OsMOTio Phenomena 27 



nitrate. The term "semi-permeable," therefore, must be 

 used with reference to a particular solute and its solvent. 



Such membranes as that of copper ferrocyanid are termed 

 "precipitation membranes;" they are formed by precipitation 

 from two solutions which react chemically. If a solution 

 of potassium ferrocyanid and one of copper sulphate be 

 brought together within the walls of a porous clay cup, such 

 a membrane (composed of copper ferrocyanid, CugFe 

 (CNjs) will be precipitated within the clay walls. The mem- 

 brane is then supported by the clay, and the whole cup may 

 be used for osmotic determinations. Another precipitation 

 membrane is that formed by gelatin and tannic acid. 



6) Electrolytes. — Osmotic pressure is found to be abnor- 

 mally high in solutions of electrolytes. This is one of the 

 facts from which the conclusion was drawn that in these 

 solutions the diffusion tension of the solute is abnormally 

 great, and hence that dissociation occurs. When the amount 

 of ionization which takes place in any solution is taken into 

 account, it is found that these solutions are only apparent 

 exceptions to the general rule of osmotic pressure. In this 

 case we can no longer say that the pressure is due to the 

 bombardment of the membrane by the solute molecules, but 

 by the solute particles, meaning thereby both molecules and 

 ions. Solutions having the same number of solute particles 

 per unit volume have, at the same temperature, the same 

 osmotic pressure. As far as it has been carefully tested, this 

 principle has been found to hold for all somewhat dilute 

 solutions. 



c) Colloids. — According to their behavior when in solu- 

 tion, substances have been classified as crystalloids and col- 

 loids. Crystalloids produce an osmotic pressure which is 

 practically equal quantitatively to the gas pressure which 

 would be produced by the same number of gas particles as 

 there are of solute particles, occupying the same volume as 



