THE WORK OF LEAVES 167 



changed to sugar by the diastase (ptyalin) of the 

 saliva. Mix a little starch paste with saliva, warm it 

 to body temperature (by placing it in a tube and hold- 

 ing the latter in the hand) . Allow it to stand at this 

 temperature for a few minutes; then add Fehling's 

 solution and boil. Do you get any indication of grape- 

 sugar? Test the starch paste and the saliva separately, 

 before mixing, as a control. 



Starch is changed to grape-sugar by uniting with 

 water : such union of substances with water is called 

 hydrolysis, and the ferments which bring it about are 

 called hydrolyzing ferments. It can be brought about 

 by boiling starch in water for a long time, or, in a 

 much shorter time, by the addition of acid to the 

 water. Make a very thin starch paste in water, add 

 hydrochloric acid (a few drops of acid to 100 cc. of 

 water; the more acid used the quicker the reaction). 

 At intervals of about five minutes stir the mixture and 

 take out two samples 1 ; test one of these (after cooling) 

 by adding a few drops of iodine solution ; test the other 

 by adding two or three times its own volume of Fehl- 

 ing's solution and boiling. At first we get a blue 

 coloration with iodine; after boiling for a time, a 

 brownish color is obtained; still later we find a yel- 

 lowish coloration. 



The brown and yellowish colorations are due to the 



1 If a strongly acid solution is used, the samples should be treated before 

 testing by adding lye until they turn red filter paper blue. 



