160 Intelligence and Miscellaneous Articles. 



1 829 by M. Becquercl, owes its constancy to the employment of 

 sulphate of copper ; the sulphates of zinc and cadmium have been 

 made use of by MM. du Bois-Reymoud and J. Regnault for the 

 construction of impolarizable electrodes and constant elements. 

 Notwithstanding the importance of its applications, this property 

 appears to have been but little studied. The experiments I am 

 about to describe have made evident an essential condition of the 

 phenomenon. 



It is the following : in order that the electrode may be depo- 

 larized, it must be formed of the same metal as is contained in the 

 solution. Thus copper is the only metal which becomes depolarized 

 in sulphate of copper, while gold, silver, and platinum are polarized 

 in that solution. Inversely, copper polarizes in sulphate of zinc, 

 cobalt, &c. A salt depolarizes only its own metal. To make the 

 experiment, operating for example on sulphate of copper and plati- 

 num, two strips of platinum are to be immersed in the liquid, and 

 put into communication with the poles of a capillary electrometer. 

 The mercury column is then at zero. A feeble current is then 

 caused to pass into the liquid so as to employ one of the strips as ne- 

 gative or exit electrode. The electrometer shows a deflection, which 

 remains even after the interruption of the current, thus proving 

 that polarization is produced in the sulphate of copper, as it might 

 have been produced in pure or acidulated water. For the same 

 reason, a couple formed of strips of copper and platinum dipping 

 in sulphate of copper furnishes only a current of brief duration, 

 the platinum receiving, owing to its polarization, an electromotive 

 force equal and opposite to that of the copper. One may even go 

 further and communicate to the platinum, by means of an exterior 

 pile, an electromotive force superior to that of the copper, so that 

 then the platinum will behave like a more negative, more oxidizable 

 metal than copper. 



Similar experiments have been made with strips and solutions of 

 silver, mercury, lead, cobalt, and zinc. 



An application readily presents itself. Since the property of 

 depolarizing a metal belongs exclusively to its salts, it permits us 

 to detect the presence of that metal in a solution. Taking copper 

 as an example, if we dip into the liquid to be tested a copper wire 

 which we use for the negative electrode of a feeble current, it will 

 be polarized if there is no dissolved copper, it will not be polarized 

 if the solution contains--^ g- - of sulphate of copper. It is possible, 

 therefore, thus to detect the presence of copper in a mixture of 

 metallic salts. With a silver wire we can in the same way test 

 for silver. The delicacy of this electric process appears to be 

 still greater for silver than for copper ; but it has not yet been 

 measured. — Convptes Rendus de VAcademie dcs Sciences, June 24, 1878, 

 tome lxxxvi. pp. 1540, 1541. 



