TREATMENT OF ORES. 251 



the solution and connected through iron conductors to the 

 negative pole of the machine. 



The baths are joined in tension or in quantity, more fre- 

 quently in tension, so as to obtain the greatest possible metallic 

 precipitation with a minimum of electricity. 



Messrs. Bias and Miest, starting from this erroneous principle, 

 that the maximum of utilisable electricity occurs when the 

 external resistance is equal to the resistance of the machine 

 itself, have made, on the motive power necessary for the treat- 

 ment of a few simple ores, some calculations which are naturally 

 incorrect. We only intend to rectify one out of some of the 

 examples given by them in their pamphlet, and, in order to have 

 a point of comparison with M. Letrange's process, we will select 

 the treatment of blende. 



Under the action of the current, sulphate of zinc is decom- 

 posed into metallic zinc, which is deposited on the cathode, 

 and sulphuric acid, which attacks the blende of the anode, 

 dissolves it with formation of sulphate of zinc and sulphur 

 deposition, as indicated in the following equations : 

 ZnS0 4 + galvanic current = Zn + SO 4 . 

 Work absorbed : 53 5 calories. 

 ZnS + S0 4 = ZnS0 4 + S. 

 Work produced : 53 '5 - 21 -5 calories. 



The heat of formation of ZnS being 21*5 calories, the 

 chemical work of electricity is therefore 21*5 calories per 

 equivalent, or 658 calories per kilogramme of zinc, which 

 corresponds, per kilogramme of precipitated zinc per hour, to 



21500 x 424 



32-7 x 75 x 60 x 60 



= 1-04. 



Theoretically, 1 horse-power is sufficient for precipitating 

 1 kilogramme of zinc per hour; in practice, if a possible 

 efficiency of 70 per cent, is admitted, 1 49 horse-power will be 

 required ; whereas, according to the inventors, the maximum 

 efficiency being only 30 per cent., 3*47 horse-power would be 

 required. With 1 horse-power, 19 kilogrammes of zinc could 

 therefore be precipitated in 24 hours by means of the Bias and 



