Prof, von Kohell upon Gahanography. 237 



vinced me that by the use of pulverized dry black lead, we com- 

 pletely attain the end in view. In the absence of other auxiliary 

 circumstances, for instance, the good conducting power of silver 

 plate beneath, the action of the plumbago would not suffice for 

 the production of the film of metal, more especially as the colors 

 are so little of an unctuous nature that the plumbago when dry 

 will adhere to them in only very minute quantity. 



In order to apply this process to a picture, some finely pow- 

 dered black lead is worked into a bit of soft untanned leather by 

 rubbing it upon a piece of paper, and then gently dabbed on to 

 such places as require it, these parts being in a suitable state of 

 dryness. 



The picture by this means is invested with an almost imper- 

 ceptible gray coating, so fine that the design of the plate is no 

 ways injured thereby, (as may be seen by examining it with a 

 magnifying glass, or as proved by the subsequent impressions 

 thrown off,) but which is nevertheless sufficient for facilitating the 

 deposition of the precipitated metal as much as is necessary. In 

 cases where it is requisite to deepen certain shades, as described 

 above under (4), it is to be done subsequently to the operation 

 just mentioned. 



It might be imagined that the application of these disconnect- 

 ed particles of black lead would have but little influence in con- 

 tributing to facilitate the precipitation, whereas we easily see how 

 this substance favors the deposit upon a wax surface when well 

 rubbed into it — an operation to which we cannot submit these 

 pictures without injuring them. The reason why the minutely 

 divided plumbago is of service, is due partly to the fact that 

 the atoms of copper thrown down, as they form a mass by ag- 

 gregation, are brought into contact with neighboring particles of 

 plumbago, thus making a conducting connection, and by a re- 

 petition of this operation causing the surface to be coated over : 

 and partly, as shown by the experiments above performed, to this; 

 that the solution of sulphate of copper bringing these conducting 

 particles into connection with the uncoated places of the copper 

 and silver plates, serves as a base beneath, without being in im- 

 mediate contact with metals, just as a silver plate becomes coated 

 over when allowed to rest upon a glass plate (a non-conductor) 

 on the bottom of a copper vessel, forming a closed circuit with 

 the zinc plate. This kind of picture may however be laid on 



Vol. xLviii, No. 2.— Jan.-March, 1845. 31 



