MORDANTS.

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acid, such a nitrate of iron is less likely to prove dangerous. Hence nitrates,with the mixture of acetate, are sometimes used as Burling Irons , for the fol-lowing purpose :—In dyeing woollens a black, or indeed other shades, it is verycommonly found, when the goods leave the dye-beck, that they are sprinkledover with small specks which have failed to take the colour, and have remainedwhite, or rather a dirty grey. These spots—technically called “ burls ”—arecaused by portions of cotton which have become intermixed with the wool,and which, not having the same affinity for colours as the wool, escape undyed.In case of black goods, if these spots are very numerous, the pieces are sub-mitted to a formal cotton-dyeing process, with an astringent and with nitrateof iron. In the selection of the latter much care is required. If too sharp italters or destroys the shade of the wool. If the iron is in excess the goodswill be disfigured with rusty stains. The above-mentioned black iron, withthe addition of acetate of lead, is preferred by many practical men for thispurpose.

The “ common ” or “ saddening ” irons are sharper than those used forblack dyeing, in order that the oxide of iron may be deposited very slowly,and may combine more firmly with the tannin of the sumac, myrobalans, &c.,and may the better resist treatment with alum or tin mordants. They are, asfar as possible, perfedt per-salts of iron.

The following receipt is in extensive use:—Prepare a nitrate of iron by theprocess given for “ blue irons ” on cotton-yarns. The weight of nitric acidto 5 cwts. of copperas may be 130 to 135 lbs. After the iron has been workedas above diredted, the liquid is run off from any residue of undissolvedcopperas, and then diluted with water. By this means no unconverted proto-sulphate of iron is dissolved as such and mixed with the nitrate of iron.

Common Iron for Drabs.

Let down nitric acid (double aquafortis) to 34° Tw., and of it take 100 lbs.Dissolve in it 4 lbs. of clean scrap iron. Then add by degrees as muchcopperas as it will take up, which will be about 84 lbs. Let it down with waterto about 6o° Tw.

Muriatic acid should never be used in the manufacture of nitric acid. Thenitric acid employed should be free from this impurity, and if nitrate of sodais used it should be freed from common salt by re-crystallisation. Muriaticacid not only retains the iron more tenaciously than nitric, but delivers it uponanimal fibre.

In examining samples of nitrate of iron the following instrudions mayprove useful:—Mix a portion with pure hydrochloric acid, and dilute with dis-tilled water. If a white precipitate is formed on adding a solution of chlorideof barium, sulphuric acid is present. This shows either that the sample hasbeen made—wholly or in part—from copperas, or that sulphuric acid, as such,has been added. Dilute another portion of the sample slightly with distilledwater, and add pure ammonia in excess. Filter off the precipitate, evaporatethe clear filtrate to dryness, and heat the residue to redness in a small porce-lain capsule. If any fixed matter remains the sample has been got up withnitrate of soda. Another portion is diluted with distilled water, a few dropsof pure nitric acid being added to prevent turbidity. A solution of nitrate ofsilver is then added. If a white curdy precipitate is formed, either some