222 Wisconsin Academy of Sciences , Arts and Letters. 
tribute the incoming ether throughout the cell and present a 
fresh surface to the layer B. When stirring is omitted, this 
work of keeping the contents of B of uniform concentration has 
to be performed by diffusion, a process which is very slow, and 
consequently a lower pressure is obtained. It is clear that for 
similar reasons the contents of B ought in general also to be 
stirred in attempting to measure the maximum osmotic pres¬ 
sure, as should also the liquid C. But when B is quite thin, 
as is the case with some membranes, stirring its contents is of 
less consequence. Furthermore, when practically very little of 
A enters C (i. e. when B is a so-called semi-permeable septum) 
it is obviously not so necessary to stir the contents of C in 
osmotic pressure measurements. But the liquid in the osmotic- 
cell must always be stirred, though it is recognized that in 
some forms of cells this is far more imperative than in others. 
Attempts to make direct measurements of osmotic pressures- 
without stirring the contents of the cell are comparable with 
attempts to make a saturated solution of a salt by placing an 
excess of it in a dish, pouring the solvent upon it and allowing 
the whole to stand without agitation. The method of stirring 
the contents of the osmotic cell will be described below in con¬ 
nection with the quantitative measurements of osmotic pres¬ 
sures. 
QUALITATIVE EXPERIMENTS. 
The following experiments which are largely of a qualitative 
nature were performed in the course of the investigation in 
order to determine the influence which the character of the 
membrane has upon the permeability of the latter, and also 
upon the main direction of the osmotic current. Unless other¬ 
wise stated, the osmometer used was a very simple one, con¬ 
sisting of an ordinary thistle tube, the mouth of which was 
closed with the membrane employed. The latter was tied on 
with thread, in which process great care was used to secure a 
perfect contact between the glass and the membrane. To ac¬ 
complish this a thistle tube was selected with a flare of about 
forty-five degrees at the mouth. On such a tube the mem- 
