SULPHUR As A MonnArrr.—Hyposulphite of soda was proposed some time ago for fixing certain aniline greens, and was used with success. The suggestion was due to a photographer, who, having been accustomed to employ hyposulphite of soda as a fixing agent in his own department, concluded that it would "fix anything." This case is interesting as an example of practical success reached by a false theory. There are many such.
It has since been found that the value of hyposnlphite of soda depends on the fact that tissues saturated therewith, and afterwards taken through an acid, or even exposed to the air, become coated with finely divided sulphur, which, in this state, absorbs colours by surface attraction, very similarly to amorphous silica. Sulphur in this condition is a useful mordant for many coal-tar colours, but is of little value for the fixation of cosine.
TIN MORDANTS.-Till within the last twenty years, tin mordants might, perhaps, claim the supremacy over all others, since the brightest and most beautiful colours, whether upon cotton, silk, or wool, were fixed by their means. Their importance is now diminished, inasmuch as many of the finest shades formerly obtained from natural colouring matters with their intervention are obtained in even greater purity and brightness from the aniline colours. Still the applications of tin in dyeing and tissue-printing remain extensive and manifold. To enumerate and describe all tho tin compounds which have been, and still are, in use, would require a volume of fair size. They comprise both stannous and stannic salts, or, as they are still technically named, protomuriate, protosulphate, &c., and on the other hand, permuriate, nitromuriate, or oxymuriate. Lastly, in the stannate of soda, the tin appears not as a base, but as an acid. The affinities of tin, both for the fibre and for the colours, are strong, and the "lakes " which it forms with the latter are rich and bright. Though not exempt from the action of sulphur, it blackens much less rapidly than lead.
The so-called protochloride of tin, protomuriate, or simply muriate (stannous chloride), is made by allowing hydrochloric acid at about 32° Tw. to act upon granulated or feathered tin, with the
aid of heat. The process is generally conducted in large stoneware vessels heated by hot water or steam. In some establishments, hydrochloric acid gas, just as evolved by the action of sulphuric acid upon common salt, is allowed to pass over granulated tin in stoneware cylinders, down which water is constantly allowed to trickle. Muriate of tin is sold in three states, differing merely in concentratiou.
Single muriate is a solution ranging in strength from 40° to 60° Tw., and containing from 1 to 2 oz. metallic tin in the lb. It is used by woollen dyers. Double muriate of tin varies in strength from 70° to 120° Tw., and contains proportions of tin from 2i to 5 oz. a lb. The weaker and more acid kinds are used in woollen-dyeing; and the more neutral and thoroughly saturated kinds, from 110° to 120° Tw., for cottons. The mere specific gravity, however, gives no certain clue to the composition of a sample, as, even in the absence of all impurities, a variety in proportion may occur. Some makers dissolve the tin in undiluted acid at 32° Tw., whilst others add water before dissolving. The higher the original specific gravity of the acid taken, the smaller will be the pro portion of tin needed to bring it up to any given degree of the hydrometer. The chief impurities which may bo met with are sulphuric acid and sulphates of zinc and magnesia.
Tin crystals, sometimes called salts of tin, are merely muriatc of tin evaporated to crystalliza tion. They should contain about 52 per cent. of metallic tin. A good sample dissolves in about ten times its weight of water, with little or no turbidity. If a few drops of pure hydrochloric acid are added to this solution, no precipitate should be formed on dropping in a solution of barium chloride. Should a white precipitate appear, sulphuric acid in some combination is present.