[CLARK] CAPILLARY ELECTROMETER 83 



Extent of the Double Layer 



The capacity increases as the meniscus is operated nearer the 

 end of the tube, which is contrary to what might be expected. This 

 fact alone suggests that the double layer is not confined to the meniscus. 



Lippmann ^ calculated the thickness of the double layer from 

 observations on the surface tension and the corresponding potential 

 difference which give the surface density o. The thickness of the 



double layer is then found from o = . 



47rd 



The calculated value is d = 1 -4 (10)~^mm. — of molecular dimens- 

 ions. If we calculate the thickness of the double layer from the capacity 

 and the dimensions of the meniscus, we obtain a very much smaller 

 value, i.e., of the order lO'^^mm. Then, either the doubb layer has no 

 such existence, as assumed by Lippmann, or the acid-mercury surface 

 is much greater than the area of the nearly hemispherical meniscus. 

 For example, the capacity of capillary No. 6 was measured in the 

 usual manner and found to be approximately • 25 mf. The diameter 

 of the tube was about -035 mm. The area of the meniscus is then 

 approximately 2(10)"^ sq. cm. Then, either the double layer is only 

 6.8(10)"^^cm. thick or its area is 0-039 sq, cm. 



This raises the question of the creeping of the acid solution be- 

 tween the glass and mercury. To test this point, a fresh capillary 

 was drawn (No. 11) and operated as follows: The large tube, which 

 tapered below into the capillary, was filled with mercury in the usual 

 manner. Then it was immersed in the acid and the mercury forced 

 to run out under pressure to exclude air. Then the pressure was 

 reduced gradually until the acid entered the tube a distance of only 

 2 • 75 mm, and the capacity was measured at a point 2 • 5 cm. from the 

 end. Next, the acid was allowed to wet the tube for a distance of 

 3-00 mm. and the capacity again measured at a point 2-5 mm, and 

 so on. The following values of potentials show that the progressive 

 wetting of the tube with acid does not change the capacity. 



Capacity in terms of 

 Potentiometer wire 



4-68 

 4-64 

 4.75 

 4-89 

 4-68 



Lippmann. Compt. rend. 95, 686, 1882. 



