Theory of Electroeapillarity , 369 



If 7 is zero, ZJ must also be zero, consequently, if 



the concentration of ions of mercury in the solution corre- 

 sponds to the maximum value of the surface-tension, the 

 potential of an insulated mercury mass does not change when 

 its surface is increased, i. e. such a solution is a " null" one. 

 The denominator of the right-hand side of equation (3) is 



always negative ; in fact, Jr;<0, and as Grouy's * numerous 

 determinations have shown =r-r3<0: ^ and ^ - are 



therefore quantities of the same sign and the potential of 

 mercury approaches, when its surface is increased, the value 

 which corresponds to the maximum surface-tension. 



Up to the present, we have considered a solution of uniform 

 composition, depending on the value of s ; if matters were 

 adequate to our supposition, we could, by increasing the 

 surface, approach the maximum of the electrocapillary curve 

 as closely as we wished and thus convert the solution into a 

 " null" one. 



In reality a dropping electrode changes the composition 

 of the solution only in its immediate neighbourhood and a 

 steady state soon results, determined by the rate of increase 

 of the surface and by the rate of diffusion. If the concen- 

 tration of mercury in the solution is low and the end of the 

 continuous part of the jet is just in the surface of the 

 solution, the influence of diffusion nearly vanishes and the 

 potential of the dropping electrode must correspond to the 

 maximum of the electrocapillary curve. 



To verify this inference I measured both quantities in 

 different solutions of organic substances with anomalous 

 electrocapillary curves. The results are ^iven in Table I. 

 Column I. contains the maximum E.M.F., determined with 

 a sensitive capillary electrometer of the form used by Grouy, 

 the large mercury electrode being immersed in a n/10 solution 

 of KC1; column II. the E.M.F. of the cell : Paschen dropping 

 electrode solution/decinormal calomel electrode, both in volts. 



Table I. 



I. II. 



n/1 NaCl + paraldohyde (saturated) 0225 0216 



n/1 NaCl+ethyl acetate „ 0257 0-259 



n/1 NaCl-f-isoaniyl alcohol „ 0307 0-315 



»/4H01 -fmetachloraniline „ 0612 0-582 



n/1 NaCl+pyrogallic acid (M) 0-809 0770 



»/2KCN+ „ „ „ 0-900 0-889 



* Arm. chim. phys. (7) xxix. p. 230 (1903). 



