MECHANICS AND USEFUL ARTS. 91 



of sal-ammoniac over it, after which the solution should be boiled. 

 The article to be coated should be first dipped in weak chloric acid, 

 and then put into the boiling- sal-ammoniac and zinc, and in a few min- 

 utes it will be covered with a brilliant coating of zinc, very difficult to 

 remove by friction. The galvanic action in this case is thus explained. 

 The double chloride of zinc and ammonium formed is decomposed 

 by the zinc and the plate of copper. The chlorine disengaged from, 

 the sal-ammoniac goes to the zinc, and the ammonium escapes in gas, 

 while the undecomposed sal-ammoniac, combining with the chloride of 

 zinc, forms the double chloride, a very soluble and easily decomposed 

 salt. If, then, an excess of zinc exists in the solution in contact with 

 the electro-negative copper, the salt is decomposed into its elements, 

 and the reduced zinc is deposited on the negative copper. 



PLATING BY HEAT. 



M. HESSENBERG, of Leipsic, has recently discovered a new process 

 for plating by heat, and he has read a paper descriptive of it to the 

 Polytechnic Society of that city. The metal to be plated is first well 

 cleansed and polished, and its surface is moistened with salt water by a 

 camel's-hair pencil, after which it is covered w r ith a powder made as fol- 

 lows. Silver is dissolved in nitric acid, and precipitated by the intro- 

 duction of a slip of copper; the precipitate must then be washed and 

 dried. Next, one part of this precipitate or powder, one part of the 

 chloride of silver, and two parts of calcined borax, are carefully mixed 

 together in a porcelain mortar and washed through a fine silk sieve. 

 This powder having been placed on the metal in such a manner that a 

 layer of it covers the whole surface, the metal is put in a clear char- 

 coal fire and heated to redness. It is then removed and immersed in 

 boiling water, either pure or having a small quantity of the tartrate of 

 potash in it. After this a stiff brush is rubbed over every part, and it 

 appears to be already entirely silvered, which is very important, as 

 this is the basis of the art, and the silver in this way penetrates the 

 metal for the following operations. 'T'he metal is now again covered 

 with a paste made of equal portions of silver powder, pure sal-ammo- 

 niac, pure salt, sulphate of zinc, and clear ox-gall. All these ingre- 

 dients are ground together, adding a little distilled water with a small 

 quantity of dissolved gum in it, and then the paste is laid on with a 

 pencil, after which the metal is again placed in a clear charcoal fire, 

 heated cherry-red, plunged in boiling water, and well rubbed as soon 

 as it is cool. This process is gone through with four or five times, 

 when the metal is sufficiently silvered, and is therefore fit to receive 

 the burnisher's lustre. Articles plated in this way show when broken 

 that the silver has evidently penetrated the copper, thereby insuring 

 the durability of the plating. The points or edges of goods from 

 which the silver has been worn off may be re-plated by this means, and 

 it is necessary to apply the process only to those parts which require 

 renewal, a silversmith's forge being used as the furnace. 



