i] ACTION OF ENZYMES 15 



iodine in alcohol or potassium iodide solution be added, 

 an intense blue colour will be produced : this is a test 

 from starch that we shall have to use repeatedly. 



When the starch paste has cooled down to 50° C. or 

 so, and the germinated barley extract has been filtered 

 clear, pour about 100 c.c. of starch paste into a beaker 

 and add to it 10 c.c. of the barley extract, stirring them 

 well together. Keep the mixture on a warm bath at 

 50° to 60° C, stir, and watch the result : almost instantly 

 the starch paste will begin to get thin and limpid, and 

 if at intervals a little is taken out and tested with iodine, 

 the blue colour will begin to give place to purple, and 

 will finally cease to appear. When this is the case, 

 taste the mixture ; it will be faintly sweet, and on testing 

 it will be found to contain sugar. About one-quarter 

 fill a test-tube with Fehling's solution, add half as much 

 of the converted starch paste, and bring to the boil : 

 the blue colour disappears, and there is a copious 

 precipitate of bright red oxide of copper. This is a 

 test from sugars we shall often have occasion to use ; 

 if it is tried on the unconverted starch paste, no change 

 takes place. Clearly, then, we have extracted from the 

 germinated barley some soluble substance which is 

 capable of transforming the thick starch paste into 

 freely soluble sugar ; it will do the same to solid starch 

 grains, as may be seen under the microscope. Dab a 

 tiny bit of cut potato on a glass slip, the little smear 

 it leaves consists of starch grains, flood them with a drop 

 of the barley extract, put on a cover slip, and place 

 under the microscope. In a few hours the grains will 

 begin to get etched, and will finally break up and 

 disappear. Now repeat the first experiment with the 

 starch paste, but this time boil the barley extract before 

 adding it — no change to sugar will take place ; the boil- 

 ing has destroyed the substance in the barley extract 



