THE VOLATILE PAET OP PLANTS. 69 



and potato tubers, for dextrin, found it only in old potatoes 

 and young wheat plants, and there in very small quantity. 

 — Jahresbericht fur Chemie, 1866, p. 664. 



Dextrin is easily prepared artificially by the transforma- 

 tion of starch, and its interest to us is chiefly due to this 

 fact. Wlien starch is exposed some hours to the heat of 

 an oven, or 30 minutes to the temperature of 415° F., the 

 grains swell, burst open, and are gradually converted into 

 a light-brown substance, which dissolves readily in water, 

 forming a clear, gummy solution. This is dextrin, and thus 

 prepared it is largely used in the arts, especially in calico- 

 printing, as a cheap substitute for gum arable, and bears 

 the name British gum. In the baking of bread it is form- 

 ed from the starch of the flour, and often constitutes ten 

 per cent of the loaf. The glazing on the crust of bread, 

 or upon biscuits that have been steamed, is chiefly due to 

 a coating of dextrin. Dextrin is thus an important ingre- 

 dient of those kinds of food which are prepared from the 

 starchy grains by cooking. 



British gum, or commercial dextrin, appears either in 

 translucent brown masses, or as a yellowish-white powder. 

 On addition of cold water, the dextrin readily dissolves, 

 leaving behind a portion of unaltered starch. When the 

 solution is mixed with strong alcohol, the dextrin separates 

 in white flocks, which, upon agitation, unite to translucent 

 salvy clumps. With iodine, solution of commercial dex- 

 trin gives a fine purplish-red color. Pure dextrin iSj how- 

 ever, unaffected by iodine. 



Exp. 28. — Cautiously heat :i spoonful of powdered starch hi a porce- 

 lain dish, with constant stirring- so that it may not burn, for the space 

 of five minutes ; it acquires a 3'ellow, and later, a brown color. Now 

 add thrice its bulk of water, and heat nearly to boilin;j. Ohscrve that a 

 slimy solution is formed. Pour it upon a filter; the liquid that runs 

 through contains dextrin. To a portion, add twice its bulk of alcohol ; 

 dextrin is precipitated. To auotlier portion, add solution of iodine; this 

 shows the presence of dissolved but unaltered starch, which likewise re- 

 mains solid in considerable quantities upon the filter. To a third portion 



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