THE CARBOHYDRATES. 13 



(b.) To a portion of the solution, add a solution of iodine 

 = reddish-brown colour, which disappears on heating and 

 returns on cooling. [The student ought to use two test- 

 tubes, placing the dextrin solution in one, and an equal 

 volume of water in the other. Add to both an equal volume 

 of solution of iodine, and thus compare the difference in 

 colour.] 



(c.) Precipitate some from its solution by adding alcohol. 



(d.) Render some of the dextrin solution alkaline by 



adding caustic soda solution. No red-brown colour is 



obtained with iodine. Acidify and the reddish-brown colour 

 appears. 



5. III. Glycogen or Animal Starch, 6 H 10 O 5 . 

 Prepare a Solution (see " Liver "). 



(a.) Take a portion of the solution ; note its opalescence ; 

 add solution of iodine (made by adding iodine to water in 

 which a crystal of potassic iodide is dissolved) = red-brown or 

 port-wine-red colour. As in the dextrin test, use two test- 

 tubes, one with water and the other with glycogen, to com- 

 pare the difference in colour. The colour disappears on 

 heating, and reappears on cooling. 



6. IV. Glucose, Dextrose, or Grape-Sugar, C 6 H 12 O 6 . 



In commerce it occurs in warty uncrystallised masses, of a 

 yellowish or yellowish-brown colour. It is readily soluble in 

 water. Prepare a solution by dissolving a small quantity in 

 water. 



(a.) To the solution add iodine = no reaction. 



(6.) To the solution add a trace of a dilute solution of 

 cupric sulphate, and afterwards add caustic soda (or potash) 

 until the precipitate first-formed is redissolved, and a clear 

 blue fluid is obtained. Boil gradually ; if grape-sugar be 

 present, the blue colour disappears, and a yellow precipi- 

 tate of hydrated cuprous oxide is obtained. It is well to 

 boil the surface of the fluid, and when the yellow precipitate 

 occurs, it contrasts sharply with the deep blue-coloured 



