Copper hy Voltaic Electricity . 



311 



A very convenient form and arrangement is the following : 

 ^ is a glass or earthen vessel containing a quantity of a sa- 

 turated solution of sulphate of cop- 

 per; B a piece of gut or tubular 

 membrane formed into a bag by be- 

 ing tightly tied at the lower end and 

 secured in a vertical position in the 

 middle of the jar A^ by means of a 

 stick, which passing through two 

 holes in it, rests upon the top of 

 the jar. This bag is filled with 

 dilute sulphuric acid, and contains 

 the zinc rod C, which is likewise 

 supported by the stick. D is the 

 mould to be copied, and E the me- 

 tallic wire connecting it with the 

 zinc. Matters being thus arranged 

 the precipitation goes on rapidly, 

 and all that is requisite is to take 

 care that the solution of sulphate 

 of copper does not become too 

 weak ; when this happens, the cop- 

 per comes down in a pulverulent and finely divided state, 

 without any cohesion ; falling off" from the surface of the 

 mould in the form of a bulky powder, and rapidly reoxidizing. 

 This effect also happens when the surface on which copper is 

 to be precipitated is very small, compared with the size of the 

 zinc and the strength of the acid. In the first case it is easily 

 prevented by always keeping excess of undissolved sulphate of 

 copper in the solution to supply that which is decomposed. 



When the deposited metal has acquired sufficient thickness, 

 it is easily removed from the surface of the mould, by gently 

 loosening its edge all round with any sharp instrument, after 

 which it may be readily separated. Mr. Spencer has ingeni- 

 ously availed himself of the diff'erent expansibility by heat of 

 different metals, in removing the deposited metal from the 

 mould. When copper is to be precipitated in a copper 

 mould, he recommends rubbing into the surface of the mould 

 a vei'y small quantity of beeswax, the copper being previously 

 warmed. In similar cases I have found that a small quantity 

 of plumbago well rubbed over the surface completely pre- 

 vented adhesion. 



The form of apparatus above described, has I believe an 

 advantage over that first proposed by Mr. Spencer, in the 

 vertical position of the mould. When the mould is placed 

 horizontally beneath the zinc rod, it is more liable to become 



