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THE EARL OF BERKELEY AND MR. E. G. L HARTLEY ON THE 
(2) All other direct methods relate to what may be called equilibrium pressures ; 
they depend on the measurement of the pressure to be applied to the solution to 
bring about a state of equilibrium between it and the solvent, when the two are 
separated by a semi-permeable membrane. The difference between the pressure on 
the solution and that on the solvent is the equilibrium pressure of the solution, under 
the conditions of the experiment. It is evident that, in general, for the same 
solution the equilibrium pressure will vary, on account of the compression of the 
solution and solvent, according to the pressure on the solvent. It is, therefore, 
necessary to state what pressure was on the solvent when the particular equilibrium 
pressure was observed. 
We have made some attempts to obtain comparative values for the osmotic 
“ force ” by a method analogous to that indicated in (1), but these attempts, although 
not entirely unsuccessful, were not satisfactory, and will not be alluded to further. 
The following account refers only to equilibrium pressures, observed when there was 
a pressure of one atmosphere on the solvent. 
General Description of the Method used. 
A gradually increasing pressure is placed upon the solution, which is separated 
from the solvent by a semi-permeable membrane, until the solvent, which at first 
flows into the solution, reverses its direction and is squeezed out. The pressure, 
when there is no movement of the solvent, is considered to be the sum of the 
equilibrium pressure and a pressure equal to that on the solvent. 
Owing to the difficulty of determining the exact point at which no movement 
takes place, and for other reasons, the experiments were arranged so as to enable an 
observation to be made of the rate of movement of the solvent both when the 
pressure on the solution was just below and when just above the turning-point 
pressure. The turning-point pressure was deduced from these two rates, or, 
sometimes, from a series of rates corresponding to slightly different pressures both 
above and below the turning-point. 
The Osmotic Apparatus. 
The apparatus used is shown in fig. 1. AB is a porcelain tube # , 15 centims. long, 
2 centims. external and 1*2 centims. internal diameter; the vertical ends are glazed. 
This tube carries the semi-permeable membrane as close to the outer surface as 
possible. CC is a gunmetal cage against the ends of which the dermatine rings DD 
are compressed, when the two parts E and F of the outer gunmetal vessel are 
screwed together. The ends of this cage have shallow radial grooves cut out of 
them, so as to prevent the dermatine rings from rotating and rubbing the membrane 
during the operation of screwing E and F home. The length of the cage is such that, 
when finally set up, the dermatine rings just overlap the ends of the porcelain tube. 
* The porcelain tubes are similar in all respects to those described in our preliminary communication. 
