VEGETABLE THICKENINGS.

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ordinary temperature of the air. The manufacture of starch may therefore beconsidered as a process of mechanical separation, aided by weak solvents ofan acid or alkaline nature. It is very hygroscopic, and may retain even asmuch as 45 per cent of water (the so-called green starch) ; but in dry starchthe quantity of hygroscopic water generally varies from 3 to 16 per cent. Ina perfectly dry and pulverulent state starch exhibits a very mobile powder,devoid of smell, taste, and colour. It is, as far as any practical purposes areconcerned, insoluble in cold water; but some chemists and botanists averthat when starch is rubbed for some time in a mortar with very sharp sandand cold water, and then filtered, a clear solution is obtained, which iscoloured blue on the addition of tincture of iodine, and is precipitated byalcohol and subacetate of lead. In this case every granule of starch shouldconsist of an insoluble and a soluble portion, both isomeric as regards theirchemical constitution. This view is strongly supported by the experimentsof M. Bechamp, who has succeeded in effecting a molecular transformation ofstarch, whereby it is made completely soluble, while it retains the property ofbecoming blue on the addition of iodine. This transformation is effected—(1) By heating starch and monohydrated acetic acid to 212 s F. (roo° C.); (2) Byadling upon starch with ordinary acetic acid, and with concentrated sulphuricand nitric acids; this operation requires great care; (3) By reducing xylo'idineor nitramadine by means of chloride of iron. This intermediate state beingthe first stage in the conversion of starch into dextrine, by careful manage-ment this so-called soluble modification of starch may be obtained in all caseswhere its conversion into dextrine takes place. By polarised light solublestarch exhibits a very marked right-handed rotation of the ray. When starchis made into gum (dextrine) by roasting, and this gum viewed under themicroscope with a power of about 300 diameters, after having been moistenedwith alcohol, the granules remain perfedt, the hilum only appearing a littlemore marked ; but when to the dry gum water is added the granule cracks atthe hilum, and the dextrine may be seen oozing out rapidly into the water.The empty starch envelopes may then be seen floating about, very light,retaining the general appearance of the original granule, but with a largeaperture with ragged edges where the dextrine has escaped. These envelopesgive no colours when viewed with polarised light. The adtion of boiling wateru pon starch is simply a disintegration, not a solution, as is proved by theopalescence of the liquid after the addition of a quantity of boiling water,and also by the fadt that, on being frozen and again becoming fluid, the wholethe starch is precipitated, and the supernatant liquor on being carefullydecanted does not become blue when a solution of iodine is added. A mostinteresting phenomenon is the conversion of starch into dextrine and sugarunder the agency of certain bodies, which, according to Berzelius, appear toa & catalytically; that is to say, they induce a molecular conversion, and also,as regards the sugar, the taking up of a molecule of water or hydratation atthe same time. Dextrine has exadtly the same composition as starch, whileglucose differs from it by containing another atom of water. Starch is con-Ve rted into either dextrine or glucose by the following readtions (x) By thea <Aion of pure water at a temperature of 302° F. (150° C.), which if sufficientlyprolonged produces glucose ; (2) A dry heat of 392° F. (200° C.) convertsstarch into dextrine only ; (3) When to boiling water two or three hundredparts of its weight of sulphuric, hydrochloric, or any other strong mineral

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