190
VIII
EVAPORATION OF LIQUIDS
evaporating surface (for this pressure is really the interesting
point), I now fused the manometer on at A, as shown in the
figure, so that the retort itself formed the open limb. It
turned out that there was a very perceptible pressure; it
amounted to 2 to 3 mm. when the thermometer stood at 160° to
170°, and the evaporation went on at such a rate that a layer
0.8 mm. deep evaporated per minute. Hence there was no
difficulty in seeing that in its condensation the vapour might
produce a temperature exceeding 100°; however, it became
clear that the simple method which had been tried would not
lead to the desired result, but that direct measurements would be
necessary. The apparatus shown in Fig. 21 was therefore used.
A is again the retort. The heating vessel (only indicated in
the diagram) in which it was contained consisted of a hollow brass
cylinder 1.5 cm. thick, closely surrounding the retort and covered
over with asbestos. It was heated by a ring gas-burner, and
had in it a vertical slit through which the level of the mercury
could be observed. Bis again the tube in which the condensa-
tion takes place; the manometer tube is shown in perspective
at C. The magnifying power of the cathetometer telescope
used was such that it could be set with
certainty to within 0.02 mm. The pres-
sure, i.e. the difference of level between
the two surfaces, was measured by a
micrometer eye-piece with two threads:
the absolute height of the surface, i.e.
the rate of evaporation, was read off
on the scale of the instrument. The
temperature was varied by altering the
gas supply. The apparatus was at
first quite free from air: by admitting
varying, but always small, quantities of
air different pressures could be obtained
at the same temperature. If the pressure of the air
introduced amounted, say, to 1 mm. no evaporation in the
sense here considered could take place so long as the pressure
of the saturated vapour above the surface did not exceed 1
mm., i.e. so long as the temperature of the surface did not
exceed 120°; but when this temperature was exceeded the air
retreated into the condensing tube, and evaporation began;
FIG. 21.