ELECTRO-METALLURGY 



211 



poured upon a slab or board and stirred together till about to set ; the film of dross 

 is then quickly cleared from the surface with a card, and the cold medal is either 

 projected upon the bright metal, or being previously fitted in a block of wood is 

 applied with a sudden blow. Moulds of wax or stearine variously combined, or 

 more recently and better in many cases, moulds of gutta-percha, are applicable to 

 many purposes. But, as none of these latter materials conduct electricity, it is ne- 

 crssary to provide them with a conducting surface. Plumbago or black-lead is 

 almost universally employed for this purpose ; it is rubbed over the surface of the 

 mould with a piece of wool on a soft brush, care being taken to continue it as far as 

 to the conducting wire, by which the mould is connected with the zinc. With 

 moulds of solid metal, the deposit of copper commences throughout the entire sur- 

 face at once ; but, with moulds having only a film of plumbago for a conductor, 

 the action commences at the wire and extends itself gradually until it has been de- 

 veloped on all parts of the surface. 



The nature of the electro-chemical decompositions that are due to the passage of 

 voltaic currents through liquids, especially through liquids in which metal is con- 

 tained in certain forms, can be best understood by studying the arrangement that is 

 most commonly used in the arts, wherein the voltaic apparatus, from which the 

 electric current is obtained, is distinct and separate from the vessel in which the 

 electro-metallurgical operations are being brought about. Such an arrangement is 

 shown in Jiff. 794, where A is a Daniell's cell, as in Jiff. 792; ands a trough filled with 



an acid solution of sulphate of copper ; m is a metal rod, on which the moulds are hung ; 

 and c a metal rod, upon which plates of copper are hung facing the moulds ; the 

 copper-plates are connected by the wire z with the copper of the battery cell, and 

 the moulds by the wire x with the zinc rod. The voltaic current is generated in the 

 coll A, and its direction is from, the zinc rod, through the solutions to the copper of 

 the coll ; thence by the wire z to the plates of copper c ; through the sulphate solu- 

 tion to the moulds m and thence by the wire x to the zinc rod. In this arrangement, 

 no shelf is necessary in the trough B for crystals of sulphate of copper to keep up the 

 strength of the solution ; for the nature of the electro-chemical decompositions is 

 such, that in proportion as copper is abstracted and deposited upon the moulds m, 

 other copper is dissolved into the solution from the plates c. Water is the prime 

 subject of decomposition. It is a compound body, consisting of the gases oxygen 

 and hydrogen, and may be represented by jig. 795, 

 where the arrows show the direction in which the cur- 

 rent, by the wire p, enters the trough B of Jig. 794, by 

 the plate of copper c, and passes through the water in 

 the direction shown, and leaves it after traversing the 

 mould by the wire n. Two atoms of water o H and o' 

 H', as bracketed 1 and 2, are shown to exist before the 

 electric current passes ; and two atoms, one of water 

 H p' (bracketed 1'), and one of oxide of copper o c, 



exist after the action. On the one hand, an atom of copper c has come into the solu- 

 tion ; and, on the other hand, the atom of hydrogen H', belonging to the second atom 

 of water, is set free and rises in the form of gas. The explanation is to show that 

 oxygen is liberated where the current enters, and combines there in its nascent state 

 with copper; it would not have combined, for instance, with gold or platinum. We 

 might easily extend this symbolical figure, and show how that, when free sulphuric 

 acid is in the solution, the oxide of copper on its formation combines with this acid 

 to produce the sulphate of copper required ; and how, when free sulphate of copper 



p2 



