202
IX
VAPOUR-PRESSURE OF MERCURY
A
FIG. 23.
B
manometers A and B (Fig. 23) communicate with one another.
They contain air of low pressure,
-about 10 to 20 mm. The closed
limbs are quite free of air. The
manometer A is kept in a water-
bath at the temperature of the room.
The manometer B was heated in a
vessel of thick cast-iron in a paraffin
bath, but never so far as to allow the mercury in the closed
limb to sink below the level in the open limb. Thus the
pressure of the mercury-vapour was smaller than the pressure
of the air present (at the time) in the open limb; so that no
evaporation, excepting by diffusion, could take place. Hence
the pressure in the open limbs of both manometers was the
same; and the difference of the readings of the two mano-
meters, reduced to mercury at 0°, gave the difference between
the saturation-pressure at the temperature of the hot mano-
meter and that of the cold one. But, according to the results
of this investigation, the pressure of the mercury-vapour in
the latter can be put equal to zero. The temperature of the
bath was read off on a very good Geissler thermometer; and
I compared the indications of this with a Jolly air thermo-
meter. The difference of level was measured by means of a
micrometer eye-piece in the cathetometer microscope. The
adjustment of the cross-wires upon the top of the meniscus
was facilitated by means of a wire grating placed behind
it, the wires being inclined at 45° to the horizontal. The
manometer tube had a clear bore of 20 mm. The pressure of
the air in the open limbs was varied. Lastly, after each heating,
I convinced myself afresh of the absence of air in the closed
limbs by producing electric discharges in them; the tubes
then exhibited a green phosphorescence, and only this, so that
the pressure of the air in them could not have exceeded one to
two hundredths of a millimetre. The result of the experi-
ments was as follows. Up to 50° I could perceive no pres-
sure exceeding the limits of error (0.02 mm.) of a single
experiment. At 60° the pressure was about
70° 0.05 mm., at 80° 0.09 mm. From here
were small compared with the whole values.
130° the observed pressures can be taken as correct, since
0.03 mm., at
on the errors
From 120° to