THE COMBINING VOLUMES OF HYDROGEN AND OXYGEN. 
403 
(1’6 cm.), was also provided with a glass point. The vertical distance between the 
glass points was very nearly 760 mm., and this distance was maintained constant by 
means of a stout glass rod sealed at its upper end to the manometer chamber and at 
its lower to the elbow of the capillary connecting the bulb and the 3-way tap. The 
whole apparatus was rigidly fixed to a heavy iron bar with a tripod base, and the top 
of the bar was clamped to a staple driven into the ceiling. Any displacement from 
the vertical could be detected by the plumb-line, E, E, and corrected by means of 
levelling screws in the tripod base. The manometer head communicated with a 
mercury pump through the tap F, and the phosphoric oxide tube, G, so that the vacuum 
could be verified from time to time. Sealed to the capillary between the bulb and 
the 3-way tap was a capillary T-piece expanding into a tube of a few cubic centimetres 
capacity, H, which terminated at its lower end in another capillary, furnished 
with a tap, J. This tube, which was normally filled with mercury to the top, may be 
conveniently referred to as the volume-adjuster, since the capacity of the pipette could 
be increased to a small extent by withdrawing mercury from the tap, J. 
The bulb, the volume-adjuster, and a portion of the manometer tube were enclosed 
in a tinned-iron bath with a stout brass base. The upper portion of the manometer 
was also enclosed in a bath constructed of a wide glass tube closed at the bottom by 
a rubber cork through which the glass rod and manometer tube passed. The lower 
bath somewhat overlapped the upper, so that by filling both baths with melting ice 
the whole system could be cooled to 0° C., with the exception of the dead-space and 
a piece of the capillary above it and the short column of mercury in the manometer 
from K to L. 
On account of the difficulty of seeing the point it was inpracticable to surround 
the dead-space with ice. The small bath, M, was therefore filled with cooled brine by 
means of the funnel, N. It was found quite easy to secure a temperature within 0°'5 C. 
of zero when the final setting was made. Because of the small volume of the dead-space, 
(almost exactly 1 c.c.) a variation of this order did not affect the results. The mercury 
required for displacing gas from the bulb was contained in the reservoir, O. An air- 
catch, P, protected the pipette from any air leak at the rubber junction. 
Calibration of the Pipette. 
The volume of the bulb, dead-space, and connecting capillaries, from the tap, C, to 
the level of the glass point in the dead-space, was calibrated by means of mercury 
after the apparatus had been set up, but before the side tube leading to the manometer 
had been sealed on. For this purpose a stop-cock on capillary tubing was temporarily 
fused to the glass tubing below the dead-space. 
The dead-space was calibrated separately. Mercury was introduced through the 
stop-cock until the meniscus was set to the point, the dead-space and tubing below it 
3 I 2 
