350 BEPORT — 1886. 



The bro^vll deposit of oxide appears as soon as tlie polarisation exceeds '28 volt ; 

 the polarisation increases first very rapidly, then slowly, and its limit is very nearly 

 the number (]'428 volt) which corresponds to the decomposition of Na.jSO^ be- 

 tween copper electrodes. (It is slightly sophisticated by the extra resistance of gas 

 bubbles then given off.) To sum up : first is decomposed OuSO^, which theoretically 

 consumes no energy (the electrodes being copper), then comes in the decomposition 

 of acid water (-28 volt), finally that of sulphate of soda (1-424). 



As for the conductivity of the mixture it i-emains perfectly invariable in spite of 

 the variability of the electrolytic reactions. 



M. Bouty then quotes a saying of Wiedemann, that from a mixture of any of 

 the following metals, Zn, Cd, Pt, Cu, Ag, Au, any metal which follows in the list 

 is deposited to the exclusion of any which precedes. This is manifestly in accord 

 with the above law, for the metals are in order of thermal equivalents. But the fact 

 is only true for feeble currents. With strong currents a mixed deposit is obtained. 



To sum up : liquids have, like metals, ordy one tnode of conductiny electricity. 

 Also they have, like metals, only one contact E.M.F. icith an electrode of invariable 

 composition. But the result of electrolysis being to modify both electrode and 

 liquid round it, their contact E.M.F. alters in a variable manner — whence polarisa- 

 tion. 



Sur la Condudibilite lEledrique des Dissolutions Salines tres Etendues. 

 Par M. E. Bouty.' Abstract bij Oliver Lodge. 



I. Historical. 



' The electric conductivity of salts dissolved in water varies with concentration 

 in a manner extremely complex and differing for different salts. One possesses 

 neither o-eneral law nor empirical formula, of however limited an application. One 

 conceives a priori that this conductivity depends on the chemical nature of the 

 salt on the hydrates it can form, and on their stability; experience establishes 

 also that it is'not without relation to some physical properties of the solution, in 

 particular its viscosity. Bat the separation of these circumstances has not yet been 

 made. There seemed to me room first to simplify the problem by considering Only 

 solutions of identical physical properties. I have therefore chosen solutions so 

 dilute that their density and viscosity are the same as pure water ; their conductivity 

 is yet relatively enormous, and can be measured easily by an electrometric method 

 derived from that of M. Lippmann.' 



In this method as now applied the tapping electrodes are zinc in sulphate of zinc, 

 communication being established between the experimental fluid and the sulphate 

 of zinc bv a pair of capillary openings in the experimental tube. The difference of 

 potential between these tapping electrodes is either measured by a Lippmann electro- 

 meter and compared against another difference taken at the ends of a known wire in 

 the same circuit, chosen so as to be as nearly equal in resistance to the liquid as 

 possible ; or, what is plainly better, it is compensated by an auxiliaiy wire, and the 

 electrometer brought to zero. 



The author quotes Kohlrausch's views as expressed in his paper in Wiedemann's 

 ^ Annalen,' vi. pp. 1, 51, 145, 210 ; but he objects to them as founded too much on ex- 

 trapolation, the conclusions being stated for extremely dilute solutions, while those 

 experimented with contained ^th of their weight of salt. 'So, although my results 

 present a general agreement with those of ]M. Kohlrausch within the limits in which 

 he has himself worked, I find myself led for their interpretation to conclusions 

 absolutely different from that of the learned German professor.' 



11. Method flf Measurement. 



The liquids to be compared are contained in two long vertical inverted U tubes 



' Annales de Chimie et de Physique, 6e serie, 1884, t. ill. ; also Journ. de P/iy*. 

 ■2e serie, t. iii. p. 325. See also Foussereau, Journ. de Phys. t. iv. 



