Prof, von Kohell upon Galvanography. 229 



copper, — 100 parts of a solution of Glauber's salt 27-5, — 100 

 parts of a solution of potash alum 27*4, — 100 parts of nitrate of 

 potash 25-4; but 100 parts of sulphate of zinc only 7 parts of 

 sulphate of copper. 



As a precipitation fluid, there are two mixtures which I usually 

 employ, the first consisting of two parts by measure of a satu- 

 rated aqueous solution of sulphate of copper, mixed with one part 

 by measure of a similar solution of the sulphate in a solution of 

 Glauber's salt ; the second consists of the same quantity of the 

 solution of the sulphate of copper, with the addition of one part 

 by measure of a solution of the sulphate in a solution of sul- 

 phate of zinc. There are however other proportions, for instance 

 equal parts by measure of the various solutions, which afford sat- 

 isfactory results ; indeed a simple solution of sulphate of copper 

 in a solution of Glauber's salt will afford an extremely malleable 

 copper, which, when as much as half a line thick, is perfectly 

 flexible, and presents a very even surface. 



The vessel in which the copper plate and the painted plate lie, 

 is to be filled with the precipitation fluid, till the parchment of 

 the tamborine standing therein is covered to the depth of some 

 lines. The tamborine must be so constructed, that on dipping it 

 down into the liquor, no air may be retained below the parch- 

 ment. After the lapse of two or three days the liquor must be 

 changed, and fresh sulphate of copper be dissolved in the fluid 

 removed. The vessel therefore is to be of a sufficient size to ad- 

 mit of a suitable quantity of copper being furnished for that pe- 

 riod. If the bottom of the vessel is about three times as large 

 as the copper plate upon which the picture rests, the quantity 

 of the solution it holds will be sufficient ; it may, however, be 

 larger. 



The precipitation of the copper is carried on without any mo- 

 tion in the fluid, so that under certain circumstances the density 

 thereof may be very variable. On one occasion I employed a tall 

 cylindrical glass vessel for making such a galvanic deposition, and 

 so placed the plate on which the copper was precipitated, that 

 there were equal quantities of the solution both above and below 

 it. As I allowed the action to go on without interruption, I 

 found the whole of the copper precipitated out of the fluid be- 

 tween the zinc and copper plates, the liquor losing its color alto- 



Vol. xLviii, No. 2.— Jan.-March, 1845. 30 



