ABNORMAL OSMOTIC EFFECTS 139 



apparently ahiiorinal osmosis. Collodion nicmbrancs, for instance, 

 are practically indifl'erent as regards electrical charge. Water passes 

 through these membranes into solutions of non-electrolytes and 

 of electrolytes more concentrated than M/8 at a rate in accord- 

 ance with the van't Hoff theory of osmotic pressure, i.e. a linear 

 function of the concentration of the particles (colloidal aggregates, 

 molecules or hydrated ions) in solution. Treatment of the 

 membrane with an amphoteric colloid 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, w^hile 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, 

 (6) 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 a 

 membrane may undergo alterations. This is a general statement 

 in which is included the effect of alterations of hydrogen ion 

 concentration on permeability. The diffusion of water through an 

 indifferent membrane depends on two forces, («) 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 increasing with the valency and 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 



