98 THE CARBOHYDRATES 



Properties. 



Glucose separates from alcoholic solution or from concen- 

 trated aqueous solutions at 30-35° in needle-shaped crystals, 

 which are anhydrous ; from cold aqueous solutions, however, 

 it crystallizes with one molecule of water (CgHiaOe . HgO) in 

 the form of plates. It is readily soluble in water, but only 

 very slightly soluble in absolute alcohol. It is readily fer- 

 mented by yeast. 



Glucose is dextro-rotatory, a^ = 52-3° ; it is sometimes 

 known as dextrose to distinguish it from the laevo-rotatory 

 sugar levulose with which it is frequently found associated 

 in ripe fruits. 



Reactions. 



1. In the presence of ammonia, glucose can reduce silver 

 from its salts. A little glucose is added to a solution of silver 

 nitrate to which have been added a few drops of caustic 

 potash and just sufficient ammonia to redissolve the brown 

 precipitate. On warming the mixture the silver is deposited 

 on the sides of the test tube, forming a mirror. 



2. Nylander's Test. — When boiled with a solution of glucose 

 Nylander's reagent turns brown and finally black owing to the 

 precipitation of bismuth oxide and metallic bismuth. 



The reagent is prepared by dissolving 2 grams of bismuth 

 oxynitrate and 4 grams of Rochelle salt in 100 grams of 10 

 per cent caustic soda solution. 



3. Add to the solution basic lead acetate and ammonia. 

 If glucose be present, a white precipitate comes down, which 

 turns red. This reaction is not given by cane sugar. 



4. Add to the solution a little copper sulphate solution 

 and an excess of caustic potash. On warming, a yellow to red 

 precipitate is formed. This reaction also is given by levulose, 

 maltose and other reducing sugars. 



5. On warming with Fehling's solution, a red precipitate 

 is given by dextrose, levulose, maltose, and other reducing 

 sugars. 



6. Add a little Barfoed's reagent and warm. A red 



