394 RECENT PROGRESS IN PHYSICS. 



the deposit of copper, to obviate difficulty in the subsecLuent separa- 

 tion of the plate. 



In order to separate the copper-plates they were put into a vice 

 and filed around their edges, and when a cleft was observed a horn 

 spatula or other similar tool was introduced to produce the separation, 

 which was assisted sometimes by a gentle heat. The plate first obtained 

 serves again as a mould, but the separation from this is accomplished 

 without difficulty. 



Sometimes an apparatus, similar to the one just described, is used, 

 in which several smaller objects are placed at the same time upon the 

 lower copper-plate. "When the process is continued for several days, 

 the apparatus has to be taken apart every day in order to remove pro- 

 jections upon the precipitated copper by the file, to renew the acid- 

 ulated water at the zinc, and also to remove impurities deposited from 

 the solution of the sulphate of copper and from the zinc itself. 



But the electric copying of copper-plates was attended by several 

 difficulties. Gerlach, for instance, (Dingler's Journal, vol. 82, and 

 Journal fur Practische Chemie, September, 1841,) observed that the 

 galvanic copper-plates are very brittle, and that although this pro- 

 perty could be removed by the application of heat, yet in this case the 

 linear expansion amounts to -^2} which, when unequal, might even pro- 

 duce distortions. Gerlach, from some of his experiments^ seems to 

 think that raising the temperature of the plates only to the heat of 

 melting tin might remedy the brittleness without producing the 

 objectionable expansion, but gives no exact information on this point. 

 It was also found that the copy sometimes firmly adiiered to the 

 original, or caught in lines which were undercut, and Eisner, there- 

 fore, (Dingler's Journal, vol. 80_, and Journal fur Practische Chemie^ 

 1841, vol. 22,) considered it hazardous to subject very valuable plates 

 to this process. The contractions which take place in the moulds, 

 especially when of plaster of Paris, the risk of bending the precipi- 

 tated deposit on removing it from the mould, or in filling up the 

 back, &c,, do not admit of applying the electrotype for copying scales 

 or other measuring instruments. Besides, according to De la Rive, 

 (Dingler's Journal, vol. 99,) such plates stand but few impressions, 

 and especially the lines of wood cuts copied by means of galvanism 

 are said to be frequently hollow, because in deep traces the solution 

 of the sulphate of copper is not properly renewed as it is decomposed, 

 and therefore becomes exhausted, and the deposition ceases until the 

 line overgrows laterally ; it is also said that vermilion cannot be used 

 for printing from these plates, because mercury separates, which is 

 not the case with ordinary engraved plates. Vogel, of Francfort, on 

 this account, (Dingler's Journal, 110 vol.,) made impressions, espe- 

 cially of steel plates, in wax, powdered over with graphit, which pro- 

 ceeding was repeated after the impression was made, and removed the 

 excess with a bellows. 



An original idea of Vogel was to cover the etched plate after remov- 

 ing the varnish, (an engraved one had previously to be treated with 

 dilute nitric acid,) entirely with the precipitate, then to grind it off 

 again to the original surface of the plate in order to obtain all the 

 traces filled in the manner of inlaid work, (Dingler's Journal, 106 



