80 



SCIENCE 



[N. S. Vol. LIII. No. 1361 



"attraction" of the Jf/192 solution of 

 Na^SO^ for water had to have a concentra- 

 tion of about or over Jlf/4. If the gas pres- 

 sure effect alone determined the relative at- 

 traction of the two solutions for water the 

 concentration of the sugar solutions required 

 to osmotically balance the M/192 solution of 

 NagSO^ should have been 2lf/64 (or slightly 

 less). Hence the sugar solution balancing 

 osmotically a M/192 ISTa^SO^ solution was 

 found to be 16 times more concentrated than 

 the theory of van't Hoff demands. This high 

 concentration of cane sugar was needed to 

 overcome the powerful " attractive " influence 

 of the anions of a M/192 solution of ISTa^SO^ 

 for the positively charged particles of water. 

 Table I. shows the results of a few such ex- 

 periments. The solution of the electrolyte 

 was in these experiments always theoretically 

 isosmotic with a ilf/64 cane sugar solution 

 (on the assumption of complete electrolytic 

 dissociation). The data contained in Table I. 

 have only a qualitative value since no attempt 

 at an exact determination of the concentra- 

 tion of the balancing sugar solutions was 

 made. The data show, however, that the 

 "attraction" of ilf/128 KCl for positively 

 charged particles of water is eight times as 

 great, that of K^SO^ sixteen times as great, 

 and that of ilf/256 Kg citrate almost forty-eight 

 times as great as that of lf/64 cane sugar; 

 while the " attraction " of a lf/192 solution of 

 a salt with a bivalent cation and monovalent 

 anion, like MgClj, for water is not greater 

 than that of a M/64: solution of cane sugar. 



These experiments then prove that the rate 

 of diffusion of water from the side of pure 

 water through a collodion membrane into a 

 solution of an electrolyte increases with the 

 valency of the anion and diminishes with the 

 valency of the cation. They give also a 

 rough idea of the relative influence of these 

 ions upon the rate of diffusion of positively 

 charged water through the pores of the collo- 

 dion membrane from the side of pure water 

 to the side of the solution. 



A second fact brought out in these experi- 

 ments was that the relative influence of the 

 oppositely charged ions of an electrolyte in 



solution upon the rate of diffusion of posi- 

 tively charged water from the side of pure 

 water to the side of the solution is not the 

 same in all concentrations. Beginning with 

 the lowest concentrations the " attractive " 

 effect of the anion for positively charged 

 water increases more rapidly with increasing 

 concentration than the " repulsive " effect of 

 the cation until the concentration of the 

 electrolyte is about lf/256; from then on the 

 " repulsion " of the cation upon positively 

 charged water increases more rapidly than the 

 " attractive " effect of the anion. As a con- 

 sequence we can say that in concentrations of 

 neutral salts between ilf/256 and M/8 the 

 " attraction " of the solution for water dimin- 

 ishes with increasing concentration. This is 

 the reverse of what we should expect if the 

 gas law alone determined the attraction of 

 water by solutions of electrolytes. When the 

 concentration of the solution is M/8, the ap- 

 parent electrostatic effects of the ions upon 

 the positively charged particles of water dis- 

 appear and for concentrations above M/8 the 

 curves for the attraction of water by electro- 

 lytes and by sugar solutions show less differ- 

 ence. 



We have already mentioned the fact that 

 the valency of the ion is not the only quan- 

 tity which determines its influence on the 

 rate of diffusion of water through a collodion 

 membrane. In addition to the valency (or 

 the nimiber of electrical charges) a second 

 quantity of the ion enters which may be 

 designated provisionally as the influence of 

 the radius of the ion. In the case of mono- 

 valent and nionatomic cations the retarding 

 influence on the rate of diffusion of positively 

 charged particles of water through the collo- 

 dion membrane from the side of pure water 

 into a solution increases inversely with the 

 radius of the ion, namely in the order 

 Li > Na > K > Eb, where the retarding effect 

 is greatest in the case of Li and least in the 

 case of Rb; while in the case of monatomic 

 monovalent anions the accelerating effect 

 upon the rate of diffusion of jwsitively 

 charged particles of water from the side of 

 pure water through the membrane into the 



