724 ELECTRIFICATION AND DIFFUSION OF WATER 



osmotic pressure was approximately the same and we shall see later 

 that the phenomenon in question is widely independent of differences 

 in the osmotic pressure of electrolytes. On the other hand, water 

 diffuses more slowly against a m/128 solution of CaCl2 than against 

 a m/64 sugar solution. 



These facts may serve as an illustration of our statement that in 

 the case of electrolytes with univalent and bivalent cation water 

 behaves as if it were positively charged and attracted by the anion 

 and repelled by the cation of the salt, and that both attraction and 

 repulsion increase with the number of charges of an ion. In the case 

 of NaCl the attractive action of CI is for some reason greater than the 

 repelling action of Na, hence the water diffuses more rapidly towards 

 m/128 NaCl than against m/64 cane sugar. It diffuses still more 

 rapidly against m/192 Na2S04 than against m/128 NaCl because the 

 SO4 ion has two charges united in one ion. In the case of CaCIo the 

 two positive charges united in the one Ca ion suffice to annihilate 

 practically completely the attractive action of the two negative CI 

 ions. As a consequence m/128 solutions of CaClo act almost like dis- 

 tilled water on the rate of diffusion of water. Solutions of MgCl2, 

 SrCl2, BaCl2, C0CI2, and MnCl2 act similarly to solutions of CaCl2. 

 Fig. 3 shows that the rate of diffusion of distilled water under a pres- 

 sure head of about 110 mm. towards m/192 MgCl2 is practically 

 identical with the rate of diffusion of water under the same pressure 

 head against distilled water and that a m/4 solution of MgCl2 is 

 required to accelerate the diffusion of water through the collodion 

 membrane to approximately the same amount as is done by a 

 m/128 solution of NaCl. 



Fig. 4 shows that the attractive action of anions of salts with uni- 

 valent cations (Na and K) increases with the valency of the anion. 

 Very dilute solutions of oxalates, phosphates, citrates, and the tetra- 

 valent anion Fe(CN)6 attract water so violently that it diffuses 

 extremely rapidly through the membrane. Hence an increase in the 

 number of charges of a cation lowers the attraction of the electrolyte 

 for water, while an increase in the number of charges in an anion in- 

 creases the attraction. All this agrees with the assumption that the 

 molecules of water possess a positive charge. 



