OSMOTIC PRESSURES DERIVED FROM VAPOUR-PRESSURE MEASUREMENTS. 299 
the reading of the mercury level in the piezometer. Direct observation showed that 
when a certain rate of increase of pressure was exceeded, the mercury thread might 
move rapidly enough to leave some of the solution behind clinging to the walls of 
the capillary, and this was the more marked the stronger the solution.* To decide 
the maximum rate of change of pressure that would be safe, experiments (which 
are not detailed) were made with the most viscous solution (960 gr. of cane sugar 
per litre) about to be examined. Judging by the information thus obtained, the 
method of changing the pressure described below seems free from objection. 
A light metal cup was placed on the top of the floating weights of the Schaffer 
and Budenberg gauge. Above this was suspended a hollow cylinder with a conical 
end truncated so as to leave a horizontal opening of about 0'8 cm. in diameter. 
When it was desired to increase the pressure, a quantity of small lead shot equivalent 
in weight to one of the large weights supplied with the pressure apparatus was run 
into the cylinder. On opening the orifice the shot were delivered at a constant rate 
into the metal cup below, f 
When all the shot had run through, a needle valve, situated so as to cut off the 
compressibility vessel from the pressure apparatus, was closed ; and the metal cup 
with its contents was replaced rapidly by one of the weights, after which the needle 
valve was opened. This process was repeated until the mercury in the piezometer 
stem showed in the field of view of the adjacent window above. It was found that 
the pressure could be lowered slowly enough by means of two needle valves in 
series. 
Method Followed in the Determinations of the Compressibilities .—In the present 
research no attempt was made to get air-free solution. In our work on calcium 
ferrocyanide we found that this precaution was unnecessary. 
The method followed was to fill the piezometer by gentle suction, taking care that 
no visible bubbles of air were left behind. When full, the piezometer was inverted 
and some mercury run into the open tube above the tap, which, when cautiously 
opened, allowed a thread to flow downwards into the capillary (CD of fig. 2), the flow 
being stopped when the meniscus reached a part of the capillary known to be visible 
through the upper window (C of fig. l). 
If the determinations were to be made at 0° C., tlie piezometer when full was 
placed with its stem immersed in the solution, and the twin bulbs were then cooled by 
an ice pack until a temperature slightly above that of the bath had been reached; 
* This effect is partly dependent on viscosity ; for where some solution has been left in the annular 
space between the mercury and the walls, this annular space is in effect a capillary tube through which 
the liquid has to be forced by the hydrostatic pressure of the mercury acting at the bottom of the enclosed 
solution. 
t The rate at which the shot fall is independent of the head of shot and depends on the diameter 
of the orifice. The correct diameter was found by trial, and was such as to cause the mercury in the 
stem of the more sensitive piezometer to rise 1 mm. in 25 seconds. 
