GALVANIC GILDING AND SILVERING. 75 



brilliant white coating of metallic silver. Eisner observes that 

 it is necessary to let the liquid remain until all the brown sedi- 

 ment has separated, in order to avoid spots on the silver surface. 

 Tourasse protects the back of the silver with varnish. The 

 cost is said to be J of that of the amalgamated tin, a very 

 thin coating of silver being sufficient. Reichardt dissolves 

 1 oz. lunar caustic in 2 oz. water, adds J oz. caustic ammonia, 

 and 3 oz. strong alcohol containing 30 drops of oil of cassia, 

 filters, and lets the whole stand 3-6 days. The liquid is 

 poured on a glass plate cleaned by potash. He employs the 

 oil of cloves in vapor, by dropping a little into a warmed 

 vessel, which is turned a little to spread the oil, and is then 

 inverted over the glass plate. The coating is made in J hour. 



Silvering glass by gun-cotton has been effected by Vohl, 

 by dissolving the cotton in caustic potassa, adding a little of 

 nitrate of silver, and then sufficient ammonia to redissolve the 

 oxide of silver, while the whole is kept warm. The whole of 

 the silver is precipitated as a brilliant coating on the sides 

 of the vessel. Other analagous nitric compounds of sugar, 

 manna, and gums produce a like result. (Technologiste, Lond. 

 Journ. 1849, and Amer. Journ. (2) viii. 117.) 



Speculums. — T. Fletcher's patent (Ch. Gaz. vi.) for making 

 speculums, is to take a glass which has been silvered as for 

 mirrors, and to coat the metallic side with a varnish composed 

 of 2 oz. shellac, | oz. lamp-black, and J pint absolute alcohol 

 as a protective against dampness and the action of acid. As 

 the coating becomes dry, it is dusted over with finely-powdered 

 plumbago, and the glass is then submitted to the electrotype 

 process, by which means a thin coating of metal will be pre- 

 cipitated over the whole back. 



Galvanic Crilding and Silvering. — (Communicated to Eisner 

 by Mr. Brauns.) — Smee's battery is best adapted to the pur- 

 pose, as it is simple in construction (consisting of platinized sil- 

 ver-foil, surrounded by an amalgamated sheet of zinc) ; requires 

 only one liquid (1 pt. sulphuric acid to 8 pts. water) ; con- 

 tinues long in action, and gives off no gas during the operation. 

 The cells consist of leaden vessels internally pitched. The 



