404 RECENT PROGRESS IN PHYSICS. 



In this same way gold, &c., may also be removed from objects when 

 used as anodes. This may be done with advantage when an object, 

 by improper management of the gilding, assumes a blackish color. 

 But cyanide of potassium is also a solvent for gold without the gal- 

 vanic current. 



As to the strength of the current, experience has taught that it 

 must be very small, so that no bubbles of gas, or scarcely perceptible 

 ones, should appear on the metal to be coated. Deposits slowly formed 

 are more compact and adhere more firmly to the surface beneath 

 them.* The durability of the galvanic coating was at first doubted, 

 and Becquerel even proposed (Dingler's Journal, vol. 89, from Comptes 

 Kendus, July, 1843) to first amalgamate the piece to be gilt with ni- 

 trate of mercury, to gild it and then subject it to the common coloring 

 process, which requires a degree of heat sufficient to expel the mer- 

 cury. The want of durability in galvanic gildings seems, at first, not 

 to have been attributed to its true cause, viz: the thinness of the de- 

 posit. Baral (Dingler's Journal, vol. 105, from Comptes Rendus, 

 May, 1847,) dissolved objects which had been subjected to the process 

 of fire gilding, and also such as had been gilded by means of gal- 

 vanism, in order to examine in both cases the film of gold, and found 

 the galvanic coating to have more continuity, but believes that it is 

 less adherent, because in fire gilding a double amalgam is formed, 

 and after the expulsion of the mercury a part of one metal is left 

 intermixed with the other. But if we take into consideration that 

 the coating produced by fire gilding is more porous and consequently 

 more readily worn away, we shall feel inclined to trust in the dura- 

 bility of the galvanic precipitate. But, besides, experience has shown 

 that galvanic gilding is as durable as that produced by the old process, 

 provided it is of the same thickness. Objects silvered by means of 

 galvanism often turn yellow after a while, but this may be prevented 

 by soaking and washing them carefully in warm water, when taken 

 out of the silvering solution, (Dr. Philipp, Dingler's Journal, vol. 92, 

 from the Berliner Gewerb-Industrie und Handelsblatt, 1844, No. 2.) 

 Objects that have become yellow can be made white again by coating 

 them with a paste of charred cream of tartar and water, and when 

 this is dried by heat, boiling them in an aqueous solution of cream of 

 tartar ; they must then be well washed and dried. — (Eisner, Dingler's 

 Journal, vol. 93.) Mourey recommends coating the objects with borax, 

 and heating to a temperature just sufficient to luse the borax. — (Ding- 

 ler's Journal, vol. 97 ; from the Technologiste, June, 1845.) 



With reference to the means of supplying the electric current, we 

 must take into consideration not only their constancy, but also the 

 expense and the inconvenience produced ; for instance, by the nitrous 

 fumes of Bunsen's or Grove's elements. For small objects a simple 

 apparatus is commonly used with an animal membrane as a diaphragm, 

 and with a solution of chloride of sodium or cyanide of potassium at 

 the zinc pole ; the object to be coated is then used as the other pole ; 



*I have precipitated gold with violent evolution of gas to the amount of 1 milligramme 

 per square centimetre, and afterwards rubbed it with the linger with a piece of linen cloth, 

 and lastly, with a wire-brush, without taking off more than J^ of the deposited gold. Of 

 2 milligrammes per square centimetre, ^ of the gold was removed by a similar procedure. 



