116 MEMBRANES (PLASMAHAUT) 



obvious that the hydrogen ion concentration must increase at the 



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cathode and decrease at the anode. (Pole finding paper is blotting 

 paper soaked in phenolphthalein an indicator which while 

 colourless in neutral solutions becomes red in distinctly alkaline 

 solutions). 



Water passes through the membrane in the reverse way when 

 the solution on both sides of the membrane is alkaline. A dilute 

 acid solution separated from a dilute alkaline solution of the same 

 relative strength by an amphoteric membrane will produce a 

 passage of water from the anodal to the cathodal side due to the 

 greater speed of the positive ion. 



Polarisation. When a current is passed between two electrodes 

 immersed in an aqueous solution, the potential difference between 

 the electrodes tends to decrease and will in time fall off altogether 

 on account of the deposition of ions of the opposite sign on the 

 surface of the electrode. This polarisation of the electrode may 

 be prevented by physical or chemical means (cf. various types of 

 concentration cells). A similar ionic layer forms on membranes 

 when a current is passed through them for some time (see also 

 Chap. XII., Polarisation Current). 



Selective permeability of membranes has often been noticed in 

 electrical transference experiments. The classical experiments 

 of Hittorff are now known to be, in some cases, vitiated by his 

 use of ox-gut membranes to prevent connection currents. For 

 instance, such membranes are much more permeable to SO 4 ions 

 than to Cu ions. A large error is thus introduced into electrical 

 diffusion experiments with CuSO 4 due to the adsorption of the 

 copper ions on the substance of the membrane. 



Till more is known of the physical state of the cell and its 

 environment, definite statements cannot be made concerning the 

 causes of alterations in permeability of membranes. Phrases 

 like " selective " adsorption should meanwhile be avoided, as they 

 postulate intelligence in the cell to " select." Although the 

 unknown must be explained in terms of the known, the day is 

 surely past when it is necessary to assume a Maxwellian " demon ' 

 or a cellular intelligence. It is certainly not unscientific to admit 

 the possibility that the unknown is similar to the known or may 



i *J 



be explained-.by analogy to known physical processes. 



