96 THE CARBOHYDRATES 



It is of course obvious that correct values can only be 

 obtained by this method on the assumption that the liquid 

 contains only a single optically active substance. 



Some substances, e.g. glucose, exhibit the phenomenon of 

 muta-rotation, that is to say, the rotation of their solutions 

 varies according to the length of time that they have been 

 made up ; the maximum rotation is given by a freshly-made 

 solution, but the rotatory power gradually decreases until it 

 finally becomes steady. The attainment of the final condition 

 is greatly accelerated by warming the solution in the presence 

 of a little alkali, but the solution must of course be cooled 

 before a reading is taken. 



POLYSACCHARIDES. 



The second great group of carbohydrates, namely the non- 

 sugars or polysaccharides, are substances of high molecular 

 weight, mostly amorphous and insoluble in water. Like the 

 di- and tri-saccharides, the polysaccharides on hydrolysis break 

 up into sugars containing five or six carbon atoms, and they 

 may therefore be looked upon as anhydrides of these substances. 



In the absence of any exact knowledge regarding their 

 molecular weights, their formulae are written (C 6 H 10 O 5 ) n or 

 (C 5 H 8 O 4 ) n according as they give rise to hexoses or pentoses 

 on hydrolysis. The value of " n " has not been determined as 

 yet for any particular case, but there is reason to believe that 

 it is fairly high. The various methods adopted for the eluci- 

 dation of this point have led to such widely different results 

 that a description of them here would not serve any useful 

 purpose. 



CLASSIFICATION. 



The polysaccharides may be classified as follows : 



I. Starches and Dextrins, including Glycogen, Inulin, 

 Mannane and Galactane (C 6 H 10 O 6 ) n . 



II. Gums, which comprise (a) Natural Gums and Pento- 

 sanes ; () Mucilages and Pectic Bodies. 



III. Celluloses CHOn. 



