348 CONSIDERATION OF CARBON COMPOUNDS. 



Groups of carbohydrates. 



Origin | Vegetable 



Animal 



Grape-sugar, C 6 H 12 O 6 (Ordinary glucose, Dextrose). This sub- 

 stance is very abundantly diffused throughout the vegetable kingdom, 

 and is generally accompanied by fruit-sugar. It is contained in 

 large quantities in the juice of many fruits ; the percentage of grape- 

 sugar in the dried fig is about 65, in grape 10-20, in cherry 11, in 

 mulberry 9, in strawberry 6, etc. 



Grape-sugar is found also in honey and in minute quantities in the 

 normal blood (0.1 per cent, or less), and traces occur, perhaps, in 

 normal urine, the quantity in both liquids rising, however, during 

 certain diseases, as high as '5 per cent, or higher. 



Grape-sugar is produced in the plant from starch by the action of 

 the vegetable acids present; it may be obtained artificially from 

 starch (and from many other carbohydrates) by heating with dilute 

 mineral (sulphuric) acids, which convert starch first into dextrin and 

 then into grape-sugar. Corn-starch is now largely used for that pur- 

 pose, the excess of sulphuric acid being removed by treating the solu- 

 tion with chalk ; the filtered solution is either evaporated to a syrup 

 and sold as "glucose," or 'evaporated to dryness, when the com- 

 mercial " grape-sugar " is obtained. 



Experiment 52. Heat to boiling 100 c.c. of a 1 per cent, sulphuric acid and 

 add to it very gradually and under constant stirring a mixture made by rub- 

 bing together 25 grammes of starch and 25 grammes of water. Continue to boil 

 until iodine no longer causes a blue color (which shows complete conversion of 

 starch into either dextrin or glucose), and until 1 c.c. of the solution is no longer 

 precipitated on the addition of 6 c.c. of alcohol (which shows the conversion of 

 dextrin into sugar, dextrin being precipitated by alcohol). Apply to a portion 

 of the glucose solution thus obtained, and neutralized by sodium carbonate, the 

 tests mentioned below. To the remaining solution add a quantity of precipitated 

 calcium carbonate sufficient to convert all sulphuric acid into calcium sulphate. 

 Filter, evaporate the solution to a syrup and notice its sweet taste. 



Glucose is met with generally as a thick syrup which crystallizes 

 with difficulty, combining during crystallization with one molecule of 

 water; but anhydrous crystals, closely resembling those of cane- 

 sugar, are also known. Glucose is soluble in its own weight of 



