218 Wisconsin Academy of Sciences, Arts and Letters. 
ether—chloroform solution in A for additional ether is insuf¬ 
ficient to extract further ether from the aqueous layer B. In 
other words, the affinity of water for ether aided by the hydro¬ 
static pressure developed in A (which militates against the 
ether passing into A) just balances the affinity of the chloro¬ 
form—ether solution in A for additional ether. And yet the 
ease is„ after, all not quite so simple, for it must, be remembered 
that chloroform is somewhat soluble in water saturated with 
ether at atmospheric pressure; now this solubility is increased 
with increase of pressure, so that as the hydrostatic pressure 
in compartment A increases due to the influx of ether, the out¬ 
flow of chloroform is continually slightly increasing, which 
tends, of course, to relieve the pressure. The actual final max¬ 
imum pressure reached is therefore determined by the relative 
influx of ether into compartment A and the outflow of chloro¬ 
form from that compartment. When this outflow is practically 
nil, or at any rate very slight, we should be dealing with a so- 
called semi-permeable membrane; when the outflow is not a neg¬ 
ligible quantity, as in the case of most septa, the final maximum 
pressure attained is materially influenced thereby. 
Now, it is easy enough to imagine the aqueous layer B firmly 
held in place as we have done, but to realize this experimen¬ 
tally presents great difficulties. It may, however, readily be 
demonstrated that such hydrostatic pressure is actually pro¬ 
duced, without to be sure making an attempt to furnish any¬ 
thing more than a qualitative proof that such pressure is really 
formed. The apparatus used for this purpose is shown in Fig. 
3. In this glass tube, D represents mercury, A, chloroform, 
C, ether, and B is a slice of an excellent piece of cork which 
had been kept under boiling water for some time so as to drive 
the air out of it and thoroughly inject it with water; during 
this process much soluble material was also extracted from the 
cork. The cork thus thoroughly soaked with water was firmly 
pushed into position. It is, of course, somewhat difficult to 
avoid having air bubbles just above and below the cork, but 
by careful manipulation it is possible to secure the arrangement 
as shown in Fig. 3. The chloroform and ether used in this case 
were each first saturated with water, S 9 that they would not 
