46 CASSEL, ELECTROLYTICALLY DEPOSITED METALS. 



It will be Seen that the total amount of hydrogen increased 

 with the current while the relative amount as compared with 

 the deposited metal steadily decreases, and that a good deposite 

 is not obtained betöre hydrogen practically ceases to be 

 liberated. 



It has long been known, that potassium and sodium can be 

 obtained from a aqueous Solution by using a high density of 

 current and it was by the same means that Bunsen ^) succeded 

 in depositing chromium and manganese. He used concentrated 

 sodution of chioride and a density of current of 6,700 ampere 

 per m-. If the density of current by any reason deereased, 

 nothing but spoungey oxidated metal Avas obtained. 



The second class, containing metals less positive than hydro- 

 gen and represented by copper, cannot be obtained in a good 

 condition from a Solution of their sulphates, nnless the density 

 of current is kept comparatively low. In copper refineries the 

 density varies between 50 and 170 ampere per m-, dependant 

 on other circumstances such as temperature, condition of electro- 

 lyte etc., and if these limits are exeeded, hydrogen would be 

 liberated, and uniting with the metal the deposite becomes more 

 or less spoungey. 



If a Solution of AgNOg is electrolysed under a high density 

 of current hydrogen is evolved and a dark spoungey deposite is 

 received on the cathode.-) If an aqueous Solution of auric 

 Chloride be electrolysed a good metal is obtained by low density 

 of current, while a high density gives a noncoherant and spoungey 

 deposit.3) 



The difFerence between the two classes in this respect is 

 easily explained. The metals belonging to the flrst class must 

 be deposited rapidly, in order to prevent the water, the free 

 acid and the hydrogen from uniting with the deposit, while the 

 metals belonging to the second class must be deposited slowly 



') The chemist N:o XI Au-. 1854, s. 685, 686. Pogg. Ann. Bd VIC, s. 619. 

 2) PoGGENDORF, Pogg. Ann.' 75, s. 337, 848; Fechner, Pogg. Ann. 47, s. 2, 1839; 



Pristley, Nichols Journ. 1, b. 198, 1802. 

 ') WiNKKLMAN, Handbuch der Physik, Dritter Band, Erste Abtheilnng, s. 481. 



