338 Prof, von Kohell upon Galvanography. 



altogether in plumbago of various degrees of fineness, and when 

 thus executed there is no difficulty in causing the copper to form 

 upon it. 



When the picture, ready for receiving its metallic coating, is 

 placed in the apparatus, care must be taken when letting it doAvn 

 into the fluid to detach the bubbles of air which adhere to the 

 paint, and this before closing the circuit ; for should this precau- 

 tion be neglected, these bubbles become themselves coated with 

 copper, and holes are thus formed in the plate. These bubbles 

 may be removed with a brush, or by alternately dipping the 

 plate into the fluid and again withdrawing it ; but it is still bet- 

 ter to prevent the bubbles from forming, which may be attained 

 by exposing the plate to the vapor of hot water prior to its in- 

 sertion in the fluid. There is a condensation of water upon the 

 plate, and as soon as the uncoated spaces thereon are observed to 

 be covered with these globules, it is to be lowered into the fluid, 

 after which it is but seldom that air bubbles make their appear- 

 ance. 



As was observed in (3), under the hesid of galvanic apparatus, 

 the picture must lie upon a plate of copper larger than it, half an 

 inch or an inch every way. The back part of the edges of this 

 base should receive a coat of melted wax, laid on with a brush, 

 so as to produce insulation ; the connecting strip too is to be 

 treated in a similar manner, excepting of course the portion pro- 

 truding above the fluid where the binding screw is attached. 

 Were the base only of the size of the painted plate, or smaller 

 than it, the latter would have thick knobs formed along its edges, 

 and shoots of metal would push out from its corners, which it 

 would not be easy to remove with the file. When the base has 

 been in use for some time, similar knobs and nodules of copper 

 form upon the waxed portions thereof, more especially towards the 

 edge. These are to be removed with a chisel from time to time, 

 and if the coat of wax on the edge is not too thin, they may be 

 very easily detached. 



Before inserting the plate in the fluid, it is a good plan to set 

 the apparatus in action for half an hour or so, by connecting the 

 strip of lead with the copper strip proceeding from the base. 



The method in which the copper spreads over the surface of a 

 picture prepared as here described, differs to some extent from the 

 way in which that metal spreads itself over a cast in stearine that 



