
of Mercury at Ordinary Temperatures. 665 
Plaats are right there would have been no difficulty in detect- 
ing it even at 30°. 
‘[ have made two series of determinations of the vapour- 
pressure of mercury, one before the publication by van der 
Plaats, and one recently. A known velume of a dry and inert 
gas was passed through a weighed quantity of mercury in 
such a way as to saturate it with the vapour. The volume 
of the gas when reduced to the temperature of the mercury 
gave the volume of the saturated vapour ; the loss of weight 
by the mercury gave the weight of this saturated vapour. 
From this can ‘be computed the pressure of the vapour with 
sufficient approximation. 
In the earlier observations the mercury was kept in a 
room whose temperature changed but little, and this tempera- 
ture was learned from the trace made by a registering ther- 
mometer. The mercury had been repeatedly treated with 
nitric acid and repeatedly distilled in a vacuum. It was put 
in a spiral absorption-tube of the form devised by Winkler. 
The counterpoise was made up of an equal volume of glass 
and an equal volume of mercury. Carbon dioxide was 
obtained from a drum containing the liquid, dried with phos- 
phorus pentoxide, and passed through the mercury at the 
rate of not more than two litres an hour. The gas escaping 
from the absorption apparatus was measured with an experi- 
mental gas-meter. Hach experiment continued about two 
weeks ; the loss of weight of mercury in this time was from 
2°5 me. to 4:0 mg. The values obtained for the vapour- 
pressure of mercury at a temperature a little below 16° C. 
were 0°0009 mm., 0:0010 mm., 0:0012 mm., and 00010 mm. 
This being sufficient for my purpose at the time, the experi- 
ments were discontinued. 
Recently they have been resumed and carried to tempera- 
tures as high as 70°. In this series, a large vessel of water was 
kept at a constant temperature by means sof a thermostat, the 
water being thoroughly stirred” by a small screw-propeller 
driven by an electric or a hydraulic motor. In the water 
was a metallic enclosure containing the two Winkler absorp- 
tion-tubes filled with mercury ; the mercury was purified as 
in the earlier experiments. Carbon dioxide was produced 
from hydrochloric acid and marble, washed in a saturated 
solution of sodium acid carbonate, and dried with phosphorus 
pentoxide. The rate of flow was regulated by fusing into 
the glass part of the connecting tubes a capillary tube of 
such diameter and length as was found suitable, since a stop- 
cock changes its rate of delivery too quickly. After the 
gas had passed through the two absorption apparatus, its 
