242 Prof, von Kohell upon Galvanography. 



and rubbing it with a piece of wash-leather, it presents the ap- 

 pearance of copper again. 



A similar coating of platina may be obtained by adding to a 

 similar solution of salt, (till the liquor assumes a pale wine-yel- 

 low color,) a few drops of a solution of platina as neutral as pos- 

 sible. About one eighth of an ounce of the solution of platina 

 is required for a pound of the solution of salt, Tt takes about 

 two hours for the film of platina to form completely upon the 

 copper plate, but it adheres as well as the silver if not better. 

 In order to clean properly the plates thus coated, they are to be 

 laid for five or six minutes in diluted muriatic acid, and then 

 washed in warm water. If, when thus washed, they assume 

 a bluish or yellowish hue, they must be again dipped in the 

 diluted muriatic acid, and washed in cold water. The deposition 

 of this film of silver or of platina is very materially influenced by 

 the quality and the nature of the copper surface ; but it is pre- 

 cisely upon this galvanic copper, owing to its purity, that these 

 films are thrown down in the most perfect state, and as they form 

 most readily upon smooth surfaces they prevent the plates from 

 growing together at the very spots where their tendency to adhere 

 is the greatest. Any thing approaching a modification in the 

 copy is out of the question by this process ; for, strictly speaking, 

 these films are not coats upon the copper, but rather they replace 

 a portion of that metal from the surface of the plate, inasmuch as 

 there is an interchange between the atoms of platina or silver in 

 solution and those of the copper. 



In order therefore to copy a galvanographic plate with correc- 

 tions, it must first receive a film of silver or platina, as has been 

 described, (the surface having been previously cleansed with 

 ether or with quick-lime carefully rubbed on with soft leather;) 

 a relief of it is then to be made, which in its turn is to be coated 

 with platina or silver ; this is then to be painted afresh when- 

 ever necessary, or altered in any manner that is desired, and a 

 second sunk plate is to be formed thereupon. This last plate 

 upon its removal presents a pure copper surface, provided the 

 relief was duly coated with platina or silver, and the energy of 

 the current was of sufficient intensity. From the power of the 

 current depends the facility with which the plates may be sep- 

 arated when the silver or platina is used, or whether, when nei- 

 ther of those substances are employed, the plates can be parted 

 at all or not. 



