326 Dr. S. L. Hart on some Capillo-electric Effects, 



The positive potential obtained must then depend on the 

 degree of oxidation possible — that is, on the rate of fall in 

 some way or other, and upon the medium around the drops, 

 as well as the degree of insulation obtained, &c. This was 

 found to be the case — so much so that, by increasing the oxi- 

 dation of the drops by fanning or breathing, a much greater 

 deflection (*8 D) was obtained, while by substituting coal-gas 

 for the air around the drops the deflection could be reduced 

 to zero. 



It thus appears that, in air, a convex advancing thread of 

 mercury is reduced, this being accompanied by a positive 

 charging of the mercury column, the reverse taking place in 

 the retreating end. 



This behaviour is precisely similar to that observed in capil- 

 lary electrometers ; and the tube with the divided column of 

 mercury bears a strong resemblance to the electrometer of the 

 Scott-Dewar form. Thus another point of analogy can be 

 traced between the action of liquid and that of " gaseous elec- 

 trolytes " on the metals placed in them ; and it is to attempt 

 to refer these two sets of phenomena to the same cause, and to 

 discover, if possible, the same laws which govern them, that 

 this paper has been written. 



The influence of the atmosphere upon the difference of po- 

 tential supposed to be set up between metals has been recog- 

 nized. Thus, Professors Ayrton and Perry remark that only 

 in a vacuum could the true contact-potential be determined. 

 Further, this influence has been clearly shown to exist, and to 

 bear not only on the quantitative measurements, but also upon 

 the actual order of the series, as was proved by Brown (Phil. 

 Mag. August 1878) in the case of an iron-copper couple placed 

 in air and also in hydrogen-sulphide gas: a " gaseous electro- 

 lytic 1 , cell " is thus formed; and its behaviour is similar to that 

 of a "liquid cell," the electrolyte being water in the one case 

 and hydrogen-sulphide solution in the other. 



The analogy is sufficiently striking to invite one to seek for 

 some one general law which may lead more or less satisfac- 

 torily to the one cause of these electrical phenomena] but the 

 real justification of such a step will depend on the measure in 

 which it enables one to explain already known cases and also 

 to predict others. 



The ordinary way in which a body acquires a certain poten- 

 tial is by receiving an electric charge. Have we not sufficient 

 ground to refer the potential-differences of metals and electro- 

 lytes in question to the charges accompanying the electrolytic 

 ions as these go to one or other of the metals, and thus to lay 

 down the following rule ? — 



