[SHAW] IMPROVED METHODS IN HYGROMETRY 
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sample of air.of known volume can be drawn in through the three-way 
stop cock at the top on the right by lowering the mercury on the left. 
When this tap is turned the sample can be transferred to the upper 
bulb on the right and brought into contact with concentrated sul- 
phuric acid which floats above the mercury on that side. This oper- 
‘ation is performed simply by raising the mercury on the left and lower- 
ing that on the right. In the middle is a tube and stop cock leading 
to a xylol pressure gauge. The extra bulb connected to the other side 
of the gauge enables errors due to small temperature or pressure 
variations to be eliminated. To measure the original vapour pressure 
of the enclosed sample it is necessary to adjust the mercury level on 
the right to the known position it would have taken if a dry sample 
had been maintained at constant pressure, and at the same time to 
adjust the mercury on the left until the pressure as indicated by the 
gauge is the same as that of the extra compensating bulb. The 
absorption will then be indicated by the rise of the mercury in the 
tube above the bulb. From this the external vapour pressure can 
at once be calculated. x 
The whole apparatus can be constructed and calibrated by an 
amateur glass-blower in a few hours. It requires only four equal 
pipettes (20-50 cc.), three stop cocks, tubing, mercury, sulphuric 
acid and xylol. Once installed the acid will last for several hundred 
determinations and each observation requires only from 2 to 4 minutes. 
The calculation of a result is very simple. 
If x is the existing vapour pressure, v the volume of one cm. 
of the tube up which the contraction is arranged to take place, V 
the volume of the sample drawn in, b the barometric pressure, and 
r the height of the contraction, then applying Boyle’s Law to the 
volume of dry air we see that 
(b — x) V = b(V — vr). 
= 
hence x =' — Dr: 
V 
vi, : : 
The constant V can be determined when the apparatus isconstructed, 
and b and r read from the barometer and the instrument itself. With 
the aid of curves for different pressures, or with the slide rule, the 
vapour pressure can be calculated from the reading in a few seconds. 
A series of comparisons with the chemical method of weighing 
gave, when precautions were taken to use samples of air of the same 
humidity, no case of a divergence as great as 2%. The majority 
of observations were within 1% whether the temperature or humidity 
was high or low. 
