MORDANTS.

277

Some of the uses of nitrate of alumina depend more on its adling as a conve-nient acid salt than as an aluminous compound : thus it is used in blockingupon chrome oranges, to change them into yellow : its utility, in some steamcolours, seems to be referable to the oxidising adtion of the nitric acid uponthe colouring matter. On account of this ready splitting up of the elementsof nitrate of alumina into acid and base, it must be sparingly and cautiouslyused as an ingredient in printers’ colours. Chloride of aluminium, or muriateof alumina, resembles the nitrate with regard to stability, its solution adtingnearly as powerfully as an acid, as if no alumina were present to neutralisethe acid contained.

Acetate of alumina, or red liquor, made by mixing together acetate oflead and sulphate of alumina, or alum, is a very important mordant. When toa concentrated solution of sulphate of alumina a solution of acetate of leadis added, there is formed a precipitate of sulphate of lead, while the resultingliquid retains some of that salt in solution, which is got rid of by the aid ofsulphuretted hydrogen and acetate of baryta. The solution of the acetate ofalumina so obtained smells so strongly of acetic acid that it seems as if aportion only of the acetic acid had entered into chemical combination withthe base. If spread out in thin layers, on plates of glass or porcelain, andheated to 37 0 , care being taken to keep the fluid constantly stirred, the saltdries up to a gum-like mass, easily re-soluble in water, but not exhibiting anylonger the slightest smell of acetic acid. This produ< 5 t is considered byMr. Walter Crum as a biacetate, while the liquor just alluded to is consideredto be a mixture of acetic acid and this biacetate in aqueous solution. Thesolution of gelatinous alumina in boiling acetic acid, applied in excess, alsoyields the acetate here alluded to. The strongest solutions of saturatedacetate, obtainable by double decomposition, contain about 5 per cent ofalumina. When that solution, or also the solution obtained by dissolvingalumina in boiling acetic acid, is left for several days standing at a temperatureof 15° to 20 0 , it deposits on the sides of the vessel a solid substance, whichafter drying has the appearance of porcelain ; and its composition is, accordingto Mr. Walter Crum, an insoluble biacetate containing five molecules of water.Another insoluble biacetate is obtainable in the shape of a crystalline precipi-tate. When a concentrated solution of red liquor, as first obtained by doubledecomposition, is heated, the supernatant liquid is almost entirely freed fromalumina. The solutions of acetate of alumina obtained by double decompo-sition, and containing less than 3 per cent of alumina, are not precipitated byebullition when they are freshly prepared, but after having been left standingfor some weeks they acquire this property as soon as they contain as muchfree acetic acid as is equal to that contained in a saturated solution of acetateof alumina containing 4 per cent of alumina. When to red liquor such saltsas, for instance, sulphates of soda, potassa, ammonia, magnesia, commonsalt, or alum, are added, they become far more readily turbid by the adtion ofheat than when pure ; but the solution of pure acetate of alumina is precipi-tated,—that is to say, becomes decomposed, and a basic salt thrown down—when heated to ioo°, while acetic acid is set free : the precipitate thus formedis re-dissolved on cooling, if care has been taken not to apply the heat so longas to expel the acetic acid which was set free.

Red liquor prepared with alum and acetate of lead, instead of sulphate of