THE CARBOHYDRATES 



137 



constituents. Prepare a strong warm solution of cane sugar. 

 Place it in a large beaker standing upon a plate, and pour in 

 concentrated sulphuric acid. Note that charcoal is set free, 

 the combined water being withdrawn by the acid. Analysis 

 shows that each substance consists of hydrogen and oxygen, 

 in the proportion in which they form water, combined with 

 carbon. They are therefore termed carbohydrates. 



Examine some starch grains under a microscope. Note the 

 shape, the nucleus, and concentric markings. To a little 

 starch add a drop of solution of iodine in solution of iodide 

 of potassium. Note the deep blue coloration, due to the 

 formation of an iodide of starch. Test for starch by this 

 method in a sliced potato, wheat grains, a turnip, and any 

 other vegetable substance, and note in what variable quantities 

 the starch is present. Shake a few grains of starch with cold 

 water in a test-tube, then boil till the starch granules, which 

 swell and burst, have become completely mingled with the 

 water, and a thin starch paste or mucilage has been produced. 

 Dilute a portion of this with water, and add iodine. Note that 

 a blue solution is formed. Heat nearly to boiling : the solution 

 becomes colourless. Allow to cool : the colour reappears. 

 It is clear that the blue compound of iodine and starch is an 

 unstable compound easily dissociated by heat. 



To another portion of the starch mucilage add some dilute 

 sulphuric acid and warm gently. Observe that the starch dis- 

 solves and the liquid becomes clear. Test a few drops of the 

 solution with iodine in another test-tube, and note that the 

 " soluble starch " formed still gives the blue compound. Now 

 boil the main solution for a few minutes and test a few drops 

 again with iodine. Note that a red coloration is now produced, 

 showing that the starch has been converted into a new sub- 

 stance. This new product is dextrin^ or "British gum," of 

 which in the impure state this reaction with iodine is charac- 

 teristic. Continue boiling the main solution, until on again 

 testing a few drops with iodine no coloration whatever is 



