112 MEMBRANES (PLASMAHAUT) 



aggregates, molecules or hydrated ions) in solution. Treatment 

 of the membrane with an amphoteric eolloid like gelatine or 

 haemoglobin causes an anomalous osmotic pressure. These 

 colloids, as we have seen, form salts with either acids or bases. 

 One may prepare, for instance, gelatine hydrochloride or sodium 

 gelatinate. In the first instance, cationic gelatine has a + charge, 

 while in the second case it acts as an anion and so has a charge. 

 The result of this is that when the membrane has a positive charge 

 it will attract water as if the water had a negative charge, and vice 

 versa. That is, the rate at which water will pass through the 

 membrane will depend on the intensity of the charge in the 

 membrane, not on the sign of the charge. 



(2) The material presented to the membrane may undergo 

 changes : 



(i) Its particles may be increased in size, 



(a) by adsorption of other material, 



(b) by combining with similar particles, 



(c) by hydration. 



An increase in size, if sufficiently great, will prevent passage 

 where previously passage was free. 



(ii) The converse may take place, i.e. the particles may be 

 dissociated and so be able to pass through interstices previously 

 too narrow for them. 



(iii) The electrical state of the material on either side of this 

 membrane may undergo alterations. This is a general statement 

 in which is included the effect of hydrogen ion concentration on 

 permeability. The diffusion of water through an indifferent 

 membrane depends on two forces, (a) pure osmosis, (b) electrical 

 osmosis caused by the presence of electrolytes. The intensity 

 of the electrical forces depends on the nature of the electrolytes. 

 Neutral salts of mono- or di-valent cations influence the rate of 

 diffusion as if they conferred a positive charge upon the water 

 molecules. In other words, the molecules of the pure solvent are 

 attracted by the charge on the anions and repelled by the charge 

 on the cations of the electrolyte, the attractive and repulsive 

 forces obviously increasing with the valency of the ion and 

 diminishing inversely with the radius of the ion. Alkalies act in 

 the same way. If, however, one considers neutral and acid salts 

 of tri- or tetra-valent cations then one finds the reverse to be the 

 case. The water molecules 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 electro- 



