228 Prof, von Kohell upon Galvanography. 



For large or heavy plates it is necessary to use a frame of cop- 

 per wire, so suspended from the sides of the tamborine, that the 

 due distance of the zinc above the membrane may be secured. 



(5.) A strip of copper or lead, half a line thick and an inch or 

 an inch and a half wide, terminating in a round plate of half the 

 size of the zinc plate. This plate is six or seven inches long, 

 and is used for closing the circuit. For that purpose, the broad 

 end of the strip is to be laid upon the zinc plate, the other end 

 thereof being connected by a binding screw, with the strip of 

 copper described under (3). This plate of copper may also be 

 made to terminate in two strips, in which case it is to be con- 

 nected with a similar double strip, proceeding from the bottom 

 plate, (3). 



(6.) The solution of copper from which the galvanically pre- 

 cipitated copper is derived. This I term the precipitation fluid. 

 In my early experiments I employed a saturated solution of the 

 sulphate of copper, and round about the plate I laid crystals of 

 that salt, so as continually to renew the quantity of the sulphate 

 decomposed in the fluid. I then, however, frequently obtained a 

 very brittle copper, which, at an early stage of the process, be- 

 came covered with nodules, and I have found it a better plan to 

 employ a saturated solution without the crystals as above de- 

 scribed, and to renew the solution in a day or two. Such a fluid 

 is weakened, it is true, by the continual deposition of the metal, 

 and its conducting power, therefore, keeps gradually diminishing. 

 In order to attain a uniformity in the current, I tried what would 

 be the effect of adding to the solution of the sulphate of copper, 

 other salts which resist the action of such feeble currents as are 

 here called into play, and I thus obtained a copper of the most 

 admirable quality. The salts 1 thus employed were Glauber's salt, 

 sulphate of zinc, potash alum, and saltpetre. Chlorides are not 

 available, for they become decomposed, and thus act on the silver 

 on which the pictures are painted. These salts, or rather their 

 saturated aqueous solutions, with the exception of the sulphate 

 of zinc, take up as much sulphate of copper as common water 

 does. I have determined the amount of oxide of copper con- 

 tained in an ascertained quantity of those solutions, and have 

 ascertained therefore by calculation what is the quantity of sul- 

 phate of copper which they take up. I find that 100 parts by 

 weight of spring water dissolve about 37 parts of sulphate of 



