234 Prof, von Kohell upon Galvanography. 



tic mass with which these colors are ground up and prepared in 

 the manner of oil colors, consists of a solution of wax in oil of 

 turpentine, to which is added a certain quantity of Damara-gum 

 varnish. Of this preparation only so much is to be used that 

 the color when applied to glass may be dull when dry, and yet 

 enough that it may adhere firmly after being allowed to he 

 twenty four hours in water, or in a solution of sulphate of cop- 

 per. The coloring material itself is not altogether a matter of 

 indifference ; oxide of iron, or most of the ochres, Cassel brown, 

 charcoal, coke, or Frankfort black, are the colors that answer 

 best. I had prepared in the same way plumbago in powder, 

 and also silver and iron-dust, and found that as they dried, these 

 substances made their way enough to the surface to prevent their 

 conducting power being altogether suspended by the thin film of 

 wax with which they were invested.* 



The best of these substances is plumbago, and from its color 

 it is peculiarly suited to deepen the shades, which are the only 

 portions (owing to their forming the thickest coats) that re- 

 quire a conducting power being given to their surface ; all the 

 parts where the coat of paint is but thin become coated over 

 with copper without help. This last remark, however, applies 

 only to certain substances, the colors enumerated, for instance; 

 where to a certain extent the deposition of the metal is furthered 

 by the porosity of the surface, aided by the circumstance of their 

 not being altogether complete non-conductors. Whereas, in con- 

 sequence of its complete power of insulation, a film of mica, be 

 it ever so thin, receives no deposit of copper if placed upon a 

 silver plate, while a far thicker coat of a color prepared as above 

 becomes gradually coated with metal. 



The picture is simply to be painted with one of the above 

 colors, which are used with oil of turpentine. The oil should 

 have a little resin or wax dissolved in it, in order to give the 

 color more consistency, for it is of course above all things requi- 

 site that it should adhere firmly to the ground on which it is 

 laid. The color that I have oftenest used is oxide of iron. The 

 picture is executed in the Indian ink manner, and the places left 

 bare on the metal upon which one paints, correspond therefore 



* Paints thus prepared are to be had in bladders, of Kern of Munich, under the 

 name of galvanograpliic colors. 



