2l6 
CHEMISTRY: J. LOEB 
Proc. N. a. S. 
tions the reverse occurs. The turning point Hes for a number of electro- 
lytes at a molecular concentration of about M/256. Positively charged 
particles of water in the pores or interstices of the membrane will be driven 
to that side of the membrane which is more negatively charged. Since 
this is usually the solution side, water will be driven from the side of pure 
water into the solution. ' When, however, the membrane has a higher 
negative charge on the side of pure water, the flow of the positively charged 
liquid through the membrane will be from the side of the solution to that 
of pure water (negative osmosis). This occurs in the case of solutions of 
(CaOH)2 and Ba(0H)2 when the concentration exceeds M/256. 
When the collodion membrane has been treated with a protein, it is 
also generally negatively charged except when the solution contains hydro- 
gen ions or simple trivalent or tetravalent cations beyond a certain con- 
centration (whicn for H is 10"^ N) ; in this case the membrane is positively 
and the watery phase is negatively charged. If we add in this case an 
electrolyte to the water the charge on the membrane is increased by the 
cations and diminished by the anion of the electrolyte, both influences 
increasing with the valency of the ion, and another property which is 
still to be defined. In lower concentrations the influence of the cation in- 
creases more rapidly than that of the anion, in higher concentrations of 
the electrolyte the reverse is true. Whenever the positive charge on the 
solution side of the membrane is greater than on the opposite side, the 
negatively charged particles of water will diffuse from the side of pure 
water to the side of solution. When, however, the positive charge on the 
solution side of the membrane is less than that of the side of pure water, 
the negatively charged layer of liquid will flow through the membrane 
from the side of the solution to that of pure water (negative osmosis). 
This will happen especially in the case of dibasic or tribasic acids when the 
concentration exceeds a critical limit. (Since the collodion membrane is 
permeable for electrolytes we are dealing in reality not with pure water 
separated by the membrane from solution, but with the condition of weak 
solution separated from a more concentrated solution.) 
It is obvious that with increasing concentrations of the solution the op- 
posite effects of the oppositely charged ions of an electrolyte on the density 
of charge of the double layer tend to become equal and then the gas pres- 
sure effect becomes the main or only driving force for the diffusion of water 
into solution. For lower concentrations of electrolytes the algebraic sum 
of both forces, the differences of potential on both sides of the membrane 
and the gas pressure effect, determine the rate of diffusion of water through 
a membrane into a solution. 
The new experiments on which these conclusions are based will appear 
in the Journal of General Physiology. 
1 Morse, H. N., "The osmotic pressure of aqueous solutions". Carnegie Inst. Washing- 
ton, Puhl., 198, 1914. 
