CARBOHYDRATES. 535 



converted into lactic acid. By hydrolysis, lactose is split into dextrose 

 and galactose. 



Milk-sugar resembles dextrose in its action on alkaline solution of 

 copper, from which it precipitates cuprous oxide ; it differs from it by 

 not fermenting with yeast, and in forming mucic acid when heated with 

 nitric acid. 



Polysaccharid.es. 



To the poly saccha rides belong the starches, gums, cellulose, glyco- 

 gen, etc. They differ from the two previous groups by being insoluble 

 in water or soluble with difficulty ; by not crystallizing and not being 

 diffusible. These latter properties are generally characteristic of sub- 

 stances of high molecular weight. By hydrolysis polysaccharides split, 

 forming dextrins, disaccharides, and monosaccharides ; their general 

 composition is indicated by (C 6 H 10 O 5 ) X , which means that the mole- 

 cules are made up of an unknown multiple of C 6 H 10 O 5 . The consti- 

 tution is unknown. 



Starch, Amylum, (C 6 H 10 O 5 ) X . Starch is very widely distributed 

 in the vegetable kingdom, and is found chiefly in the seeds of cereals 

 and leguminosa?, but also in the roots, stems, and seeds of nearly all 

 plants. 



It is prepared from wheat, potatoes, rice, beans, sago, arrow-root, 

 etc., by a mechanical operation. The vegetable matter containing 

 the starch is comminuted by rasping or grinding, in order to open 

 the cells in which it is deposited, and then steeped in water; the 

 softened mass is then rubbed on a sieve under a current of water 

 which washes out the starch, while cellular fibrous matter remains on 

 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 water, 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 



