314 CONSIDERATION OF CARBON COMPOUNDS. 



current of water which washes out the starch while cellular 

 fibrous matter remains on the sieve; the starch slowly deposits 

 from the washings, and is further purified by treating it with 

 water. 



Starch forms white, amorphous, tasteless masses, which are 

 peculiarly slippery to the touch, and easily converted into a 

 powder; it is insoluble in cold water, alcohol, and ether; when 

 boiled with water, it yields a white jelly (mucilage of starch) 

 which cannot be looked upon as a true solution, but is a suspen- 

 sion of the swollen starch particles in water; by continued boil- 

 ing 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, Amylum iodatum, 

 U. S. P., an unstable dark-blue compound of the doubtful com- 

 position C 6 H 9 IO 5 L 



Starch is an important article of food, especially when asso- 

 ciated, as in ordinary flour, with albuminous substances. 



Dextrine, C 6 H 10 5 (British gum). Obtained by boiling starch 

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

 or by the action of diastase (infusion of malt) upon hydrated 

 starch. 



Dextrine is a colorless or slightly yellowish, amorphous pow- 

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

 water, reduces alkaline copper solutions, and is colored light 

 wine-red by iodine. 



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 boiling with diluted sulphuric acid. 



Gum-arabic consists chiefly of the calcium salt of arabic acid, 



