Pt'of. von Kohell upon Galvanography. 231 



proof that the solution of copper has found its way through the 

 sides of the tamborine, or that there is a rent or minute holes in 

 the parchment. Such a tamborine is to be laid aside, as must 

 also those on the parchment of which there is an abundant form- 

 ation of nodules of copper. 



These nodules are not produced when the glass triangle below 

 the zinc is made of a cane of sufficient thickness, nor unless the 

 action is allowed to continue till the zinc plate is entirely con- 

 sumed. I always put on a new zinc plate when the old one be- 

 gins to be thin ; and this, with such plates as I use, takes place 

 about every other day. What remains of the plates may be re- 

 melted for forming new ones. 



The strip proceeding from the leaden plate, as also that portion 

 of the copper plate which is screwed thereto, are to be kept 

 bright, and the leaden plate from time to time is also to be made 

 bright by the use of a file. 



The galvanic copper is in the best state when of a pale flesh 

 color, and when, if on being rapidly dried, it assumes a dull me- 

 tallic lustre ; if brownish red or dull, it is brittle and friable, and 

 there is then something wrong either in the tamborine* or in the 

 state of concentration of the precipitation fluid, or perhaps the 

 zinc plate is too much consumed, or the leaden plate and the con- 

 necting strips are not clean enough, or may be the tamborine is 

 inserted too deep in the fluid. 



In a case like this, the plate is to be dried and rubbed with a 

 scratch-brush, till it is again bright, and after correcting such 

 fault as there may happen to be in the apparatus, the deposition is 

 to be allowed to proceed. 



It must be further observed, that what has been here said re- 

 specting the fluids employed, the distance between the plates, the 

 connecting slips, and so forth, is applicable only to such an ar- 

 rangement of the apparatus as is here described ; any essential 

 modifications thereof would naturally carry with them various 

 modifications of the details here given. 



2. Of the colors and the method of paiiiting. — A color used 

 for the preparation of a picture to be galvanically copied, must 

 possess the following qualities. 1st, it should adhere well to the 



* If a clean stripofzincinsertedinto the tamborine turns grey within five minutes, 

 there is something wrong in the tamborine itself or in the membrane. If the strip 

 remains bright, the fault is in some other part of the apparatus. 



