of the saturated
vapour at the lower temperature, viz. that
of B; and assumed that it would suffice to vary the latter
temperature only in order to obtain corresponding values of
the pressure.
It soon became clear that this assumption was
erroneous; for when the temperature began to exceed 100°,
and the evaporation became fairly rapid, the vapour did not
condense in the cold tube B, but in the neck or connecting
tube at C. This became so hot that one could not touch it; its
temperature was at least 60° to 80°. This cannot be explained
on the assumption that the vapour inside has the exceedingly
low pressure corresponding to 0°; for in that case it could
only be superheated by contact with a surface at 60°, and
could not possibly suffer condensation. In order to measure
the pressure I introduced at C the manometer tube shown in
the figure. But this did not show any change from its initial
position when the rate of evaporation was increased. It was
certain that the vapour moved with a certain velocity, so that
its pressure upon the surface from which it arose must be
different from the pressure which it would naturally possess.
It could easily be seen that this velocity was very considerable;
for when the drops of mercury on the glass attained a certain
size they did not fall downwards from their weight, but were
carried along nearly parallel to the direction of the tube. In
order to see whether the vapour exerted a pressure upon the