522

DYEING AND CALICO PRINTING.

more or less seriously with the uses of this acid. Arsenious acid, which isoften present, is dete&ed by means of Marsh’s apparatus, as diredted forhydrochloric acid, the sulphuric acid being previously diluted with ten timesits volume of water. Nitric oxide or other nitrogen compounds are alsocommon, and may be detefled by putting a little of the suspe&ed sample in atest-glass, and dropping into it a clean fragment of the protosulphate of iron(copperas). If a trace of nitric oxide be present, a slight reddish-browncolouration will appear in the liquid, and will become by degrees more intense.Dry sulphate of soda (salt-cake) is sometimes added to increase the apparentstrength of diluted acids. To deteft it a portion of the acid is evaporated todryness, when sulphate of soda, if present, remains behind.

The purity of the water is far from being an unimportant point. Rain-water, condensed steam-water, or the surface drainage of uncultivated moor-lands, should be seledted. Well- and river-waters, holding in solution lime,magnesia, iron, or alumina, may produce serious mischief.

It must not, however, be supposed that purity of ingredients and striftattention to the proportions laid down in approved receipts will suffice to pro-duce satisfactory results. The initial temperature, the order in which thematerials are mixed, the speed at which the operation progresses, and thehigher or lower degree of heat obtained in the process,—all modify the result.It is very easy to find two samples of a tin solution made with the same ma-terials in the same proportions, yet where the one gives an excellent resultthe other is pradtically worthless, having affinity neither for the fibre nor forthe colour.

I. Proto-Salts of Tin and their Solutions.

Protochloride of tin, or stannous chloride, unmixed with any other body,is used in dyeing and printing in various forms. These are tin-crystals,sometimes called tin-salt, or salts of tin ; double muriate and singlemuriate. The preparation of these compounds is very simple. To obtaintin-crystals, hydrochloric acid, at 32° T\v., is allowed to aft upon an excessof feathered tin, with the aid of heat. The operation is generally per-formed in large stone-ware vessels, heated by means of a water- or steam-bath, and fitted with covers to prevent needless waste of hydrochloricacid vapours, whilst allowing the hydrogen gas generated to escape. Somechemists use copper vessels for this purpose, which, so long as any tin remainsundissolved, are not in the slightest affedted by the acid. A very commonarrangement is to effedt the solution in a series of large three-necked receivers,connedted together by stone-ware pipes, so that any acid escaping from thefirst receiver is caught and utilised in the next. The most economical arrange-ment on the large scale is that devised by Noellner, who places the featheredtin in stone-ware cylinders, allows water to trickle down through the massfrom above, and drives in from below hydrochloric acid gas as it issues fromthe ordinary retorts in which it is generated. The saturated solution of tin,however obtained, is rapidly evaporated down, during which process some freshgranulated tin must be constantly kept in the liquid, otherwise oxychloride andbichloride of tin will be formed, and the properties of the preparation will bemodified. The liquid, when sufficiently concentrated, is drawn off, and allowedto crystallise.

The crystals, which should contain 52 per cent of metallic tin, are white,