1898-99.] Prof. C. Brown on Nernsfs “ Osmotic Experiment." 439 
On Nernst’s “Osmotic Experiment” and a Definition 
of Osmotic Pressure. By Prof. Crum Brown. 
(Read February 6, 1899.) 
In vol. vi. of the Zeitsclirift fur Physikalische Chemie (1890), 
pp. 16-36, Nernst demonstrates the relation between the osmotic 
pressure of a given solution of X in A and the difference of 
concentration of two solutions of A in B, the one made by shaking 
up B with A and the other by shaking up B with the solution of 
N in A; where A and B are two liquids miscible with each other, 
but not in all proportions, as, for 
instance, water and ether, and N a 
substance soluble in A but not in B. 
Immediately after this paper, Hernst 
describes ( l.c ., pp. 37-40) an osmotic 
experiment in which the “ semiperme- 
able membrane ” is a layer of the 
liquid B held in its place by capil- 
larity. Through this layer no hi can 
pass, because hi is insoluble in B, but 
A will pass from what we may call 
the A side, on account of the concen- 
tration gradient, the layer of B con- 
taining more A dissolved in it on the 
A side than on the solution side. At 
the same time a pressure is developed 
on the solution side equal to the osmotic 
pressure of the solution of 1ST. 
So far as the diffusion of A through 
the layer of B from the A side to 
the solution side is concerned, Hernst’s experiment can be shown 
without fixing the layer of B. In the form exhibited to the 
Society, A is water, B phenol, and hi calcium nitrate. The 
solution was taken of such concentration that its density is con- 
