74 THE CARBOHYDRATES 



freshly prepared potato or maize starch with dilute sulphuric 

 acid in sealed copper vessels under 3 atmospheres pressure. 

 When the hydrolysis is complete, the acid is removed as 

 calcium sulphate by the addition of powdered chalk, and the 

 filtered solution, after being decolorized by means of animal 

 charcoal, is evaporated in a vacuum ; a little anhydrous glucose 

 in then introduced, and the syrup is allowed to crystallize at 

 a temperature of 40. Prepared in this way, the glucose forms 

 a rather soft cake of small crystals ; it is not a pure product, 

 being contaminated with maltose, isomaltose (p. 86), and 

 dextrin ; it may, however, be purified by recrystallizing from 

 aqueous alcohol. 



Commercial dextrose is employed as a substitute for cane 

 sugar for the sweetening of cheap jams, etc., but its sweetness 

 is only about three-fifths that of cane sugar. The use of im- 

 pure sulphuric acid containing arsenic for the hydrolysis of 

 starch, and the subsequent employment of the glucose in the 

 preparation of beer, has been the cause of the numerous deaths 

 from arsenical poisoning. 



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 (C 6 H 12 O 6 . H 2 O) in 

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

 very slightly soluble in absolute alcohol. It is readily 

 fermented by yeast. 



Glucose is dextro-rotatory, a D = 52*5; 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. 



I. 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. 



