Prof, von Kohell vpon Galvanography . 243 



I have instituted many experiments upon this point. I weak- 

 ened the current by laying a glass disc upon copper rods, so that 

 the fluid could circulate above and beneath it, about half an inch 

 above the copper plate ; and I found that then the precipitation 

 proceeded so tardily as not to give a metal of a serviceable qual- 

 ity. I next substituted a parchment frame for the glass plate. 

 In this case the deposit was likewise slow, while the metal 

 thrown down was brittle, and here and there the two plates ad- 

 hered. I now brought the drum within half an inch of the base, 

 and by this means obtained, generally speaking, a tough deposit 

 of copper, sometimes however it was brittle ; but the two plates 

 could always be easily separated, and the smooth surfaces were 

 always faithfully given. This I have had frequent occasion to 

 notice. At the first formation of the deposit therefore, it is always 

 advisable to bring the zinc tamborine near to the base, or in other 

 words, to increase the energy of the current. 



As some skins of parchment, ccBteris paribus, answer better 

 than others for producing copper of an excellent quality, I always 

 employ for the commencement of the deposit, a skin which I 

 find by experiment to possess this property, and when the pro- 

 cess has proceeded for a certain time, I substitute an inferior 

 tamborine. 



In giving the above mentioned film of platina or silver, the 

 plates may be taken out of the liquor several times, dried, gently 

 rubbed with leather, and again inserted into the fluid. The pro- 

 cess will be successful, provided the plates are not left very 

 much too long in either of the salt mixtures mentioned, and un- 

 less, in the case of platinizing the plates, too much of the platina 

 solution should be used. 



Common copper will seldom take this film as perfectly as gal- 

 vanic copper does, and on many plates apparently pure, there are 

 brought out patterns similar to those on what are termed watered 

 ribbons. 



It is superfluous to remark, that copper plates coated as above 

 described with platina or silver, may be employed as a base for 

 the reception of the picture in place of a silvered plate, and cop- 

 per plates formed by galvanism are the best for this purpose. 



From what has been said, it is evident that the cost of the ap- 

 paratus for forming galvanographic plates, is but moderate. For 

 producing one pound of galvanic copper, four pounds of sulphate 



