440 
CHEMISTRY: /. LOEB 
In figures 2 and 3 the crosses indicate the position of stars of the 
sub-types BO, Bl, and B2, and the dots refer to the sub-types B3 and 
B5. Stars of the first group of sub-types are beheved to be intrinsi- 
cally brighter than those of the second. For a given apparent mag- 
nitude, therefore, their distances are greater — possibly even twice as 
great. It is to be noted that the crosses have completely left the sec- 
ondary circle in figure 3, indicating that the local cluster is not large 
enough to include objects as distant as Bl stars must be when they 
appear fainter than magnitude 7. 
^ Shapley, Harlow, these Proceedings, 4, 1918, (224-229); Mt. Wilson Communications, 
No. 54. See also Mt. Wilson Co?itr., No. 157, and a small modification of the original state- 
ment of a star-streaming hypothesis, Mt. Wilson Contr., No. 161, section IX. 
2 For other properties of the local cluster and for a discussion of the peculiar value of 
B-type stars in describing its extent, form, and orientation, reference may be made to Mt. 
Wilson Contr., No. 157, part II. and No. 161, section IX. 
3Stromberg, Gustaf, Astrophys. /., Chicago, III., 47, 1918, (7-37); Mt. Wilson Contr., 
No. 144. 
INFLUENCE OF IONS ON THE ELECTRIFICATION AND RATE 
OF DIFFUSION OF WATER THROUGH MEMBRANES 
By Jacques Loeb 
The Rockefeller Institute for Medical Research, New York 
Communicated, August 27, 1919 
1 . When pure water is separated from a watery solution by a strictly 
semipermeable m^embrane more molecules of water will diffuse through 
the membrane from the pure solvent to the solution than will diffuse 
simultaneously in the opposite direction; and this difference in the rate 
of diffusion of water in the two opposite directions will be the greater 
the higher the concentration of the solution. When the solution is put 
under pressure, the number of molecules of water diffusing in the unit 
of time from the solution into the pure solvent will be increased and if 
this pressure reaches a certain value the number of molecules of water 
diffusing simultaneously in opposite directions through the membrane 
will become equal. We may therefore define the osmotic pressure of the 
solution as the additional pressure which has to be applied to the solu- 
tion in order to cause as many molecules of water to diffuse from the 
solution to the pure solvent as will diffuse simultaneously in the oppo- 
site direction. 
van't Koff's theory of osmotic pressure assumes that this quantity de- 
pends exclusively upon the concentration of the solution. For some 
