146 ROYAL SOCIETY OF CANADA 



The molecules whase pasisage through tlie meuibrane is prevcated, 

 exercise a kinetic effect with tJio result tliat tliey eskiblish a pressure, 

 knowTi as osmotic pressure, in proportion to the strength of tlie solutiofn 

 in the casei of a non-electrol \ t.e, but in pi-oportion to its concentration 

 andj ionisation in the case tlie solute is an electrolyte. 



This conoeption of a semi-permeable membrane was first introduced 

 by the botanist, Moritz Traube,^ who, in 1865, began the study of what 

 are now known as precipitation membranes. These may be made by add- 

 ing tannin to gelatine, wkich has Ixh,^ boiled for a long time in order to 

 convert it into /? gelatine. This results in the formation of tannate of /? 

 gelatine which constitutes a membrane. A more typical form of such 

 1 semi-permeable membrane may be obtained in the following fashion, 

 first proposed by Traube. A piece of narrow glass tubing about six or 

 more inches in length, open at one end, but co^•ered at the other by a piece 

 of rubber tubing compressed with a clip, is taken, and into this is drawn 

 a few drops of a ^.8 per cent, solution of cupric acetate. The open end 

 is now lowered into la test tube containing a quantity of a 2.4 per cent, 

 solution of potassi'c ferrocyanide. The liquid in the inner tube is, by 

 s-hiifting the pressure on the rubber tubing, made to form a plane surface 

 at the mouth of the tube and cupric ferrocyanide is deposited there as a 

 fine transparent fihn which closes the opening. This film or membrane 

 prevents the diffusion of the copper salt downwards and of the potassimn 

 ferrocyanide upwards, as shown by the facts that the cupric ferrocyanide 

 does not oocur above or below the film, and that the film remains for a 

 considerable time transparent and of very great tenuity, but, yet of 

 sufficient density and grain to prevent either salt passing through it. 



That ■wTater, however, does pass through sucli a membrane can be 

 demonstrated by showing that the refractive index lof the solution in the 

 immediate neighbourhood of the film is increased, this indicating that the 

 concentration of the solute is increased at that point, or it may be demon- 

 strated by placing a crystal of tlie chloride or sulphate of copper in a 

 moderately concentrated solution of potassium ferrocyanide. In this 

 latter case there is formed a precipitate of cupric ferrocyanide in the 

 form of a membrane about tlie crystal which distends gradually, obviously 

 only by the penetration of waiter alone, for the distension may result in a 

 rupture, and then there escapes a drop of concentrated solution of 

 chloride or of sulphate of copper which in contact with the external fluid 

 gives a new precipitate repairing and thus closing the rupture in the 

 original membrane. Through this also the water passe=? and disibension 

 continues till the osmotic pressure inside and outside is the same. 



1 Arch. fUr Ana.t. und Physiol., 1887, p. 87. 



