OSMOTIC PRESSURES DERIVED FROM VAPOUR-PRESSURE MEASUREMENTS. 297 
r 
D 
C 
to pressure apparatus 
steel tube 
1 
tri 
piezometer. The windows are tapered plugs cut from one-inch plate glass ; they were 
inserted into dermatine washers moulded inside and outside to the same taper. The 
openings for the windows in the casting having been machined to this taper (large 
end inwards), the glass together with its 
dermatine wrappers was placed in position 
from the inside and pushed home as well as 
could be by means of wedges. On putting a 
high hydrostatic pressure on the apparatus a 
final “ leak-tight ” seating was secured. 
The Bath .-—The whole apparatus (about two 
feet high) is immersed in a water-bath, which 
is furnished with a thermostat and stirrer; 
submerged electric lamps supply heat, and 
two of the lamps are under the control of 
the thermostat. The front of the bath is 
fitted with a plate-glass window, so that the 
graduations on the piezometer stem are seen 
clearly when a small 4-volt lamp is placed 
behind the apparatus. 
The same bath, divided into two compart¬ 
ments by a loosely fitting vertical partition, 
was used for the observations at 0° C. One 
of these compartments contained ice, and a 
constant stream of cold water was pumped 
from this compartment into the other. During 
an experiment, which took some hours, care 
was taken that the. ice reached nearly to the 
bottom of the bath, and the supply was main¬ 
tained by hand. The temperature variations 
were observed by means of a Beckmann ther¬ 
mometer reading to 0°'01 ,C. (estimating to 
0°'001 C.) in the bath, and another reading 
to 0°'02 C. in the dome of the apparatus. 
Piezometers .—Piezometers similar to those 
of our earlier research were used in apparatus 
A, but in the course of the work we found 
that there were two drawbacks to this form : 
(l) the somewhat large bulb took a very 
considerable time to return to the temperature 
of the bath after having been heated or cooled by the alterations in pressure (at 
0 C. this effect is small); (2) it was found that for a given pressure on the 
2 e 2 
F 
dermatine 
ring 
F 
Fig. 1. 
