POLYSACCHARIDES 145 



Initial reading of polariscope, without any solution = o" 30' 

 Final ,, ,, ,, with glucose ,, = 3° 45' 



Difference (a) = 3° 15' or 3-25° 



Length of tube containing the solution (/) = 2 dms. 



Specific rotation of glucose (a^) = 52-5° 



From which m = — - — - — = -0300 

 2 X 52-5 ^ ^ 



.•. the strength of the solution is 309 per cent. 



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. Plant 

 extracts should be treated with lead acetate in the manner 

 described above. 



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. 



FURTHER REFERENCES. 



Armstrong : " The Simpler Carbohydrates and Glucosides," London, 

 1924. 



Mackenzie : " The Sugars and their Simpler Derivatives," London, 



1913- 



Cramer : " Les sucres et leurs Derives," Paris, 1927. 



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 (CgHioOs)!! or 



10 



