Prof, von Kohell upon Galvanography. 239 



has received a coat of black lead. In the latter case, the deposi- 

 tion of copper proceeds from the strip of metal by which it is 

 surrounded, and extends itself with a kind of negation as it were, 

 till the entire surface is coated over ; while in the case of a pre- 

 pared picture, though near the uncoated places something of a 

 similar course is observed, there is formed a thin film upon 

 the general surface of the plate, as if minutely divided copper 

 had been sifted over it. This film gradually thickens, until it 

 forms a plate capable of furnishing impressions. From this we 

 can clearly trace the influence which is exercised by the metal 

 surface upon which the color is spread, as also the propriety of 

 employing colors possessed of a certain amount of conducting 

 power. The copper dust just alluded to, consists of metal in an 

 infinite state of subdivision, of atoms or molecules of copper, 

 and hence we see that we can by no means supply its place by 

 mechanically prepared copper powder, which when sprinkled on 

 the picture will, it is true, unite so as to form a plate, but not 

 one which furnishes an accurate copy of the original. 



A picture rightly handled is coated over with copper in most 

 places in twenty four hours, and in two days this drawing is com- 

 plete ; on the third or fourth day little nodules or other asperities 

 begin to make their appearance. To get rid of them, the plate is 

 to be taken out of the fluid and dried, and the surface having 

 been rendered smooth with a fine file, the plate is to be replaced 

 in the apparatus. With small pictures, the copper deposited in 

 from four to five days, attains sufficient thickness to admit of its 

 being removed, but with pictures of larger size the process should 

 be allowed to go on for six or eight days. It is advisable to lay 

 a strip of silvered copper (5) in the apparatus, in the vicinity of 

 the painted plate. After the lapse of four days, the copper upon 

 this strip is to be examined with the file. If it is flexible, it is a 

 sign that the plate thrown down upon the picture is fit to be taken 

 oflf, supposing at least that it has attained a suitable thickness ; if 

 the copper on this test strip is, on the contrary, brittle, the depos- 

 ited metal upon the picture must be allowed to increase in thick- 

 ness prior to removing it. 



Should it ever happen that, after two or three days' action, 

 small spots are not covered with metal, the picture is to be taken 

 out of the fluid and dried. Silver, in the form of very fine pow- 

 der, is then to be sprinkled upon the refractory places, and this 



