IX
203
VAPOUR-PRESSURE OF MERCURY
their errors were negligible compared with those which arose
in determining the temperatures. Groups of eight to twelve
separate observations, lying sufficiently close to each other,
were then formed, the mean temperature being simply associ-
ated with the mean pressure. The six principal values thus
obtained, together with the three determined by the first
method, are given in the first two columns of the following
table. The subsequent calculations are based upon the results
given in these columns.
P
Ap
At
Sp
Δε
89.4 0.16
117.0 0.71
0.00
+0.04
0.0
+11
*184.7 11.04
+0.15 +0.4
190.4 12.89
-0.37 -0.8
154-2 3:49
+0.01 +0.1
203.0 20.35
+0.23 +0.3
*165.8 5.52 +0.04 +0.2 *206.9 22.58 -0.20 -0.3
177-4 8.20 -0.22 -0.7
In calculating out the experiments I have made use of a
formula which has not hitherto been employed for the same
purpose.¹ It can be theoretically justified and must be
correct to the same degree of approximation that the laws of
Gay-Lussac and Boyle, which apply to very dilute vapours, are
correct for saturated vapours. On the assumption that this
law holds good, the vapour possesses a constant specific heat
at constant volume. Let this be denoted by c; further let s
denote the specific heat of the liquid, and p, the internal heat
of vaporisation at the absolute temperature T. Then it
necessarily follows from our assumption that p₁ = const-
(sc)T. This can be proved as follows. Let a quantity of
the liquid at temperature T be brought to any other tempera-
At this temperature it is converted into vapour with-
ture.
Pr
1 An analogous formula, deduced by similar reasoning, has indeed been
used by Koláček (Wied. Ann. 15, p. 38, 1882) for representing the pressure of
unsaturated water-vapour upon salt solutions. In that case the theoretical
justification of the formula is much stronger than in ours, where its appli-
cability is only really proved by comparing it with the results of experiment.
With regard to Koláček's investigation, I may remark that all the experimental
data are known for applying the above formula to the pressure of vapour above
ice and above water cooled below its freezing-point down to the absolute zero.
Such an application would have to be justified by proving that the formula
obtained represents with satisfactory approximation the pressure of the vapour
for a considerable interval above 0°. For if the formula holds good for a given
interval of temperature, it must hold good for all temperatures below this interval,
inasmuch as a saturated vapour approximates more and more to a perfect gas as
the temperature diminishes.