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DYEING AND CALICO PRINTING.

a peculiar odour when the salt is submitted to combustion, and leaves a grayor black residue. Most metallic salts can be detected by treating the sampleof oxalate of potash with an excess of ammonia, and adding sulphide ofammonium.

Manganate of Potash,

Manganate of potash is obtained by heating peroxide of manganese withcaustic potash, dissolving and crystallising; it occurs in crystals of a finegreen colour. The evaporation must be conducted in vacuo . The applicationis limited to the restoration of indigo colours.

Permanganate of Potash.

Permanganate of potash, or chameleon salt, mineral chameleon, occurs inred-purple crystals. It can be obtained by heating peroxide of manganesewith caustic potash, and treating the resulting mass with boiling water.The salt can then be crystallised out by evaporation. It is a most powerfuloxidising agent, and has recently been applied to the bleaching of animal andvegetable fibres.

Tartar.

The tartar of commerce is obtained from the only available source, fromwine tuns, where it is deposited as red tartar, cream of tartar, or bitartrate ofpotash. Tartaric acid combines with potash to form two salts, the tartrateand bitartrate of potash ; the former is very little known, but the bitartrate isextensively employed under the name of argol, red tartar, or simply tartar. It isemployed in dyeing in its crude form, but for finer kinds of work it is necessarythat it should be twice re-crystallised. It is not readily soluble in water,and at the usual temperature cannot be concentrated to a higher degree than2° Tw. For cochineal-pink in calico-printing, and for mordanting woollengoods, it is largely in demand, in the latter case being employed with alum andsalts of tin. Besides its application as an acid, tartaric acid possesses theremarkable property of masking metals or preventing their precipitation underthe influence of potash; thus potash will not throw down a precipitate from asolution of sulphate of copper to which tartaric acid has been added. Manysubstitutes have been suggested and sold for tartaric acid, but their successhas always been doubtful. The purity of tartaric acid or bitartrate of potashcan be determined by calcining a sample in a porcelain crucible. If the acidis pure nothing but a black coke will remain, and this will disappear at ahigher temperature. The potash salt should leave either a white or greyresidue. An admixture of sulphates may be detected by means of chlorideof barium. Copper salts may be detected by ammonia, and salts of theprotoxide of iron may be deposited as sulphides by sulphide of ammoniumafter the addition of ammonia.

Potash Water-Glass.

Potash water-glass, K 0 ,Si 0 2 , is made of 45 parts of pulverised quartz,30 parts of potash, and 2 parts of wood charcoal strongly heated together.Potash water-glass occurs in commerce in both fluid and solid form. Fluidwater-glass is a yellow, thick, fluid, which by exposure to the air suffers littleconversion. It colours litmus paper blue. As a fixing agent water-glass islargely employed in printing.