140 MEMBRANES (PLASMAHAVT) 



the reverse to be the case. The water niolceules act as if they were 

 negatively charged and so are attracted by the cations and repelled 

 by the anions of the electrolyte. Acids act in this way and have a 

 high electrostatic effect on account of the small ionic radius of the 

 hydrogen ion. It is important to note that certain salts of bio- 

 logical interest have a marked electrostatic value — very dilute 

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

 valent ion Fe(CN)6 attract water violently. On the other hand, 

 the effect of the anion may be masked by the opposite electrostatic 

 effect of the cation. As the valency of the cation increases, the 

 attractive force of the anion decreases. Calcium chloride, for 

 instance, has little more action than distilled water, because the 

 calcium almost neutralises two positive charges on the two anionic 

 charges (cf. Hydrophilic property of Ca, Chap. IX.). 



The value of this electrical force has been determined by Loeb 

 in a very neat manner. Inside a collodion bag he placed an M/128 

 solution of KCl and outside the bag an M/64 solution of sugar. 

 These solutions are approximately isotonic, i.e. movement of 

 water through the membrane by osmotic forces is thus eliminated. 

 He found that water did diffuse from the sugar solution to the 

 KCl solution. This transport of water must be due to the electrical 

 pull of the KCl. He then raised the concentration of the sugar 

 outside the bag till its osmotic pressure just balanced the attractive 

 forces of the KCl. The sugar solution was now M/8. Therefore 

 the electrical forces which are at work correspond to an osmotic 

 pressure which is the difference between the osmotic pressures of 



7M 7 X 22-4 



an M/8 and an M/64 solution of sugar = ^j- = — = 2-4 



atmos. (approx.). 



These electrical forces also account for negative osmosis — the 

 passage of water from a more to a less concentrated solution. 

 As far back as 1835, Dutrochet observed that water diffused out 

 of a pig's bladder filled with a dilute solution of oxalic acid, into 

 pure water. Early investigators tried to explain this on the 

 assumption that there was a greater imbibition of water on the 

 acid side of the membrane and a lesser on the side in contact with 

 the pure water. In 1914, negative osmosis was observed taking 

 place through a porcelain filter, and therefore the imbibition 

 theory becomes untenable. Loeb has shown that negative osmosis 

 occurs when neutral salts as well as acids and alkalies in certain 

 well-defined concentrations are separated from water by a mem- 

 brane capable of taking up either a positive or a negative charge. 

 At these concentrations the repelling action of the ion with the 

 same sign of charge as that of water becomes greater than the 



