DEPOSITION OF VARIOUS METALS. 189 



affords more economical results ; in some cases, however, elec- 

 tricity must be preferred as giving more adhering and durable 

 deposits. 



Amongst the numerous electro-tinning processes tried with 

 more or less success, we will mention those of Messrs. Eoseleur, 

 Fearn, Lobstein, Maistrasse, and Birgham, and we will very 

 briefly analyse them. 



KOSELEUR'S PROCESS. The bath recommended by M. 

 Koseleur is composed as follows : 



Distilled water 50 litres. 



Pyrophosphate of sodium 500 grammes. 



Melted stannous protochloride .. .. 50 



The water is contained in a vat entirely lined with sheet tin 

 or tin anodes ; the pyrophosphate is poured in and the mixture 

 stirred until it is dissolved. The stannic protochloride is 

 introduced, on a brass sieve, in the midst of the bath, and 

 the solution is again stirred until a complete dissolution 

 is arrived at. The liquid then becomes clear and almost 

 colourless. 



The anodes are not sufficient for keeping the solution in a 

 state of saturation ; it is necessary, when the deposition gets 

 slower, to add, by small quantities at a time, some tin salts and 

 some sodic pyrophosphate. 



FEARN'S PROCESS. This process, which is used by the 

 Electro-Stan nous Company of Birmingham, comprises four 

 different solutions, two for thick and two for thin deposits. 



The following are the two in use for obtaining thick deposits : 



1st. A solution of stannous chloride containing 100 grammes 

 of tin per 5 litres is first prepared ; then a solution of 15 kilo- 

 grammes of caustic potash in 100 litres of water; and lastly a 

 third solution of 15 kilogrammes of sodic pyrophosphate in 

 30 litres of water. 



The liquid potash is poured into the tin solution, continually 

 stirring with a glass rod ; 15 kilogrammes of potassic cyanide 

 are then added, and lastly the 45 kilogrammes composing the 

 solution of sodic pyrophosphate, keeping the mixture in a 

 constant state of agitation. 



