THE VOLATILE PART OF PLANTS. 69 
and potato tubers, for dextrin, found it only in old potatoes 
and young wheat plants, and there in very small quantity. 
—Jahresbericht fiir 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. When 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 aclear, gummy solution. Thisis dextrin, and thus 
prepared it is largely used in the arts, especially in calico- 
printing, as a cheap substitute for gum arabic, 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 ef 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 comercial dex- 
trin gives a fine purplish-red color. Pure dextrin is, how- 
ever, unaffected by iodine. 
Exp. 2%.—Cautiously heat a spoonful of powdered starch in a porce 
lain dish, with constant stirring so that it may not burn, for the space 
of five minutes; it acquires a yellow, and later, a brown color. Now 
add thrice its bulk of water, and heat nearly to boiling. Observe that a 
slimy solution is formed. Pour it upona filter; the liquid that runs 
through contains dextrin. To a portion, add twice its bulk of alcohol; 
dextrin is precipitated. To another portion, add solution of iodine; this 
shows the presence of dissolved but unaltered starch, which likewise re- 
mains solid in considerable quantities upor the filter. To a third portion 
