Januabt 28, 1921] 



SCIENCE 



79 



first mimites accidental irregularities are 

 liable to interfere with the result, we measure 

 the rise in the level of liquid in the mano- 

 meter during the first 20 minutes. 



If the initial rise of level of liquid in the 

 solution is thus measured it is noticed that it 

 occurs approximately in proportion with the 

 concentration of the solution when the solute 

 is a non-electrolyte. The rate of diffusion 

 of pure water into a solution of cane sugar 

 through a collodion membrane is therefore 

 approximately a linear function of the con- 

 centration of the solute within the limits of 

 O and 1 M. This is what we should expect 

 on the basis of van't Hoff's theory of osmotic 

 pressure. 



If, however, a watery solution of an electro- 

 lyte is separated from pure water by a collo- 

 dion membrane, water diffuses into these 

 solutions as if its particles were positively 

 charged, and as if they were attracted by the 

 anion of the electrolyte in solution and 

 repelled by the cation with a force increasing 

 with the valency of the ion (and another 

 property of the ion to be discussed later). 



Pure water diffuses into a M/1'28 solution 

 of !N"aCl through a collodion membrane more 

 rapidly than it diffuses into a M/Qi solution 

 of cane sugar; water diffuses into a ilf/192 

 solution of ]Sra,SO^ or Na, oxalate still more 

 rapidly than into a M/128 solution of NaCl; 

 and into a M/256 solution of Naj citrate 

 water diffuses more rapidly than into a il'//192 

 solution of Na,SO^, and into a M/S20 solu- 

 tion of ]Sra^re(CN)g still more rapidly than 

 into a Jf/256 solution of ISTaj citrate. As- 

 suming complete electrolytic dissociation of 

 the electrolytes in these cases, the influence 

 of the five solutions mentioned should be 

 identical according to van't Hoff's theory. 

 We notice, instead, that the " attraction " of 

 the solutions for water increases with the 

 valency of the anion. This is true for all 

 neutral solutions of salts contained in a collo- 

 dion bag, regardless of the nature of the 

 cation. 



If a collodion bag containing a neutral 

 solution of a salt with bivalent cation, e.g., 

 lf/192 CaClj or MgCU, or with a trivalent 



cation, eg., M/256 LaCl3, is dipped into a 

 beaker with pure water we notice no rise in 

 the level of water in the manometer. In 

 solutions with bivalent or trivalent cations 

 the repulsion of the cation equals or exceeds 

 therefore the attraction of the anion for the 

 positively charged particles of water diffusing 

 through the pores of the collodion membrane. 

 Hence we conclude from these (and numerous 

 similar) experiments that the particles of 

 water diffuse through a collodion membrane 

 as if they were positively charged and as if 

 they were attracted by the anion of an electro- 

 lyte and repelled by the cation with a force 

 increasing with the valency of the ion. 



It seemed of interest to find that concen- 

 tration of a cane sugar solution which just 

 suffices to prevent the diffusion of water into a 

 given solution of an electrolyte. Into each of 

 a series of beakers, all containing the same 

 neutral salt solution, e.g., M/192 Na^SO^, was 

 dipped a collodion bag containing a cane 

 sugar solution of different concentration, from 

 ilf/128 to 1 Jf, and it was observed in which 

 of these sugar solutions the level in the 

 manometer rose during the first 10 minutes, 

 in which it fell, and in which it remained 

 constant. It was found that the cane sugar 

 solution which was just able to balance the 



TABLE I 



Approximate Concentration of a Solution of Cane 

 Sugar Required to Balance the Osmotic At- 

 traction of the Following Solutions of 

 Electrolytes for Water 



Molecular Approximate Molecular Concen- 



Concen- Electrolyte tration of Balancing Solution 



tratlon Used of Cane Sugar 



M/128....K01 3f/8 



Jtf/192 K2SO4 Betweeai M/i and M/2 



M/192 K, oxalate . . . M/2 



M/192 kT tartrate. . . Slightly above M/2 



M/256 K3 citrate SUghtly a/bove 3M/4 



K/128....RbCa M/i 



lf/128....KCl M/8 



3f/128....NaCl M/8 



M/12S .... Lia SUgMly above M/32 



M/192. . . .MgCl^ M/6i 



M/192.... C&CL Belo.w M/Gi 



M/192 SrClj Below _ M/Gi 



M/192.... BaCa, " M/Gi 



M/192 .... CoOl'j Below M/Gi 



M/1^2 . . . .MnOU Below if/64 



