708 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



ing the plate, before putting it into the bath, to the vapour of iodine. 

 In this way for example the postage-stamps are printed. Two or 

 three hundred moulds of the original engraving are joined together, 

 and plates are thus procured from which sheets may be obtained con- 

 taining the same number of stamps. To prevent imitation, which by 

 impressions on stone might easily be made, the paper on which the 

 stamps are printed is treated with a white safety ink, which would 

 be transferred to the lithographic stone with the drawing, and the 

 impression obtained would be nothing more than a uniform blot 

 covering the whole sheet. 



The stereotype-plates employed in printing the Bank of England 

 notes were made by Smee by electro-plating. To give an idea of the 

 durability of the stereotype plates we quote the following words from 

 a memoir by that physicist, in which he gives an account of the pro- 

 cesses employed in this reproduction. " Th3 electro-copper," he says, 

 is so durable, that we can scarcely assign a limit at wl>ich it becomes 

 useless." And for the Times newspaper we are told that a mould of 

 this kind has already furnished an impression of twenty millions 

 without being completely worn out. Up to the present time, the 

 limit of the durability of the electrotypes for printing the Bank notes 

 has not been reached,- and there have been already printed from them 

 a million notes without any very sensible effect. 



In Trance, M. Hulot employed electro-typing for the repro- 

 duction and printing of the Bank notes issued in 1848, and since then 

 for the figures on playing cards. 



If electro-typing renders signal service in the impression of engra- 

 vings of various kinds, it is no less useful for the correction of engraved 

 plates ; for example, in the introduction of new details in geographical 

 and topographical maps. These modifications are indispensable in 

 great works like that of the Ordnance Survey maps. Alterations of 

 roads, the addition of new roads, of railways, canals, industrial works, 

 &c., were formerly only possible by processes of retouching,, and 

 recutting, which risked the damaging of the plates. M. Georges has 

 invented a method of correction by which these great disadvantages are 

 avoided. The parts to be corrected are removed by a scraper, and a 

 deposit of copper on the spot is made by electro-plating, the necessnry 

 precautions being taken. It is then planed carefully, and a proof is 

 taken in which the parts to be altered come out blank. The artists 



