SULPHINDIGOTIC ACID.

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added. Sulphindigotate of potassa requires 140 parts of water for solution;the soda salt is somewhat more soluble ; the baryta salt is rather sparinglysoluble in cold, but more readily so in hot, water; the lime, magnesia, andalumina salt are very soluble, but the lead salt is insoluble.

All reducing agents, such as mixtures of zinc and sulphuric acid, tin-salt,hydriodic and hydrosulphuric acids (at 50°), zinc and sulphurous acid, de-colourise sulphindigotic acid. The alkaline salts, which are reducing agents,e.g., sulphites, also readily bleach solutions of sulphindigotates, but the bluecolour is restored by the adtion of the air. Hyposulphindigotic acid may beobtained from the alcoholic solution of the ammoniacal salt by precipitatingwith acetate of lead, and decomposition of the lead salt by sulphurettedhydrogen. This acid is only distinguished by the solubility of its alkalinesalts in alcohol at 83 per cent; pradtically this acid is of no importance.

The following is a brief summary of the results of the investigations madeby Berzelius and Gros-Renaud on the sulpho acids of indigo when adted uponby alkalies and alkaline earths. These researches are indeed interesting, andmight, if resumed and more fully gone into, produce useful pra&ical results,since the savans just named found that these blue substances are capable ofyielding red, yellow, green, and violet derivatives, which dye wool and silkwithout the aid of mordants. Hydrosulphindigotate of baryta in solutionbecomes green when evaporated on a water-bath; the residue, very soluble inwater and alcohol, yields with subacetate of lead a greyish green precipitate,from which, after treatment with sulphuretted hydrogen, is obtained sulpho-viridic acid, the solution of which appears green by refledted and red bytransparent light.

When sulphindigotate of potassa is mixed with 30 parts of lime-water, andthis mixture submitted to heat in a sealed tube, a purplish liquid is formed;when, after opening of the tube, the excess of lime is precipitated by means ofcarbonic acid, and the liquid, after having been filtered to remove carbonateof lime, evaporated to dryness, the residue first washed with alcohol, andnext treated with water, there is obtained a beautiful purple liquor, whichyields, when precipitated with acetate of lead, a reddish-brown deposit, which,after having been treated with sulphuretted hydrogen while suspended inwater, and after separation of the sulphuret of lead, leaves a purple liquid,from which, after evaporation to dryness, is obtained an amorphous brownmass, sulphopurpuric acid. When sulphindigotate of potassa is adted upon bylime-water in contadt with air, the liquid becomes first red, next yellow. Whenthe lime is withdrawn from the liquid while still red by means of carbonicacid, and the carbonate of lime is first filtered off, and the liquid nextevaporated, a brown residue is obtained, from which alcohol at 82 per centdissolves out a yellow compound. This yellow fluid yields, when precipitatedwith acetate of lead, a yellow mass, which, on being decomposed by sul-phuretted hydrogen, produces sulphide of lead, and sulphoflavic acid, a yellowcrystalline substance, after evaporation of its solution. The residue from thetreatment with alcohol just named dissolves in water, giving a red liquid,precipitable by basic acetate of lead; the resulting lead salt having again beentreated by sulphuretted hydrogen, yields, to absolute alcohol, after evapora-tion, sulphofulvic acid, a reddish-yellow substance; while the residue aftertreatment with absolute alcohol consists of sulphorufic acid, a red matter.Both these acids are amorphou s and soluble in water.