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DYEING AND CALICO PRINTING.

alumina, consists of a mixture of acetate of alumina and an alkaline sulphate,and hence this preparation is more readily decomposed by heat than the pureacetate, which requires longer boiling and a certain degree of concentration.

For the preparation of neutral acetate of alumina, by means of alum andacetate of lead, xoo parts of the last-named salt require 62*6 of alum for com-plete decomposition, but very frequently in practice alum is applied in slightexcess.

In Germany, a solution of alum is very frequently first converted into inso-luble (basic) alum by means of carbonate of soda, and the resulting saltdissolved in acetic acid. Acetate of alumina is usually made by the com*sumers, who often employ, instead of acetate of lead, the cheaper acetate oflime for the purpose of double decomposition, or also acetate of soda. Some-times red liquor is purposely mixed with such salts as chloride of zinc, commonsalt, or chloride of ammonium, for the purpose of preventing the too rapiddrying up of the acetate on the textile fibres. Acetate of alumina is alsoapplied, on the large scale, to render woollen fabrics water-tight. If acetateof lead be mixed with alum and hot water, and cotton cloth then passed in it,it takes up alumina in sufficient quantity to dye with. In mordanting woollencloth, cream of tartar (bitartrate of potassa) is used to mix with the alum, andhere a similar decomposition takes place ; the sulphuric acid goes to thepotassa, leaving the tartaric acid to the alumina, from which the wool can takeit up easily. The acetate of alumina does not answer so well for woollenfabrics as the tartrate, perhaps because it gives up its alumina too readily,whereby the mordanting is rendered superficial and uneven.

Aluminate of potassa is known as alkaline pink mordant. Alumina, inthe hydrated gelatinous state, is readily soluble in caustic alkalies. If causticpotassa is added to a clear aqueous solution of alum, with the precaution notto overdose the alkali, there is at first precipitated gelatinous hydrate ofalumina, which re-dissolves on adding more alkali. On the large scale thissolution is made by taking a sufficient quantity of a solution of causticpotassa, sp. gr. 1*307 to 1*41 = 40° to 50° Tw., and heating nearly to boilingin an iron pan. The requisite quantity of alum or sulphate of alumina, incoarse powder, is put in by degrees, the heat of the liquid being kept up, andthe whole well stirred. Finally, it should be boiled a short time, say fifteenminutes, and then allowed to cool. Crystals of sulphate of potassa are depo-sited, which are very sparingly soluble in the alkaline liquid. Another sediment,of alight colour, is found at the bottom of the pan, consisting of alumina notdissolved. About 3$.lbs. of alum to a gallon of caustic lye, at 40° Tw., is aproper proportion : if sulphate of alumina be used the same quantity may betaken, but the caustic liquid should be more concentrated, ranging from 50° to55 0 Tw. Practical experience has taught that ammonia-alum will not answerfor this purpose, since more caustic lye is required to drive off the ammonia.Alumina hydrate may be made first, and then dissolved in caustic lye. Thisis a more scientific as well as a more economical method, since no alkaliwould then be required for combining with the sulphuric acid of either alumor sulphate of alumina.

Aluminate of potassa should be kept in a covered vessel, since it is injuredby lengthened exposure to air: it is preferably kept in vessels made of iron,since it attacks glass, earthenware-jars, and wood. The addition of acids or acid