210 Wisconsin Academy of Sciences, Arts and Letters. 
In this later work effort in two directions is clearly discern¬ 
ible. (1) Those who have been favorable to van’t Hoff’s hy¬ 
pothesis of solutions (based on the analogy between gases and 
solutions) according to which the osmotic pressure, so-called, is 
due to the bombardment of the semipermeable membrane, by 
the dissolved molecules, have sought either to compare osmotic 
pressures of aqueous solutions with each other, or to measure 
directly the osmotic pressure of certain solutions in the hope of 
securing data to uphold the theory. It is true, however, that 
considering the vast importance of direct measurements of os¬ 
motic pressure for the van't Hoff theory of solutions, but little 
effort has been made to measure osmotic pressures directly. 
This has come about very largely because of the attitude taken 
in the matter by the main adherents of the van’t Hoff theory, 
who voiced and continually supported the dogma that the os¬ 
motic pressure is necessarily independent of the nature of the 
membrane if it be semipermeable; and that since it is very diffi¬ 
cult to measure osmotic pressures directly, it is better to con¬ 
tent one’s self with the so-called “indirect” measurements of 
osmotic pressure, namely, with a computation of the latter from 
vapor tension, freezing-point or boiling-point observations on 
solutions, which, be it remembered, involves the assumption that 
the gas laws hold for solutions. And so we have the rather re¬ 
markable situation that direct measurements of osmotic pres¬ 
sure, and indeed the general investigation of osmosis, has not 
only been neglected by the chief advocates of the gas theory of 
solutions, but they have in addition through the attitude they 
have taken actually discouraged work in this direction. They 
have even claimed to have proven by thermodynamics that the 
osmotic pressure must be independent of the nature of the 
membrane provided the latter is semipermeable. The assump¬ 
tions made in such “proofs,” and the fact that there is in real¬ 
ity no such thing as a semipermeable membrane in the strict 
sense of the word, have been passed over lightly. (2) Quite a 
different direction in the investigation of osmotic phenomena 
has been taken by those who have held van’t Hoff’s conception 
of the nature of osmotic pressure to be untenable. These men 
have continually brought forward experiments, of a qualitative 
