104 THE CARBOHYDRATES. [CH. V. 



susceptible of being oxidised to various acids, thus reducing 

 certain oxidising reagents. This reaction only takes place 

 in hot alkaline solutions, and is of great value as a test for 

 these sugars, and especially as a basis of various methods 

 of estimation. 



They react with phenyl hydrazine in excess to give 

 insoluble crystalline bodies called osazones. These are 

 of the greatest value in determining the presence of and in 

 characterising the monosaccharides, though not in dis- 

 tinguishing them from one another. 



When heated with an alkali the monosaccharides 

 become yellow and then brown, and finally decompose 

 into a mixture of acids and resinous substances. 



They are reduced by sodium amalgam to hexahydric 

 alcohols. Sorbite is formed from glucose, mannite from 

 mannose and dulcite from galactose. Fructose yields a 

 mixture of sorbite and mannite. These alcohols are of 

 considerable interest, as they are used by bacteriologists 

 for the differential diagnosis of certain pathogenic organ- 

 isms. 



On oxidation glucose gives rise to three acids 

 CO 2 H CHO CO 2 H 



I I I 



(CHOH) 4 (CHOH) 4 (CHOH) 4 



CH 2 OH CO 2 H CO 2 H 



Gluconic acid. Glycuronic acid. Saccharic acid. 



Glycuronic acid is interesting physiologically, as it is 

 frequently found in the urine in combination with various 

 drugs, such as chloral, camphor, phenol, etc. These com- 

 pounds protect the organism from the injurious effects of 

 the drugs. 



On oxidation galactose gives inactive mucic acid, 

 which is isomeric with saccharic acid. Being only slightly 

 soluble its production is used as a test for the presence of 

 lactose in urine, since lactose is hydrolysed by acids into 

 galactose and glucose. 



