414 



WELLS'S NATURAL PHILOSOPHY. 



by Fig. 343. It consists of a trough of wood, or an earthen vessel, containing 

 the solution, the decomposition of which is desired — for example, sulphate of 

 copper. Two wires, one connected with the positive, and the other with tho 

 negative pole of a battery, Q. are extended along the top of the trough, and 

 supported on rods of dry wood, B and D. The medal, or other article to be 

 coated, is attached to the negative wire, and a plate of metallic copper to the 

 positive wire. "When both of these are immersed in the liquid, the action 

 commences — the sulphate of copper is decomposed — the copper being de- 

 posited on the medal, and the liberated oxygen on the copper plate. As tiie 

 withdrawal of the metal from the solution goss on, the copper plate attach:d 

 to the positive pole undergoes Currosion by the sulphuric acid which is liber- 

 ated and attracted to it, and sulphate of copper is formed. This, dissolving in 

 the liquid, maintains it at a constant strength. "When the operator judges 

 that the deposit on the medal is sufficiently thick, he removes it from the 

 trough, and detaches the coating. The deposit is prevented from adhering to 

 the medal by rubbing its surfact, in the first instance with oil, or black-lead, 

 and if it is desired that any part of the surface should be left uncoated, that 

 portion is covered with wax, or some other non-conductor. 



FiQ. 343. 





In this way a most perfect reversed copy of the medal is obtained, — that ia, 

 the elevations and depressions of the original are reversed in the copy. To 

 obtain a fac-simije of the original, the electrotype cast is subjected to a repe- 

 tition of the process. 



In general, it is found more convenient to mold the object to be repro- 

 duced in wax, or Plaster of Paris. Tlie surface of this cast is then brushed 

 over with black-lead to render it a conductor, and the metal deposited directly 

 upon it. The deposit obtained will then exactly resemble the original ob- 

 ject. 



The pages and engravings in the book before the reader are illustrations of 

 the perfection and practical application of the electrotype process. The en- 

 gravings were first cut upon wood-blocks, and then, with the ordinary type, 

 formed into pages. Casts of the whole in wax were next made, and an dec- 



