272 Wisconsin Academy of Sciences , Arts and Letters. 
osmosis does occur, these factors also determine the direc¬ 
tion of the main current and the magnitude of the pressure 
developed. The motive force in osmotic processes lies in the 
specific attractions or affinities between the liquids used, and 
also those between the latter and the septum employed. 
These attractions or affinities have also at times been termed 
the potential energy of solution, etc., they are to the mind of 
the writer essentially the same as what is commonly termed 
chemical affinity. 
It has been emphasized that osmotic pressures , are equi¬ 
librium pressures, and that in osmotic processes there is al¬ 
ways a current in both directions; though the main current 
may in specific cases be so much stronger than the minor that 
the latter almost sinks into insignificance. In such cases the 
septum is termed “semipermeable.” 
Vulcanized caoutchouc has been found to be a “semiperme- 
able” membrane when it separates pyridine solutions of sil¬ 
ver nitrate, lithium chloride, and cane sugar from the pure 
solvent. 
The necessity of stirring the contents of the osmotic cell 
and also the outer liquid during osmotic pressure measure¬ 
ments has been pointed out, and a new apparatus for 
measuring osmotic pressures accordingly nas been devised. 
The results of the osmotic pressure measurements show 
that the gas laws do not hold; and it has consequently been 
pointed out that the latter can not serve as a basis for a sat¬ 
isfactory theory of solutions. 
The advantage of stirring in processes of dialysis has been 
indicated; and it has been shown that whether substances 
pass through membranes or not does not depend upon their 
colloidal or crystalloidal character, but solely upon their 
affinity for the membrane employed and for the liquids that 
bathe it. 
In conclusion I desire to thank Messrs. F. L. Shinn, J. H. 
Mathews, Wm. Marquette and IT. E. Eggers for assistance 
which they have kindly rendered me from time to time in 
the experimental part of the work. 
Laboratory of Physical Chemistry , 
University of Wisconsin , Madison. 
