1857.] and Electricitif to the production of Engravings. 349 



not in this art deemed to be the best mode of procedure. A device, 

 analogous to one used in type printing, is therefore adopted ; a sort 

 of stereotype process is gone through. A mould in softened gutta 

 percha, or other suitable moulding material, — possibly a compo- 

 sition of wax or stearine, is made ; this mould will of course have 

 the raised lines or dots of the original gelatine, represented by 

 grooves and cavities, apparently graven in the surface ; and here, 

 again, if the mould were firm enough, we might ink it as if it were 

 an engraved copper-plate, and print by the copper-plate printing 

 press. From these considerations, it will be evident that we have 

 only to seek to convert these yielding surfaces into enduring ones, 

 and we shall end our labours successfully. This the electrotype 

 art enables us to do. We have simply to render the mould a con- 

 ductor of electricity, by black lead, or finely divided metal, and we 

 can deposit in it copper to any amount. "We shall thus get in 

 copper a fac-simile of the original swelled gelatine plate. But 

 since this requires surface printing, and that is not to be preferred, 

 we must once more apply our electrotyping process, using this first 

 obtained and raised copper-plate as a matrix^ to produce as many 

 engraved or sunken plates, ready to be printed from, as we may 

 desire. The original matrix remains, as in Fizeau's case, unworn. 

 The above is an outline of the more important features of Herr 

 Pretsch's invention. There is one more point that deserves atten- 

 tion. In all the engraving processes hitherto described, there is a 

 difficulty in obtaining a granular surface over the etched parts 

 necessary to hold the amount of ink required by the printer. In 

 Pretsch's process this difficulty remarkably enough does not present 

 itself; the swelled surface breaks up in a direction vertical to its 

 surface into little masses which are just what is desired ; this result 

 is quite characteristic. It has been attributed to the presence of 

 particles of chromate of silver, or of iodide of silver. Would it be 

 too far-fetched to suppose that it is another beautiful instance of the 

 slaty cleavage action demonstrated by Dr. Tyndall ? However this 

 may be, the fact is very important for the success of the invention. 

 The chemistry of the processes of the first and third divisions of this 

 subject is but little advanced. M. Niepce de St. Victor has found, 

 what M. Chevreul had anticipated, that the oxygen of the atmo- 

 sphere is essential in the bitumen process. In an illuminated 

 vacuum the result could not be obtained, although ordinary photo- 

 graphic action went on quite as well as in air. With reference to 

 the gelatine processes, it might be observed that Mr. Ponton, who 

 first used bichromate of potash as a photographic agent, and M. 

 Edmond Becquerel, who extended its use on paper, both found 

 that the sizing materials became more insoluble by the photographic 

 action. It was believed that chromic acid was liberated by the 

 sun's rays, since simple mono-chromate of potash produced no effect. 

 On mentioning these facts to a friend (Dr. Hugo Miiller), the 

 speaker leamt that solutions of chromium had been employed iii 



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