620 SCIENCE PROGRESS 



and record the production in the laboratory of several cells of 

 first-class excellence. It may therefore be possible in the near 

 future, when these essential conditions are better understood, to 

 obtain trustworthy porous vessels for the demonstration and 

 measurement of osmotic pressure. 



Preparation of the Semi-permeable Membrane 



A feature of recent work on osmotic pressure is the use of 

 the electric current in the deposition, or in the " remaking," 

 of the copper ferrocyanide membrane — the membrane which 

 is almost exclusively employed in the absolute measurement of 

 the osmotic pressure of aqueous solutions. When the porous 

 walls have been completely freed from air, a result that may 

 be effected by electrical endosmose, the cell is filled with 

 a solution of potassium ferrocyanide and placed in a solution 

 of copper sulphate. The electrodes of platinum and copper, the 

 former dipping in the ferrocyanide solution and acting as 

 cathode, are connected with the terminals of a ioo-volt circuit, 

 and electrolysis is continued until a satisfactory membrane of 

 copper ferrocyanide has been deposited. Provision is made for 

 the renewal of the potassium ferrocyanide solution during the 

 electrolysis, since the alkali produced at the cathode has 

 naturally a deleterious action on the membrane. 



Unless hard-baked cells are used, such a method of procedure 

 as that just described yields a membrane embedded in the 

 interior of the porous walls, and to this there are serious 

 objections. It is preferable, if not essential, to have the 

 membrane deposited close to that surface of the porous wall 

 which is to be in contact with the solution. Pfeffer, in his 

 original investigations of osmotic pressure, laid stress on this 

 point, and deposited his copper ferrocyanide membranes close 

 to the inner surface of the porous pots employed. 



In the experiments made recently by Lord Berkeley and 

 Mr. Hartley, the porous tubes used were first immersed in 

 a copper sulphate solution in a desiccator, which was subse- 

 quently evacuated. The result of this treatment was that air 

 was completely removed from the porous walls, and the pores 

 of the porcelain were filled with the solution of copper sulphate. 

 After being superficially dried, the tubes were plugged at both 

 ends and rotated rapidly in a solution of potassium ferrocyanide, 



