IX
201
VAPOUR-PRESSURE OF MERCURY
made with the U-shaped manometer of the evaporation
apparatus shown in Fig. 21 (p. 190); but as there was now
no evaporation, the condenser and connecting tube were not
required. By boiling and pumping out with a mercury pump,
all air was removed from both limbs of the apparatus. The
temperature of the heated limb was indicated by a ther-
mometer dipping right into the mercury; the thermometer
was calibrated and its readings were reduced to those of an
air-thermometer. In determining the pressure the difference.
of level between the two limbs was read off, and then a con-
siderable correction had to be applied. The major part of
this depended upon the expansion of the mercury with heat.
In calculating this, care was taken to ascertain the distri-
bution of temperature, as determined by the law of conduction,
in the tube connecting the two limbs; and the constants
required for ascertaining this distribution were determined by
special experiment. A smaller part of the correction arose
from the difference in the capillary depressions in the two
limbs. It seemed safe to assume that this correction would
be constant for all the temperatures under consideration; so
that it was simply determined by measuring the difference
of level when both limbs were at the same temperature. Of
the pressures measured by this method, only those which
relate to temperatures above 150° were retained for the final
calculations: these were reduced to three mean values, which
are given in the table below and are marked by asterisks.
The observations below 150° were rejected because the correc-
tions were here much larger than the quantities to be observed,
so that the results were uncertain. For example, at 137°4
the pressure was found to be 1.91 mm.; but here the cor-
rection was +249 mm. and the amount directly observed
only - 0.58 mm. Allowing for these unfavourable condi-
tions the rejected observations are found to agree sufficiently
well with the values obtained by the second method and
given as correct. They never differed from the latter by more
than 0.2 or 0.3 mm. They lay between these and Regnault's
values; but were twice or three times as far from Regnault's
values as from my own final ones.
The following method was adopted as much more suitable
for measuring the lower pressures. The open limbs of two