CHAP. XL] ELECTRICITY AND MAGNETISM. 389 



electrolyte for a limited time, until the opposing electro- 

 motive force has reached an equal value. Helmholtz, 

 who has given the name of electrolytic convection to this 

 phenomenon of partial electrolysis, assumes that it takes 

 place by the agency of uncombined atoms previously 

 existing in the liquid. This assumption is virtually in- 

 cluded in the kinetic hypothesis of Clausius. 



419. Electrometallurgy. The applications of elec- 

 tro-chemistry to the industries are threefold. Firstly, 

 to the reduction of metals from solutions of their ores, 

 a process too costly for general application, but one 

 useful in the accurate assay of certain ores, as, for 

 example, of copper ; secondly, to the copying of types, 

 plaster casts, and metal -work by kathode deposits of 

 metal ; thirdly, to the covering of objects made of baser 

 metal with a thin film of another metal, such as gold, 

 silver, or nickel. All these operations are included 

 under the general term of electrometallurgy. 



420. Electrotyping. In 1836 De La Rue ob- 

 served that in a Daniell's cell the copper deposited out 

 of the solution upon the copper plate which served as a 

 pole took the exact impress of the plate, even to the 

 scratches upon it. In 1839 Jacobi in St. Petersburg, 

 Spencer in Liverpool, and Jordan in London, independ- 

 ently developed out of this fact a method of obtaining, 

 by the electrolysis of copper, impressions (in reversed 

 relief) of coins, stereotype plates, and ornaments. A 

 further improvement, due to Murray, was the employment 

 of moulds of plaster or wax, coated with a film of plum- 

 bago in order to provide a conducting surface upon 

 which the deposit could be made. Jacobi gave to the 

 process the name of galvano-plastic, a term generally 

 abandoned in favour of the term electrotyping- or 

 electrotype process. 



Electrotypes of copper are easily made by hanging a 

 suitable mould in cell containing a saturated solution of 

 sulphate of copper, and passing a current of a battery 



