40 ELECTROLYSIS. 



will attack it and form some sulphate of copper which will be 

 dissolved in the bath. If, therefore, the cathode increases in 

 weight, the anode will decrease in the same proportion. The 

 work of decomposition will be exactly compensated by that of 

 recomposition, and the electrical energy will be expended only 

 by the conductor and by some small secondary actions. 



Notwithstanding this arrangement, the liquor will, after a 

 certain time, be subject to some changes in its degree of satura- 

 tion; the sulphate will gradually decrease round the negative, and 

 concentrate round the positive pole. The remedy for this want 

 of homogeneity in the body of the liquor consists in producing 

 a continuous mechanical movement in the vat, so as to constantly 

 mix all the parts which are in a state of dissolution. 



When treating of the refining of metals we shall have an 

 opportunity of examining the work expended when soluble 

 anodes are used, and we shall see that this work is far from being 

 nil. The necessity of employing baths of very great resistance 

 and to keep the electrodes at a certain distance from each other 

 in order to obtain the metal in a state of absolute purity create 

 considerable resistances which often absorb more than 2 volts. 



This comparatively large electromotive force has the incon- 

 venience of producing decomposition of the water, which would 

 not take place under 1 J volt, and of constituting a very impor- 

 tant waste of work. The phenomena of polarisation and of in- 

 version of currents, which are so prejudicial to industrial electro- 

 lytic operations, are also caused by the decomposition of the 

 water. 



PRACTICAL DETERMINATION OF THE COUNTER ELECTRO- 

 MOTIVE FORCE AND OF METALLIC KESISTANCES. M. E. 

 Marchese, an engineer of Genoa, who makes his speciality of 

 the treatment of sulphide of copper by electricity, has quite 

 recently published a note on the determination of the resistances 

 of the circuit. We will consider its principal points. 



The difference of potential observed between the two elec- 

 trodes of an electro-chemical bath arises out of the counter 

 electromotive force which is necessitated by the dissociation of 

 the electrolyte and of the electromotive force absorbed by the 

 metallic resistances of every kind. In order to obtain an 



