MORDANTS.

521

is generally sold to dyers and printers at about 32 0 Tw. = sp. gr. 1*60. Whendiluted with distilled water and mixed with a small quantity of a weaksolution of chloride of barium it should not give a white precipitate or tur-bidity. If such appears the sample contains sulphuric acid, either fromslovenly preparation or from design, and should be rejected. If a few par-ticles of granulated tin are put in a test-glass and covered with the acid thegas given off should be free from the odour of sulphuretted hydrogen, andthe resulting liquid should be clear, not yellowish or turbid. In the lattercase the sample is contaminated with sulphurous acid, a very common im-purity when the manufacture is not conducted with due care.

An ounce of the acid should be placed in a small porcelain capsule, andevaporated to dryness at a gentle heat. If pure nothing will remain behind.If, as is sometimes the case, common salt has been added to raise the specificgravity of a weak acid, a saline residue will be left in the capsule. To detectarsenical compounds a few fragments of absolutely pure zinc are placed in aMarsh’s apparatus, and covered with the suspected acid previously let downto one-half its strength with distilled water. The gas generated is thenburned at the jet as directed in testing tin for arsenic.

Nitric acid, or, as it is technically still called, aquafortis, is sold for the pre-paration of mordants at two strengths. Double aquafortis varies in specificgravity, from 64° to 66° Tw., about 1*50 to 1*56. It maybe tested for sulphuricacid, for saline matter, and for arsenical compounds, in the same way asdirected for hydrochloric acid. In testing for arsenic, it should be previouslydiluted with four or five times its volume of water. Sulphurous acid is notlikely to be present. Nitrate of soda is a not unfrequent adulterant. Theaction of nitric acid in dissolving metals is greatly modified by the presence ofhyponitric and nitrous acids. These impurities may be removed by theaddition of peroxide of lead or red-lead. Either of these bodies gives offoxygen, so as to convert the lower oxides of nitrogen into nitric acid, whilstthe lead does not dissolve. The nitrous acid is, however, liable to be repro-duced if the stock is left exposed to the light, especially to direct sunshine.

The so-called “ single aquafortis,” or dyers’ aquafortis, is a quite peculiarpreparation. Its strength is generally 32 0 or 33 0 Tw. It is absolutely freefrom sulphuric acid and from the lower oxides of nitrogen. To free it fromthis impurity various expedients are adopted at different establishments.Sometimes it is exposed to the air for several weeks in a cool dark place.Others cause it to trickle in a thin stream down stone-ware channels filledwith clean quartz pebbles. In one large chemical-works, in Yorkshire, menare actually set to pour the acid from one receiver to another. Elsewhere astream of cold air is forced through the tanks of acid, which has the effedt ofcausing the nitrous and hyponitric acid to be expelled. Single aquafortiscontains a certain amount either of hydrochloric acid or of an alkalinechloride. This is not to be viewed in the light of an adulteration, sincewithout it the aquafortis would not answer its purpose in the preparation of“ scarlet spirits.”

Sulphuric acid, commonly known as oil of vitriol, or simply “ oil,” is gene-rally bought for mordant-making as “ double oil of vitriol,”—the strongestquality of acid which can be obtained by the ordinary process. It stands at169° to 170° Tw. A variety of impurities may be present, which interfere