VIII
193
EVAPORATION OF LIQUIDS
function has been determined. In order to find this, special
experiments were made with the apparatus shown in Fig. 22, b.
A piece of the same tube from which the manometer was
made, was bent at its lower end into the shape of the manometer
limb. This was filled with mercury to the same depth as
the manometer tube; above the mercury was a layer of water
about 10 cm. deep, and in this a thermometer and stirrer were
placed. This tube was immersed up to the level of the mercury
in a warm linseed-oil bath, the temperature of which was
indicated by a second thermometer. A steady flow of heat
soon set in from the bath through the mercury to the water.
The difference between the two thermometers gave the differ-
ence between the temperatures of the bath and of the mercury
surface; the increase of the temperature gave the corresponding
flow of heat. Of course a number of corrections were neces-
sary; after applying these it was found that the flow of heat
increased somewhat more rapidly than the difference of tem-
perature. For example, a difference of 10°0 was necessary in
order to convey to the surface per minute sufficient heat to
warm a layer of water 117 mm. high (lying above the surface)
through 0°48. I shall make use of these data for calculating
out an experiment made with the evaporation apparatus.
When the temperature of the bath was 118°0 and the differ-
ence of level was 0.26 mm., it was found that in 3.66 minutes
the mercury in both limbs sank 0·105 mm. (this was the mean
of measurements in both limbs). As the evaporation took
place only in one limb, the depth of the layer removed from
this in a minute was 2 x 0.105/3.66 = 0·057 mm. In order
to vaporise unit weight of mercury at 118° under the pressure
of its saturated vapour, an amount of heat is required which
would raise 72.8 units of water through 1º.
This value may
be used with a near approach to accuracy in calculating the
results of our experiment. Thus there must have been con-
veyed to the surface per minute enough heat to raise a layer
of water 0.057 × 13.6 × 72·8564 mm. high through 1º, or
a layer of water 117 mm. high through 56 4/1170°48.
For this, according to what precedes, there must have been a
difference of temperature of 10°-0 between the bath and the
surface; so that the true temperature of the evaporating sur-
face was 108°.0. Since the mercury in the open limb was
M. P.
O