CARBOHYDRATES. 531 



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

 pose, the excess of sulphuric aoid 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 63. Heat to boiling 100 c.c. of a 1 per cent, sulphuric acid and 

 add to it very gradually and with 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 

 water and is less sweet than cane-sugar, the sweetness of glucose com- 

 pared to that of cane-sugar being about 3 to 5 ; when heated to 170 

 C. (338 F.) it loses water, and is converted into glucosan, C 6 H 10 O 5 ; 

 by stronger heating it loses more water and forms caramel, a mixture 

 of various substances ; it turns the plane of polarized light to the 

 right. 



By gentle oxidation dextrose is first converted into monobasic glu- 

 conic acid, C 6 H 12 O 7 = C 5 H 6 .(OH) 5 .CO 2 H, and then into dibasic sac- 

 charic acid, C 6 H 10 O 8 = C 4 H 4 .(OH) 4 .(CO 2 H) 2 . Further oxidation 

 results in the formation of acids of lower molecular weight, due to 

 splitting up of the molecules. (Saccharic acid is soluble in less than 

 its own weight of water.) 



Dextrose combines with various metallic oxides (alkalies, alkaline 

 earths, etc.), and also with a number of other substances, forming a 

 series of compounds known as glucosides. 



Dextrose may be recognized analytically : 



1. By causing a bright-red precipitate of cuprous oxide, when 

 boiled with a solution of cupric sulphate in sodium hydroxide, to 

 which tartaric acid has been added. (A solution containing these 

 three substances in definite proportions is known as Fehling's solu- 

 tion. See index.) 



2. By precipitating metallic silver, bismuth, and mercury, when 



