BETWEEN THE FREEZING AND BOILING-POINTS. 
203 
In this case \ 0 = ’001293 ; P = 760 ; p = the observed barometric pressure at the 
time of weighing ; p = the vapour-tension of water at the temperature of weighing. 
This latter correction for the water-vapour is small and amounts to about 2 parts in 
100,000 on the flow at the usual room temperature. In applying this correction, it 
was necessary to have the water enter the flasks at the temperature of the room, 
which was very nearly the temperature of the balance case. This was important, 
especially when the calorimeter was at a temperature very different to the room 
temperature. A cooler, consisting of a bath through which a constant stream of water 
could be made to flow, was arranged adjacent to the calorimeter, so that the outflowing 
water from the calorimeter was passed through a spiral of copper tube immersed in 
the water before passing through the switch-over device into the flasks. The tem¬ 
perature of the cooler was maintained near the temperature of the room by controlling 
the temperature of the stream of water by a gas flame. A stirrer w T as also fitted up 
for the bath. 
A small change in temperature of the cooler, during the time of an experiment, 
required a small correction to the flow. This depended only on the readings of a 
thermometer in the cooler-water just previous to the switching over of the flow into 
the weighed flask, and just after it was turned off from the flask at the end of the 
interval. If dt represents the change in temperature obtained from the two readings, 
v the weight of water filling the copper-spiral in the cooler, and - a the coefficient of 
expansion of water, then the correction to be applied to the flow is avdt. When dt is 
of the order of a degree, this correction is just negligible for the volume of the total 
length of copper-tube used, which contained about 22 grammes of water. 
Sec. 4.— Description of the Apparatus and Method of Making the Experiments. 
The Calorimeter .—A general plan of the calorimeter is shown in fig. 1 (p. 153). The 
first three calorimeters were made at Bonn, and sent out to the laboratory unexhausted. 
We exhausted the vacuum-chamber of two of these, but the third one was not used 
owung to the adoption in later experiments of a slightly different design. They all 
had the same dimensions, with a fine-bore flow-tube 2 millims. inside diameter and 
50 centims. long, which was fused at both ends to larger tubes 25 centims. long and 
1’8 centim. inside diameter. These larger tubes wmre sealed into the vacuum-jacket 
made from a glass-tube 4 centims. in diameter. The seal at each end was made at 
about the middle of the larger flow-tubes, at a point about 11 centims. from the end 
where the fine-bore tube was sealed on. Two side tubes were sealed into the larger 
tubes at each end, but were eventually done away wfith in the later design, with the 
exception of one on the inflow end. The vacuum-jacket was exhausted on a large 
five-fall Sprengel pump with a McLeod gauge for determining the vacuum, and con¬ 
nected to the pump by a side tube fused into the glass of the chamber. When 
exhausted sufficiently, to a vacuum of about ’002 millim. as shown by the gauge, the 
2 d 2 
