GUMS.

6g

is valuable, as it enables him to convert insoluble into soluble farina ordextrine without any change of colour. This he effects by subjecting starch,farina, and other amylaceous substances to the chemical aCtion of hydrochloricor some other acid gas or vapour in a cylinder, the exterior of which issurrounded by an atmosphere of steam. This process cannot be said to havematerially extended the employment of soluble farina as a substitute for gum-arabic, as perfectly soluble white dextrine is easily made on the large scale byPayen’s process without needing the expensive apparatus used by O’Neill.

Calico printing, as at present developed, having assumed such extraordinaryproportions, thousands of tons of soluble materials for thickening the variousmordants and colours are required, and a great variety of this class of artificialgums are prepared to meet the requirements. Thus, in addition to farina,sago, rice, slimes, and wheaten starch are used. The latter substance whenheated generally bears the name of British gum, which differs from calcinedfarina in being soluble in boiling water only. Pure dextrine exhibits thefollowing properties :—In the dry state it is as white as powdered gum arabic ;it has a slightly sweet taste ; it is soluble in water in all proportions, yielding,according to the proportion of water present, a more or less viscous solution ;insoluble in alcohol, by which it is precipitated from its aqueous solution as afloceulent mass; it gives a strong right-handed rotation to polarised light, henceits name. Nitric acid and heat convert it into oxalic acid, while the native gumsare chiefly converted thereby into mucic acid. Solution of neutral acetate oflead does not precipitate a solution of dextrine unless ammonia be added, butit is precipitated by subacetate of lead ; it is also precipitated by chloride oftin, but not by solutions of salts of peroxide of iron. Solutions of iodineimpart no colour to dextrine. A hot alkaline solution of oxide of copper isreduced by dextrine, but not by native gums. Commercially, it is distinguishedas calcined farina, starch heated to 3g2° F. (200° C.), lei'ocome, roasted wheatenstarch, gomma, gommine, and gommeline, all of which are dextrine, more orless pure, and varying in colour, and are prepared by the a&ion of acids andheat upon starch, as already described.

It should be borne in mind that starch and wheaten flour, with all thesubstances derived from them or manufactured with them, exert a reducingadtion, preventing oxidation of the colours. Colours mixed with dextrine donot keep so well as those mixed with gum.

Gums.

Four kinds are chiefly used—(1) Soluble gums (arabic, Senegal, andsalabreda); (2) Gum tragacanth ; (3) Bassora gum ; (4) Inland gum (thegum from cherry, plum, and other stone fruits). Only the first two of thesehinds are used as thickeners.

Gums Soluble in Water belonging to the species of Arabic Gums.“-There are three varieties in the market—Arabic, Senegal, and salabreda;these all exude from trees belonging to the Acacia tribe.

Without practical experience it is very difficult to distinguish between thethree above-mentioned varieties: gum arabic dissolves more readily in water,an d, as its solution has less tendency to become acid, it is better suitedas a thickener for bright and clear mordants; but, on the other hand, itssolutions are more readily coagulated by basic metallic salts than those of