IX
205
VAPOUR-PRESSURE OF MERCURY
In the above table the third and fourth columns are added
so as to make it possible to compare the values calculated by
the formula with the observed values. The third column
gives the errors which must have occurred in the pressure
measurements if the observed temperatures are correct. The
fourth column gives the errors which must be attributed to
the temperature measurements if the pressures are to be
regarded as correct. It will be seen that the formula repre-
sents the observations completely, if we admit an uncertainty
of 0.02 mm. in the pressure measurements and of 0°6 in the
temperature measurements; and the disposition of the devia-
tions shows that such uncertainties must be admitted. The
measurements made below 89° agree perfectly with the formula,
as far as a comparison is possible. The following table is
calculated by means of the formula, and gives the pressure of
the vapour for every 10° between 0 and 220°-
2
P
p
P
0º
0.00019
60⁰
0.026
120⁰
0.779
1800
9.23
10
0.00050
70
0.050
130
1.24
190
13.07
20
0.0013
80
0.093
140
1.93
200
18.25
30
0.0029
90
0.165
150
2.93
210
25.12
40
0.0063
100
0.285
160
4.38
220
34.90
50
0.013
110
0.478
170
6.41
It should be noted that p = 0 when t=
=
-
273°; and that
the formula gives for the internal latent heat of the vapour
the value pr
76.15 0.0183T. The values given above
differ considerably from Regnault's as well as from Hagen's.
They are always smaller than Regnault's, but approach the
latter as the temperature rises, and almost coincide with them
at 220°. Compared with Hagen's they are smaller below
80°, nearly coincide between 80° and 100°, and above this are
larger.
The most interesting point is the pressure of the vapour
at the ordinary temperature of the air. According to the
results of our investigation this amounts to less than a
thousandth of a millimetre.¹ Hence no correction need be
1 It might be objected that this value is only calculated; whereas both the pre-
vious observers made observations at the temperature of the air, and both believed
that they perceived a pressure of a few hundredths of a millimetre. But the