674 IONIC RADIUS AND IONIC EPFICEENCY 



their radius is as follows : The efficiency of anions increases directly and 

 that of cations inversely with the radius of the ion. By radius of the 

 ion we mean the distance between the positive nucleus of an ion and 

 its outermost ring or shell of electrons. This term has a definite 

 meaning in the case of monatomic ions. 



The method of experimentation is the same as in the preceding 

 pubHcations. The solution of the electrolyte was put into collodion 

 flasks of about 50 cc. content, which had previously been filled over 

 night with a 1 per cent gelatin solution. The gelatin solution was 

 then carefully washed out with warm water. The membrane retained 

 on its inside a very thin film (probably of only one molecule in thick- 

 ness) of gelatin. This was done to enable us to reverse the sign of 

 charge of the membrane with dilute acid; when the hydrogen ion 

 concentration is below 2 x 10~^ N the membrane is negatively (and 

 the watery phase positively) charged; while when the hydrogen ion 

 concentration is above this value the membrane assumes a positive 

 and the water a negative charge. The membranes used in the experi- 

 ments to be described had therefore all been treated with gelatin. 



The collodion flask was closed with a rubber stopper, through which 

 a glass tube with a bore of about 2 mm. in diameter was pushed inside 

 the flask. The latter was filled with the solution of electrolyte and 

 suspended in a beaker with distilled water having the same hydrogen 

 ion concentration as the solution of electrolyte. The glass tube 

 served as a manometer to indicate the rate at which water diffused 

 from pure water into the solution through the collodion membrane. 

 The temperature was kept constant at 24°C. We shall treat the 

 influence of ions on negatively and positively charged membranes 

 separately. 



II. Negatively Charged Membranes. 



When an electrical double layer is formed at the boundary of a 

 collodion membrane (previously treated with gelatin as described) 

 the membrane assumes a negative and the watery phase a positive 

 charge as long as the hydrogen ion concentration is below 2 x 10"^ n 

 and no trivalent or tetravalent cations are present. If the hydrogen 

 ion concentration exceeds 2 X 10~^ n and becomes 10~^ N the sign 

 of charge on the two layers is reversed. This was proved by experi- 



