236 Prof, von Kohell upon Galvanography. 



blank spaces of considerable size are left in the picture upon the 

 latter. 



But as the process does not in any case injure a plated plate, 

 unless indeed the highest lights are scratched out with a graver 

 or other hard pointed instrument, the use of a plated plate is to be 

 preferred, inasmuch as after it has served it may, should occasion 

 require it, be cleaned with ether or rectified oil of turpentine, and 

 again polished without difficulty. 1 may here mention an inter- 

 esting case that occurred to me, on an occasion where 1 was ex- 

 perimenting upon a painted plate of pure silver. It was placed 

 on the somewhat uneven bottom of a copper vessel serving for 

 the galvanic apparatus, thus allowing the fluid to get beneath it. 

 Its under surface was not polished. The upper surface exposed 

 towards the zinc became coated with copper, while on the lower 

 side the silver was gradually taken up and again deposited upon 

 the sides of the copper vessel in the form of mossy aggregations. 

 I have never noticed any such corrosion of the lower surface 

 where pure copper or plated copper plates were employed. 



3. Of the fiirther treatment of the painted plates, and of the 

 manner of placing them in the apparatus. — Where many lights 

 or faint tints occur in a picture it is unnecessary to heighten the 

 conducting power of its surface, more especially when the deeper 

 shades have been touched up with the black-lead paint in the 

 manner above described, and when also the color adheres suffi- 

 ciently to the ground. In a picture, however, where the lights 

 are but few, or where the entire surface of the silver is coated 

 with color, it is advisable, if only indeed to a limited extent, to 

 give the picture a certain conducting power, for the quicker the 

 plate is coated over, the more perfectly the picture is copied. 



I have taken considerable pains to find out a method of pro- 

 ducing a film of metal on the painting by chemical means, but 

 there are many difficulties in the way of this, owing to the colors 

 employed not becoming melted by aqueous solutions. To this 

 end I laid the painted plates in flat vessels, and poured on to them 

 solutions of the acetate, the nitrate, and the formate of silver, 

 and then by exposing them to the light I brought about a reduc- 

 tion of the silver upon the surface of the color. The success 

 of the experiment was only partial ; for there were frequently 

 formed minute crystals of silver in groups, instead of a uniform 

 coating of that metal. Repeated experiments have however con- 



