ÖFVERSIGT AF K. VETENSK.-AKAD. FÖRHANDLINGAR 189 4, N:0 2. 47 



to enable all the. hydrogen to Substitute the metal from the 

 Solution and tiius prevent tiie formation of a spoungey deposit, 

 caused by the hydrogen uniting Avith the metal. 



II. The temperature. 



If a normal and neutral aqueous Solution of ZnSO^ is 

 electrolysed no coherant deposite is reecived, if the temperature 

 exeeds '60° C, no matter how high density of current is used. 

 The authur') has corroborated the followiiig stateraent by Nansen, 2) 

 who obtained good zinc under the following conditions: 



Ampere per m^ 



0° 



10^ 



20° 



30° 



10 



firm. 



spoungey. 



spoungey. 



spoungey. 



50 



d:o 



d:o 



d:o 



d:o 



100 



d:o 



firm. 



d:o 



d:o 



150 



d:o 



d:o 



firm. 



d:o 



200 



d:o 



d:o 



d:o 



firm. 



The second ciass of metals is quite difterent in this respect. 

 Warburg^) states that if a verv acidulated Solution of cupric 

 sulphate is electrolysed under a certain density of current, spoungey 

 copper, together with a great amount of hydrogen, is received; 

 now, if the temperature is elevated, all other conditions being 

 the same, hydrogen ceases to evolve and the metal beeoraes firm. 



Certain precious metals cannot be deposited in coherant 

 condition, unless the electrolyte is considerably heated. 



For the metals, belonging to this class, and a for all other 

 metals, deposited in the same manner i. e. secondary, as is the 

 case in nearly all electroplating processes, handbooks prescribe 

 that the temperature must be kept not under 20° C.*) 



The difference between the two classes is perhaps most 

 evidently demonstrated by an experiment made by Morris and 

 Perhause. ^) They electrolysed a mixture of zinc sulphate and 



') I used a normal and neutral Solution of Zn SO4. 



^) Borchers, Elektroiuetallurgie 1891, s. 95. 



^) Warburg, Pogg. Ann. 135, s. 118. 



••) Phanhäusek, Metallplätirung u. Galvanoplastik 1890, s. 140; Langbein, 



Galvanische Metallniederschläge 1886, s. 104. 

 ') Gore, Electrochemistry, 1888, s. 112. 



