194
VIII
EVAPORATION OF LIQUIDS
colder than that in the closed limb, the measured difference
of level (0.26 mm.) was somewhat smaller than it would
have been if both limbs were at the same temperature. An
examination of the distribution of heat in the interior gives
0.03 mm. as the necessary correction; thus the difference of
pressure in the two limbs was equal to 0.29 mm. of mercury at
118°, or 0.28 mm. of mercury at 0°. If we subtract from this
pressure the difference between the saturation-pressures at 118°
and 108°, we obtain the divergence between the pressure upon
the evaporating surface and the saturation-pressure. The
difference to be subtracted amounts to 0.27 mm.; so that
only 0.01 mm. is left. This shows that the pressure of the
vapour does not differ perceptibly from the saturation-pressure;
and the same result follows from all the observations made by
this method. At lower temperatures (90° to 100°) deviations
of a few hundredths of a millimetre, in the direction anti-
cipated, were found; but at high temperatures, on the other
hand, pressures were calculated which slightly exceeded the
saturation-pressure. Clearly there must have been slight
errors in the corrections, as indeed might have been expected
from the method of determination. But the experiments
undoubtedly prove two things. In the first place, that the
method is not well adapted for giving quantitative results,
because the constant errors of experiment are of the same
order as the quantities to be observed. In the second place,
that the positive results obtained by the earlier method had
their origin partly, if not entirely, in the errors made in
measuring the temperature.¹ For, if they had been correct,
deviations of pressure of 0.10 to 0.20 mm. must have mani-
fested themselves in the last experiments, and these could not
have escaped observation.
Thus the net result of the experiments is a very modest
one. They show that the pressure exerted upon the liquid
by the vapour arising from it is practically equal to the
saturation-pressure at the temperature of the surface; and
hence that of the two alternatives mentioned in the intro-
duction, the first is to be regarded as correct.
not show definitely the existence of the small deviation from
But they do
1 That very large errors are possible can be easily seen by calculating those
which would arise if the surface were only supplied with heat by conduction.