352 CONSIDERATION OF CARBON COMPOUNDS. 



the sieve; the starch deposits slowly from the washings, and is 

 further purified by treating it with water. 



Starch forms white, amorphous, tasteless masses, which are pecu- 

 liarly slippery to the touch, and easily converted into a powder; it 

 is insoluble in cold w r ater, alcohol, and ether; when boiled with water, 

 it yields a white jelly (mucilage of starch, starch-paste) which cannot 

 be looked upon as a true solution, but is a suspension of the swollen 

 starch particles in water ; by continued boiling with much water some 

 starch passes into solution. 



Starch, when examined under the microscope, is seen to consist of 

 granules differing in size, shape, and appearance, according to the 

 plant from which the starch was obtained. Concentric layers, which 

 are more or less characteristic of starch-granules, show that they are 

 formed in the plant by a gradual deposition of starch matter. 



The most characteristic test for starch is the dark-blue color which 

 iodine imparts to it (or better to the mucilage). This color is due to 

 the formation of iodized starch, an unstable dark-blue compound of 

 the doubtful composition C 6 H 9 IO 5 I. 



Starch is an important article of food, especially when associated, 

 as in ordinary flour, with albuminous substances. 



Dextrin, C 6 H 10 O 5 (British gum). Obtained by boiling starch with 

 diluted acids, or by subjecting starch to a dry heat of 175 C. 

 (347 -F.) or by the action of diastase (infusion of malt) upon 

 hydrated starch. Malt is made by steeping barley in water until it 

 germinates, and then drying it. 



Dextrin is a colorless or slightly yellowish, amorphous powder, 

 resembling gum-arabic in some respects; it is soluble in water, re- 

 duces alkaline copper solutions, and is colored light wine-red by 

 iodine. It is extensively used in mucilage as a substitute for gum- 

 arabic. 



Gums. These are amorphous substances of vegetable origin, 

 soluble in water or swelling up in it, forming thick, sticky masses ; 

 they are insoluble in alcohol, and are converted into glucose by boil- 

 ing with dilute sulphuric acid. Some gums belong to the saccharoses, 

 others to the amyloses. 



Acacia, Gum-arabic is a gummy exudation from Acacia Senegal ; 

 it consists chiefly of the calcium salt of arabic acid, C 12 H 22 O n . Other 

 gums occur in the cherry tree, in linseed or flaxseed, in Irish moss, 

 in marsh-mallow root, etc. 



