40

DYEING AND CALICO PRINTING.

knowledge of their properties desirable from a practical point of view. Thedarker shade of colour seen in the so-called “ nankin ” cotton is probably dueto a great excess of these colouring matters existing in the fibre. It is certainlynot caused by oxide of iron, since the ash of this kind of cotton contains nomore iron than that of ordinary kinds, and the colour is for the most part re-moved by treatment with caustic alkali.

Pedtic Acid.

A considerable portion of the organic matter extradted from cotton by causticalkali consists of a body belonging to the pedtine class. Which member ofthis class it is that exists originally in the fibre is a question on which noopinion is expressed, though there can be no doubt that the precipitate pro-duced by acid in the alkaline extradt of cotton contains pedtic acid itself.This acid, which is insoluble in water, is almost entirely converted during theprocess of purification adopted into an acid soluble in water, which seems tobe identical with Fremy’s parapedtic acid. The chief properties of this solubleacid are as follows :—

On evaporation it is left as alight yellow, amorphous, translucent substanceresembling gum or gelatine. When heated on platinum it burns with a slightflame, without previously melting, leaving a little ash, which is yellow orbrown and only slightly alkaline, and consists chiefly of alumina, togetherwith oxide of iron and lime. The watery solution is clear and colourless andreddens litmus-paper. It gives abundant white flocculent precipitates withsulphuric, nitric, hydrochloric, and acetic acids, as well as with baryta water andacetate of lead. When the watery solution is mixed with several times itsvolume of alcohol the liquid gelatinises, the jelly being perfedtly clear andtransparent. The substance is decomposed with difficulty by boiling nitricacid; and no oxalic acid can be discovered among the produces of decom-position. When treated with strong caustic potash or soda-lye it turns yellow;and on boiling, the liquid assumes a bright yellow colour similar to that ofchromate of potash solution. On continuing the adtion of the caustic alkali thesubstance is gradually dissolved; but on adding an excess of acid no precipitateis produced, showing that a complete change has taken place. In very dilutepotash or soda-lye, as well as in liquid ammonia, it dissolves readily onboiling. The solutions when mixed with alcohol give colourless jellies, con-sisting of the salts of the respedtive bases. If the precipitated ammoniacompound, after being filtered off and washed with alcohol until the excess ofammonia is removed, be treated with boiling water, it dissolves, and thesolution leaves, on evaporation, a transparent, amorphous, nearly colourlessresidue, which separates from the sides of the vessel in shining scales. Thisresidue dissolves again entirely in water. The solution is neutral to test-paper, and gives flocculent precipitates with alkaline salts, such as chlorideof sodium and chloride of ammonium, as well as with all earthy and metallicsalts which were tried, except perchloride of mercury and chloride of gold.The precipitate with nitrate of silver, which is white, does not change muchwhen exposed for several days to light, and when dissolved in ammonia yieldsa solution which on being boiled becomes yellow but deposits no metallicsilver. When a solution of the acid in a sealed tube is heated for some timein the water-bath it is found to have undergone a complete change. The