192
VIII
EVAPORATION OF LIQUIDS
As an
at 120°, 0.35 at 150°, and 0.25 mm. at 180° to 200°.
example may be given the case in which the highest rate of
evaporation was observed. In this case the vessel was quite
free from air, the temperature was 183°3, the pressure 3.32
mm., and the level of the mercury sank uniformly at the rate
of 1.80 mm. per minute. Now, since the pressure of the
saturated vapour¹ is 10:35 mm. at 183.3°, and 3.32 mm. at
153°0, we must assume that there was an error of 7 mm. in
the measurement of pressure, or of 30° in the measurement
of temperature, if we are unwilling to admit that this proves
the existence of a limited rate of evaporation peculiar to the
liquid. The first-mentioned error could not have occurred;
nor do I believe that the second could. But I could not
conceal from myself that the results, from the quantitative
point of view, were very uncertain; and so I endeavoured to
support them by further experiments. For this purpose I
made observations with the apparatus shown in Fig. 22, a.
P
FIG. 22.
The glass vessel A, shaped like a mano-
meter and completely free from air, is
contained in a thick cast-iron heating
vessel in a paraffin bath. The level of
the mercury in both limbs is observed
from the outside through a plane glass
plate. The open arm communicates with
the cold receiver B; the communicating
tube is not too wide, in order that the
evaporation may take place slowly. The small condenser inside
the heating vessel is intended to prevent condensed mercury from
flowing back into the retort. There is now no difficulty in
observing the rate of evaporation or the pressure, at any rate
if we regard the pressure of the saturated vapour in the closed
limb as known; the uncertainty comes in again in determining
the temperature of the evaporating surface. This temperature
is equal to that of the bath, less a correction which for a given
apparatus is a function of the convection current only which
supplies heat to the surface. The known rate of evaporation
gives us the required supply of heat; from this again we can
deduce the difference of temperature when the above-mentioned
1 For all data as to the pressure of saturated mercury vapour here used, see
the determinations given in the next paper (IX. p. 200).