536 CONSIDERATION OF CARBON COMPOUNDS. 



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 L 



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

 ordinary flour, with albuminous substances. In the body starch, as well as 

 other carbohydrates, must be converted into monosaccharides before being 

 absorbed. This hydrolysis of starch may be made outside the body acting on 

 starch paste with some diastatic enzyme, or by prolonged boiling with very 

 dilute (1 per cent.) mineral acid. The intermediate products of the hydrolysis 

 are the same in either case. Starch is first converted into soluble starch or 

 amylo-dextrin, which gives a blue color with iodine ; the soluble starch next 

 passes into malto-dextrin and ery thro- dextrin, giving a red color with iodine ; 

 erythro-dextrin passes into malto-dextrin and achroo-dexfrin, giving no color 

 with iodine, but forming a white precipitate with alcohol. Achroo-dextrin 

 passes into maltose, and maltose into dextrose. The hydrolysis is a progressive 

 reaction, all these compounds being present in the solution at one time. 



Dextrin, C 6 H 10 O 5 (British gum). This name is given to a mixture 

 of the dextrins just mentioned, and formed by hydrolysis of starch 

 by means of 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 

 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, re^ 

 sembling gum-arabic in some respects ; it is soluble in water, does not 

 reduce alkaline copper solution, 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 amy loses. 



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

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

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

 in marsh-mallow root, etc. 



Gum-arabic dissolves slowly in 2 parts of water ; this solution 

 shows an acid reaction with litmus, and yields precipitates with lead 

 acetate or ferric chloride. 



Cellulose (C 6 H 10 O 5 ) X , perhaps C 18 H 30 O 15 (Plant fibre, Lignin). 

 Cellulose constitutes the fundamental material of which the cellular 

 membrane of vegetables is built up, and forms, therefore, the largest 

 portion of the solid parts of every plant ; it is well adapted to this 

 purpose on account of its insolubility in water and most other sol- 



