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DYEING AND CALICO PRINTING.

Senegal gum. The solution of the latter gum in water is at first quitecolourless, but in a few hours it becomes very dark, sometimes as dark evenas an aqueous solution of Spanish juice. These gums contain a peculiarprinciple, which is known as arabine , and also a very small quantity of anorganic lime salt, and variable quantities of chlorides of calcium andpotassium and acetate of lime. Arabine is insoluble in alcohol, and even inweak spirits. Aqueous solutions of gum are therefore precipitated by thesefluids, and also by solutions of persalts of iron, nitrate of mercury, subnitrateand subacetate of lead. The precipitate contains gummic acid combinedwith metallic oxides. Boiling nitric acid converts gum into oxalic, tartaric,and mucic acids. Solutions of gum turn the plane of polarisation to theleft. Allowed to remain, they are gradually converted into glucose. Thesolutions of all varieties of gum very soon become acid, a fadt of someimportance in reference to the use of gums as thickeners. When gums areboiled for some length of time with dilute acids, glucosic acid is formed. Thesoluble gums do not reduce the potassio-tartrate solution of copper unlessthey are heated for some time with water either in sealed tubes or in a Papindigester at a temperature of 302° to 320° F. (150° to 160° C.)

Gum Arabic.—East Indian gums and an Australian article are largely usedfor manufacturing purposes, and these, together with samples of picked gumSenegal, are substituted for the genuine gum arabic, which has become tooscarce to be used as a thickener.

Gum Senegal is chiefly used as a thickener; it is dissolved in water at140° or even at 212 0 F. (6o° or ioo° C.), and allowed to stand till the impuritieshave settled, when it is passed through a fine hair sieve. It is employedespecially in printing with rollers. Gum solutions do not mix readily withother thickeners ; with pipe-clay there is, however, an exception. It happensoccasionally that larger or smaller quantities of gum, insoluble in water, arefound among the soluble gums; when this is the case the material swells inwater, but does not yield a clear liquid.

It is worthy of observation that gum neither tarnishes nor alters delicatecolours, nor weakens the mordants ; that it does not coagulate colours, andthat on being dissolved in water it thickens rapidly. The adtion of gumsupon delicate colours is best tested with cochineal pink or fuchsine upon purewool; after impression, the colour is steamed, and then washed, and if thegum is pure there will be no trace of any yellow tinge or hue.

The adtion of gum upon mordants varies greatly, and depends very muchupon the degree of acidity of the aqueous solution. For certain colours it istherefore absolutely necessary to use a very superior kind of gum; thisapplies especially to bright pink and rose-red. In order to estimate thedissolving adtion exerted by gum upon mordants, an impression is made witha mordant for bright rose-red, thickened with the gum to be tested. Afabric mordanted with this mixture after having been treated in the usualway, which is hereafter described, is placed in a madder-bath and cleansedwith soap, when it should produce a bright rose-red ; if the gum be acidthere is hardly any mordant left, and consequently no effedt of dye is after-wards to be seen.

Gums should possess the property of giving to water a certain degree ofviscosity without requiring an excess of material: this point is of far greater