520

DYEING AND CALICO PRINTING.

is a poor compensation for occasional and costly failures. The like care mustbe taken in the selection of the acids employed. Traces of iron and ofarsenic are very pernicious. The coarse hydrochloric (muriatic) acid obtainedas a secondary produdt at the alkali-works, and known in commerce as “ tower-salts,” is not trustworthy. It sometimes yields good results, but as frequentlyoccasions mischief, the more serious as the cause is not readily detected.The hydrochloric acid employed should be “ cylinder-salts,” made from cleansalt, and from sulphuric acid free from arsenic. The presence of sulphurousacid, of free chlorine, of arsenious acid, or arsenical chlorides, leads toannoying complications. Both the hydrochloric and the nitric acid usedshould be free from sulphuric acid, an agent which, indispensable as it is,should never be introduced except where directed, and then in exaitly knownquantities. The sulphuric acid employed should be prepared from Siciliansulphur, and not from iron- or copper-pyrites, which contain arsenic in greateror smaller quantities. It should also be free from nitrogen compounds.

The following instructions will be of use in judging of the quality of thematerials used in the preparation of tin mordants :—The tin should dissolvein pure hydrochloric acid without leaving a black powdery residue. If suchresidue appear it should be well washed in distilled water, dissolved in nitricacid, freed from excess of acid by evaporation, and mixed with an excess ofammonia. A blue tinge indicates the presence of copper.

For the detection of arsenic and antimony a few fragments of the sus-pected metal are placed in a Marsh’s apparatus, and dissolved in pure hydro-chloric or dilute sulphuric acid. The gas given off is burned at the jet, withthe precautions indicated in treatises on qualitative chemical analysis, andfragments of white porcelain are held in the flame. If shining metallic spotsare deposited on the porcelain, either arsenic or antimony or both must bepresent.

As regards other impurities, a portion of the solution of the metal in purehydrochloric acid is saturated with sulphuretted hydrogen gas, a current ofwhich is passed into it as long as anything is deposited, and until the liquidhas a strong and permanent smell of the gas. The precipitate formed, whichmay contain sulphurets (sulphides of tin, lead, copper, arsenic, and antimony),is filtered off, and digested in hydrosulphate of ammonia containing an excessof sulphur. Should any matter remain undissolved, which when dried andheated does not volatilise, it is probably sulphide of lead.

If zinc and iron are present, they will be found in the liquid filtered fromthe precipitate occasioned by the current of sulphuretted hydrogen. Thisfiltrate is concentrated by evaporation to a small bulk, heated with nitric acid,and, after the excess of the acid has been expelled, it is mixed with ammonia.If iron is present it will be deposited as hydrated peroxide, of a reddish-browncolour. This is removed by filtration, and the residual liquid, after furtherconcentration, is mixed with a solution of perfedtly pure carbonate of soda.A white precipitate shows the presence of zinc.

Or the liquid as filtered from the precipitate obtained by sulphurettedhydrogen is mixed with hydrosulphate of ammonia. If any precipitate offixed matter is obtained the tin contains iron, zinc, or some metal of the sameclass.

Hydrochloric acid, or, as it is still called, muriatic acid or “ spirts of salts,”