Tlieovy of Electrocapillarity . 373 



capillary electrometer the surface-tension of a zinc amalgam 

 in solutions of ZnS0 4 and Na 2 S0 4 . The results are given in 

 Table II. (the maximum surface-tension between mercury 

 and water is assumed to be 100). 



Table II. 



n. ZnS0 4 014 



1/1000 ;>.ZnS0 4 91-2 



v. ZnS0 4 + ;/. Na,S0 4 910 



1/1000«. ZnS0 4 + w. Na 2 S0 4 89"8 



The experimental values are in agreement with the theory; 

 unfortunately, the lack of mobility of the meniscus does not 

 allow of rendering these measurements more accurate or of 

 extending them to still more dilute solutions. Let us now 

 consider the mechanism of working of a dropping electrode 

 in these solution*. To simplify the problem we may neglect 

 the presence of S0 4 " ions. If a drop of amalgam is 

 immersed in a solution of zinc sulphate, zinc ions enter the 

 solution, but, as no other ions are present, they must remain 

 in the double layer to counterbalance the negative charge 

 of the amalgam. The formation of a new surface is there- 

 fore not accompanied by any change of concentration, and 

 the potential of a dropping electrode is equal to the potential 

 of a still one. The conditions are quite different when 

 there is in the solution a great excess of sodium ions; these 

 take the place of the zinc ions in the double layer, and the 

 zinc ions enter the bulk of the solution. Thus, in presence 

 of an excess of sodium ions the formation of a new surface 

 lowers the concentration of sodium ions and increases the 

 concentration of zinc ions. The result is an alteration of the 

 potential. 



Table III. contains the potential difference between 

 dropping and still amalgam in different solutions of ZnS0 4 

 and Na 2 S0 4 , the amalgam jet being surrounded by a hydrogen 

 atmosphere. 



Table III. 



0-OOIn. ZnS0 4 0-003 volt 



0-001w, ZnS0 4 -r". Na 2 SO, 0-037 „ 



0-OOOlrc. ZnSO, " 0-011 „ 



0-OOOlw. ZnS0 4 + ??.Na,S0 4 0-055 „ 



^Ve see that according to the theory, the addition of 

 ]Sa 2 S0 4 greatly increases the potential difference; the small 

 potential difference observed without Na 2 S0 4 is due probably 

 to the presence of S0 4 " ions, which we neglected in our 



