Prof, von Kohell upon Galvaiiography. 241 



removed by the application of sulphuric ether rubbed on with a 

 flock of cotton, and the surface is then to be gently rubbed with 

 soft leather dipped in calcined hartshorn shavings or finely levi- 

 gated quick-lime. The plate is then ready for the printer. The 

 press employed is the ordinary copper-plate printer's press, in 

 which the paper lying below the inked plate is made to pass 

 under a metal roller. The plate becomes slightly bent thereby, 

 not however more than it will readily bear, even should the 

 metal be brittle, if it is only of sufficient thickness. 



5. Of corrections and alterations introduced into a galvanic 

 plate. — It need scarcely be remarked, that by the use of the scra- 

 per and burnisher, or of the etching-needle and graver, a galva- 

 nographic plate may be corrected and touched up, just as an 

 ordinary etched plate may be. There is, however, another simple 

 method in which one of these plates may be corrected, and have 

 any alteration we like introduced into it, namely, by taking a se- 

 cond galvanic copy thereof. 



The picture side of the plate is to have thrown down into it a 

 coating of metallic copper, the suitable thickness of which is at- 

 tained in the space of two or three days. 



This second plate gives, in the strictest sense of the word, a 

 perfect metallic copy of the original picture. In this copy the 

 desired corrections -are to be made, either by again having re- 

 course to the paint-brush, or by the use of the scraper or bur- 

 nisher, as the case requires. A second plate thrown down on to 

 this relief as before, will naturally furnish impressions containing 

 the desired alterations. Occasionally, however, the two copper 

 plates thus prepared adhere together so firmly, that there is no pos- 

 sibihty of separating them ; but this difficulty is altogether obvi- 

 ated by the following process, which was first communicated by 

 me to the Academy of Sciences, at their meeting of the 11th De- 

 cember, 1841. 



If we form a concentrated solution of common salt, and dis- 

 solve chloride of silver in it, (this is done by adding thereto a 

 suitable quantity of a solution of nitrate of silver, shaking the 

 precipitate well up in the fluid, and then letting it settle,) and 

 into this liquor dip a clean strip of copper, it will be observed that 

 in from ten to fifteen minutes there forms upon the surface 

 thereof an infinitely thin film of firmly adhering metallic silver, 

 so thin, that on washing the plate, and drying it with a towel, 



