_Conversion of Sulphuret of Silver into Metallic Silver._--The black sulphuret of silver may be reduced to the state of metal by roasting and subsequent fusion with carbonate of soda; but it is more convenient, in operating on a small scale, to proceed in the following manner:--first convert the sulphuret into nitrate of silver, by boiling with nitric acid diluted with two parts of water; when all evolution of red fumes has ceased, the liquid may be diluted, allowed to cool, and filtered from the insoluble portion, which consists principally of sulphur, but also contains a mixture of chloride and sulphuret of silver, unless the nitric acid employed was free from chlorine; this precipitate may be heated in order to volatilize the sulphur, and then digested with hyposulphite of soda, or added to the hypo bath. The solution of nitrate of silver obtained by dissolving sulphuret of silver is always strongly acid with nitric acid, and also contains _sulphate_ of silver. It may be crystallized by evaporation; but unless the quantity of material operated on is large, it will be better to precipitate the silver in the form of chloride, by adding common salt, as already recommended. * * * * * On the Use of Test Papers. [Sidenote: ON THE USE OF TEST PAPERS.] The nature of the coloring matter which is employed in the preparation of litmus-paper has already been described at page 98. In testing for the alkalies and basic oxides generally, the blue litmus-paper which has been reddened by an acid may be used, or, in place of it, the turmeric paper. Turmeric is a yellow vegetable substance which possesses the property of becoming brown when treated with an alkali; it is however decidedly less sensitive than the reddened litmus, and is scarcely affected by the weaker bases, such as oxide of silver. In using test papers observe the following precautions:--they should be kept in a dark place, and protected from the action of the air, or they soon become purple from carbonic acid, always present in the atmosphere in small quantity. By immersion in water containing about one drop of liquor potassæ in four ounces, the blue color is restored. Test-papers prepared with porous paper show the red color better than those upon glazed or strongly sized paper. If the quantity of acid present however is small, it is not sufficient in any case simply to dip the paper in the liquid; a small strip should be thrown in, and allowed to remain for ten minutes or a quarter of an hour. If the paper, on immersion, assumes a _wine-red_ or purple tint, in place of a decided red, it is probably caused by carbonic aid gas: in that case the blue color returns when the paper is washed and held to the fire. Blue litmus-papers may be changed to the red papers used for alkalies by soaking in water acidified with sulphuric acid, one drop to half a pint. * * * * * The Salting and Albumenizing Paper.