161 



If we cut out now the potential-difference E^ the liquid will rise 

 again in consequence of osmosis. When it has reached the heiglit 

 h.^, we stop it again by introducing a new potential dilFerence (/sj 

 between P^ and P^. 



Now the equation holds: 



P-h, = kE, (2) 



From (1) and (2) it follows: 



so that the osmotic pressure to be measured is known. 



The great difficulties which arise when a semi-permeable mem- 

 brane is used for direct determination of osmotic pressure may thus 

 be avoided. These difficulties are ; 



1^^ The long time needed to reach equilibrium. During this time 

 diffusion takes place with consequent change in the concentration of 

 the solution. In the above method the measurement is made at Ihe 

 beginning when the rise is greatest and is completed in a short time 

 so that diffusion is avoided. 



2"^. That the semi-permeable wall is pressed on one side until 

 equilibrium is reached. Here, on the contrary, osmotic pressure is 

 compensated by the electro-endosmotic pressure ; consequently the 

 wall is not pressed. 



3id- By the entrance of solvent the concentration within the cell 

 changes. Here, on the contrary, this change of concentration may 

 be avoided by making h^ and h.^ and also the internal diameter of 

 the capillaiy tube small. 



One might inquire whether the laws of electro-endosmosis which 

 play a rule in the principle described, are valid when electro-en* 

 dosmis takes place through a semi-permeable membrane. It will be 

 shown later that it is possible to consti-uct the apparatus in such a 

 way that the principle holds even if this question must be answered 

 in the negative. 



Utrecht, June 1913. van 't Roff- Laboratory, 



Chemistry. — "The influence of pressure on the E. M. F. of the 

 lead-accumulator". By Prof. Ernst Cohen and G. de Bruin. 



According to chemicn-l and thermodynamical investigations which 

 have been carried out recently ^) the reactions which take place in 

 the lead-accumulator when it is discharged (charged) can be represented 

 by the equation : 



Ï) DoLEZALEK, die Theorie des Bleiakkumulalors", Halle I'JOl. 



iJ 

 Proceedings R(jyal Acad. .Anislerdam. Vol. XVI. 



