COPPER PRECIPITATED. 79 



washed. A gold solution is evaporated to dryness, the dry 

 residue mixed with 1| pts. saltpeter, projected portionwise 

 into a red-hot crucible, extracted by water after cooling, the 

 insoluble residue dissolved in nitromuriatic acid, diluted, and 

 the gold precipitated as sulphuret by sulphuretted hydrogen ; 

 or after removing nitric acid from the solution by evaporation, 

 and then diluting, it may be obtained purer by precipitation 

 with copperas. 



See, also, Berlin Gewerbe-Industrie u. Handelsblatt, Bd. 18. 



Platinizing Glass, Porcelain, and Pottery. — Ludersdorff 

 gives the following method (Verb. d. Gewerbfl. in Preussen, 

 1847). A solution of platinum in aqua regia is evaporated 

 to dryness, at a gentle heat, so that the residue appears 

 reddish-yellow, and not brown, and is immediately dissolved 

 in an equal weight of strong alcohol. 8 pts. of the solution 

 are poured into 5 pts. oil of lavender, forming a clear brown 

 liquid, containing platinum as pt-otochloride. This solution is 

 brushed upon the article to be platinized, and after drying 

 burned in under a muffle. Glass and pottery is heated to low 

 redness ; porcelain to a bright red-heat. After cooling, the 

 articles are rubbed with cotton and prepared chalk. 



3. Various other metals and their compounds, beside the 

 precious metals, have been employed for coating articles for 

 various purposes, and we offer a few suggestions on these 

 points. 



Copper Precipitated. — In the usual method of precipitating 

 copper from mine-waters by bars of iron, more iron is dis- 

 solved than necessary, as the water generally contains an 

 excess of sulphuric acid, all the copper is not precipitated, 

 and a portion of oxide of iron is lost from the subsequent 

 exposure of the solution. Napier's improvement consists in 

 acidulating the liquid with sulphuric acid, which keeps the 

 surface clean for more energetic action, and in previously 

 putting in saw-dust or other organic matter, which converts 

 the persulphate present into protosulphate, so that all the iron 

 is obtained as copperas. 1000 litres of such water are treated 

 with 2 kilogr. sulphuric acid, and 2 kilogr. saw-dust (the last 



