Prof, von Kohell upon Galvanography. 233 



printing from, unless the design upon it has been rendered dull 

 by etching, (that is to say, has a grained suface,) or unless lines 

 or strokes are cut in it, as in etching or engraving. And from the 

 condition which an etched plate presents, it is evident that metals 

 thus treated consist of infinitely minute crystals, otherwise there 

 is no reason why a smooth surface should become rough and dull 

 by that operation, as we know it does. Crystals, as we are aware, 

 are however more easily acted on upon their planes, than upon 

 their angles and edges. It is an aggregation of crystals thus un- 

 dergoing solution to a certain extent, which is the cause of the 

 roughness or dullness in question. 



Painting therefore with a color that dries glossy, as most oil 

 colors do, can only be had recourse to for obtaining plates for 

 impressions resembling etchings. If it is desired to form with a 

 glossy color a picture in the style of Indian ink, a grain must be 

 given to it previous to its receiving its coating of copper. This 

 is done by dusting it over with some substance in the form of 

 powder. I have employed for this purpose pulverized plumbago, 

 fine silver dust, and also iron in that minutely divided state in 

 which it is sold at the apothecaries under the name of ferrum 

 alcoholisatum. A tube of four or five lines in diameter, and at 

 one end covered with a bit of fine crape, either single or double, 

 is used for applying the dust. A portion of powder in the tube 

 is to be sifted on to the picture until the entire surface is thinly 

 covered over therewith; and the greasier the surface is, the more 

 readily this metallic dust adheres to it. I have thus prepared 

 several pictures painted in burnt sienna, that color being very 

 well suited to the purpose, owing to its transparency, which ren- 

 ders it necessary, where a deep tint is required, to lay it on so 

 thick as to produce a somewhat raised surface. The iron pow- 

 der, which in the fluid is very soon replaced by copper, did not, I 

 found, answer so well as the plumbago or silver — it led to the 

 formation of air-bubbles, producing little holes or indentations in 

 the plates. 



Although by suitable treatment good impressions, faithfully re- 

 producing the original, may by this means be produced, it is clear 

 that to a certain extent the original is nevertheless modified. I 

 therefore endeavored to find out colors better suited for this pur- 

 pose, and the result of my enquiries is, that those prepared for 

 encaustic painting answer better than any others. The encaus- 



