344

DYEING AND CALICO PRINTING.

obtain a white combination of hematoxylin and the alkalies, because theaffinity of that substance for oxygen is so greatly increased by the presence ofpotassa or soda, and it is, moreover, difficult to have hematoxylin in solu-tion so completely free from any trace of oxidation, that the addition even ofany alkali to a liquid containing it brings out at once a violet tint, whichdarkens rapidly on exposure to air.

A colourless alkaline hematoxylate maybe obtained by taking advantage of thesparing solubility of these salts in a liquid containing common salt in solution.A cold decodtion of logwood, or a solution of hematoxylin, is first saturatedwith common salt, and next a solution of caustic soda is gradually added;the first portion of the ensuing precipitate will be more or less coloured, be-cause it carries down the oxidised substance, but by a rapidly managedfiltration this portion of the precipitate may be eliminated, and on continuedaddition of alkali to the filtrate a white precipitate of hsematoxylate of sodawill be obtained, which becomes, however, rapidly coloured on access of air.Neutral and basic acetates of lead yield, with solutions of hsematoxylin,bluish-white precipitates, which also become Fapidly darkened by contact withair. Solutions of salts of gold and silver yield precipitates which soon alter,and deposit reduced metals; salts of copper also precipitate hematoxylinsolutions. Protochloride of tin produces a permanent rose-cofoured preci-pitate ; solution of alum gives a bright red colour, but no precipitate is atfirst formed, though after some time a slight blackish-coloured precipitateensues; acetate of alumina gives a fine purple. Dilute mineral acids haveno particular adtion upon hsematoxylin ; concentrated nitric acid converts itinto oxalic acid ; chlorine converts it into a brown non-crystallisable substance,not thoroughly examined; chromic acid and bichromates are, in contact withhsematin, instantaneously reduced, forming a deep black, which is extensivelyapplied in cotton dyeing and printing.

The charadters and reactions of the decodtion of logwood, as first describedby M. Chevreul, refer to mixtures of colourable matter (hsematin) and adtualcolour (haematein). Dilute mineral acids turn the decodtion yellow ; if theacids are concentrated a red is produced. Since sulphuretted hydrogen enters,by causing deoxidation, into a colourless combination with the already moreor less oxidised substances present, that gas or its aqueous solutions bleachthe decodtion. Sulphurous and carbonic acids turn the decodtion yellow;alkalies produce first a red, and ultimately a violet; baryta, lime, and hydratedmetallic oxides yield blue precipitates ; basic salts adt as bases, and acidsalts like acids ; aluminate of soda yields an abundant violet-blue precipitate,insoluble in excess of alkali—this test is so delicate and so charadteristic thatby means of it logwood may be detedted in a mixed decodtion with greatfacility.* Hydrated protoxide of tin combines with the colouring matters oflogwood decodtion in the same manner as the alkalies, yielding a violet-lake.Protochloride of tin yields with the decodtion a violet precipitate. Stannichydrate adts as an acid, and turns logwood liquor red. Solution of alumyields first a yellow colouration, which after a while turns red. Salts of ironyield a blue-black (hence the application of logwood for the preparation ofwriting ink). Salts of copper give a blue precipitate. Salts of zinc yield a

* E. Mathieu-Plessy, “ Bulletin de la Societe Industrielle de Mulhouse,” vol. xxvii., p. 4°3*