RUBIAN.

241

after some time a yellowish-red or brownish precipitate of purpurin is formed.The same happens when nitric acid is substituted for hydrochloric acid. Inboth instances this fluid remains orange-yellow. Oxalic acid produces noimmediate change, but after a time there is formed a pale yellow flocculentprecipitate containing oxalate of lime. The colour of the fluid is not altered.No readtion is produced by the addition of alkaline chlorides, or of thechlorides of the alkaline earths. Carbonate of soda gives no precipitation,but colours the fluid purple. Lime-water yields immediately a brownish-crimson precipitate, and the colour of the liquid turns a reddish-violet. If theprecipitate is filtered off, and heat applied, another precipitate of a rose-red isproduced. Solution of alum produces no change at first, but after a time abrilliant vermillion precipitate is produced, the supernatant liquor remaining ayellowish-red. If a large excess of solution of alum is added, and a solutionof carbonate of soda or lime-water immediately after, a rose-red precipitate isobtained, which, when dry, exhibits a very pure rose-colour, especially if lime-water has been employed. The acetate and the hyposulphite of alumina givesimilar reactions. The precipitation of this alum-lake is promoted by heat.Its shade depends very much upon the relative proportions of the ingredients,and upon the concentration of the madder-liquor. With hypochlorite of limethe liquid is bleached to a yellowish colour, and a light flocculent precipitateis deposited. A solution of protosulphate of iron gives a deep brown preci-pitate, the liquid assuming a deep—somewhat inky—colour. With perchlorideof iron no immediate precipitate is formed; the liquid turns a deep brownish-yellow, and after a time a brownish-black precipitate is deposited. Perchlorideof tin gives an orange-yellow precipitate. With acetate of lead in moderatequantity there is formed a chamois-coloured precipitate, the liquid retainingits orange-yellow colour. If the precipitate first formed is removed, and afurther quantity of the same reagent is added, a yellowish-red precipitate isproduced, and the supernatant liquid retains only a slight yellowish colour.On the addition of ammonia to this liquid a fine rose-red precipitate is pro-duced. Corrosive sublimate gives a flesh-coloured precipitate, whilst theliquid remains yellow. If the mercuric salt has been previously acidified withhydrochloric acid no immediate precipitate is formed, but after a time anorange-red precipitate is deposited, which is almost entirely soluble in boilingwater.

Secondary colouring matters, either insoluble or but very sparingly solublem water, are derived from the decomposition of the above-mentioned sub-stances. On this subjedt there has been much controversy; for while no onedenies the existence of alizarin, some assume the presence of one or moredistindl tindtorial substances, whilst others hold these to be merely alizarinin a more or less impure state. This last view gained strength from the fadtthat the physical, chemical, and tindlorial properties of alizarin are veryreadily modified and masked by the presence of small quantities of heteroge-neous matter. Thus, as long as alizarin retains traces of a resinous fattymatter which clings to it very persistently, it yields in alcohol a brown solution,and cannot be obtained in well-defined crystals. When pure it dissolves inalcohol with an orange colour, and forms long needle-shaped crystals. Thelarge number of colouring matters which, according to Schunck, are derivedfrom rubian, did not appear to many investigators of this subjedt sufficientlywell defined to be accepted without some hesitation.