74 HYDROMETALLUIIGY. [ill. 



is introduced into this mixture, which is then heated to boiling, 

 and in a short time is again- withdrawn, covered by a shining 

 coat of mercury. It is now ready to receive the amalgam of 

 gold or silver for the purpose of fire-gilding (Pogg. Annal. 

 1846). The strongly positive zinc amalgam increases the 

 electric tension between the positive iron and negative mer- 

 cury, so as to cause their union. 



Gilding Watch-wheeU. — Ph. Plantamour prepares an amal- 

 gamating fluid for gilding wheels of watches, which, being 

 alkaline, cleans and amalgamates the wheels at the same time, 

 without injuring the steel pivots. Mercury is dissolved in an 

 excess of nitric acid, and ammonia added to the solution until 

 the precipitate at first form.ed is redissolved. The wheels being 

 immersed in this solution, the ammonia dissolves fatty matters, 

 with other impurities, from the surface, and the brass is 

 amalgamated. While still moist, the wheels are covered with 

 gold amalgam, put on a drum with holes for inserting the 

 pivots, and gently heated over a spirit lamp, so that the qua- 

 lity of the steel is not impaired. (Comptes Rendus, xxiv. 784.) 



Silvering Mirrors. — For Drayton's original process for sil- 

 vering glass surfaces, see Lond. Journ. xxiv., or Journ. Fr. 

 Inst. viii. 3 ser. His improvements in the same are in the 

 Lond, Journ. for 1849, and Journ. Fr. Inst. 1850. One ounce 

 ammonia, 2 oz. nitrate of silver, 3 oz. water, and 3 oz. spirit 

 of wine are mixed together, allowed to stand for 3 or 4 hours, 

 and then filtered. A quarter-ounce sugar (grape-sugar being 

 preferred), dissolved in a half-pint of spirit of wine diluted 

 with as much water, is added to each ounce of the filtered 

 liquid, and this solution is employed for silvering, the article 

 to be silvered being kept at 160°. 



Meurer dissolves 5 grs. lunar caustic (nitrate of silver) in a 

 little caustic ammonia, and adds to it a mixture of 1 drop oil 

 of cinnamon, 2 drops oil of cloves, and 1-1 J drachms of 

 absolute alcohol. The mixture becomes gradually cloudy, 

 depositing a brown precipitate, which is filtered oiT, and the 

 clear liquid poured upon a clean glass plate, surrounded with 

 a rim. In the course of a few hours, it is covered with a 



