108 ELECTROTYPE PROCESSES. 



of the before-mentioned solvent as will dissolve it, and afterwards 

 add about ^-th part more of the solvent, so that the solvent may 

 be in excess ; I then stir them well together and let them remain 

 about 24 hours, and then filter the solution, when it will be ready 

 for use. This is what I designate Silvering Liquor. 1 



Sulphurous acid gas for making the above liquor may be prepared 

 by heating sulphuric acid, undiluted, in a flask or any convenient 

 vessel, to which should be added small pieces of copper or charcoal : 

 the gas escaping is made to pass into the solution to be saturated 

 with it. 



Fig. 40 is a very convenient apparatus for the preparation of this 

 gas for saturating solutions. 



This solution is also very easily decomposed by the electric cur- 

 rent, and serves the purposes of plating very well ; but it is also 

 liable to decomposition by light, and is not so good in practice as the 

 solution of cyanide of potassium and silver. The latter solution is, 

 however, liable to a kind of decomposition not yet fully investigated, 

 but it is wholly confined to its impurities, and it never deposits its 

 silver ; whereas the decomposition that takes place in the sulphite or 

 hyposulphite affects the silver compound, and precipitates the silver 

 from solution. 



To Recover Silver from Solution. When a silver solution gets 



out of order, and cannot be recovered so as to be fit for use again, 

 instead of throwing down the silver by muriatic acid it is better to 

 evaporate the solution to dryness, and to fuse the product. When 

 the solution has contained yellow prussiate of potash, it is found 

 that during this fusion portions of the metal sometimes form a nodule 

 which floats about the crucible, and all the heat that can be applied 

 will not fuse it. This refractory piece, when cooled, has generally a 

 rough scoriaceous surface, and is exceedingly hard. When filed it 

 has more the colour of German silver than of real silver : it has 

 considerable malleability, and retains its bright appearance for a long 

 time without tarnish. An analysis of this alloy gave 



Silver 82-15 



Copper 912 



Iron 7-50 



Carbonaceous matter . . *46 



99-23 



If we suppose the carbonaceous matter to be an accidental im- 

 purity, this alloy will nearly agree with the formula Ag 3 CuFe. 



Preparation of Articles for Plating. Articles that are to be plated 

 are first boiled in an alkaline ley, to free them from grease, then 



1 Repertory of Patent Inventions, 5th series, p. 210, 1843. 



