THE HEXOSES. 193 



unorganised ferments or by the action of hot acids. The 

 glucoses thus may be regarded as the anhydrides of the di- 

 and poly-saccharides. 



They are generally crystallizable substances, soluble in water, 

 possessing a sweet taste, and nearly insoluble in alcohol. They 

 generally exhibit optical activity, i.e., they rotate the plane of 

 polarised light. 



They show the reactions of alcohols and ketones or alde- 

 hydes, and possess the power, in presence of alkalies, of re- 

 ducing cupric oxide to cuprous oxide. With phenyl-hydrazine, 

 H 2 N.NH(C<;H 5 ), the hexoses yield, in presence of acetic acid, 

 crystalline precipitates known as osazones. 



C 6 H 12 6 +3H 2 N.NH.C 6 H 5 

 = C 6 H 10 4 (N.NH.C 6 H 5 ) 2 + 2H,0 + NH 8 + NH C 6 H 5 



Glucosazone. Aniline. 



Many of the hexoses are capable of fermentation under the 

 influence of yeast, Saccharomyces cerevisice, yielding alcohol 

 and carbon dioxide as the chief products. 



C (i H 12 6 = 2C 2 H + 2CO a . 



Small quantities of amyl alcohol, C 5 H U OH, glycerol, 

 C 3 H 5 (OH) 3 , and succinic acid, C 2 H 4 (COOH) 2 , are also formed. 

 Glucose, mannose, d-galactose, and d-fructose are thus 

 affected, while sorbose, gulose, 1-fructose, and the other 

 hexoses do not ferment with yeast. 



Mannose, C 6 H 12 0,,, is the aldehyde of mannitol or mannite, 

 C 6 H 14 6 , a sweet substance occurring in many plants, especially 

 in the manna-ash (Fraxinus ornus), the dried sap of which con- 

 stitutes manna. The sugar is obtained by oxidising mannitol 

 by means of platinum black. It can also be obtained from the 

 reserve cellulose contained in many seeds by the action of 

 dilute sulphuric acid. 



Glucose, C 6 H la 6 = CH a OH.(CHOH) 4 .CHO, also called 

 dextrose and grape sugar, occurs in many fruits, generally 

 associated with fructose. It is formed by the hydrolysis of 

 polysaccharides, e.g., starch, dextrin, and cellulose, of cane 

 sugar, or of glucosides. 



Commercially glucose is made by boiling starch with dilute 



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