106 THE HUMAN MECHANISM 



EXPERIMENT II 



Let us now inquire what has become of the starch in the second test 

 tube. The solution is clear and has a sweetish taste. Moreover, if boiled 

 with a mixture of sodium hydroxide and a few drops of copper sulphate, 

 it gives a red precipitate, indicating the presence of sugar. These sim- 

 ple tests then prove that saliva first changes starch into dextrine and 

 subsequently changes dextrine into sugar. 



EXPERIMENT III 



Dilute some starch paste with an equal volume of 0.4 per cent 

 hydrochloric acid (which will, of course, make a 0.2 per cent solution 

 of the acid). Now add a few drops of saliva. It will be found that no 

 reaction takes place. Saliva will not act in an acid medium of this 

 strength, and it can be easily shown that it acts most vigorously in a 

 neutral or faintly alkaline medium. This result is of much practical 

 importance, because the gastric juice contains approximately 0.2 per 

 cent of hydrochloric acid and may therefore be expected to interfere 

 with salivary digestion. 



EXPERIMENT IV 



Prepare five or more small beakers of starch paste and add (best 

 with a medicine dropper) to the first a drop of filtered saliva, to the sec- 

 ond two drops, to the third three drops, and so on; then observe the 

 time required in each case for the disappearance of the opalescence 

 and also of the iodine reaction. This experiment will show that while 

 a very small amount of saliva will transform an indefinite amount of 

 starch into sugar, the more saliva there is present the more rapidly will 

 the transformation occur ; and the same thing is true of all enzymes. 

 If the result is not perfectly clear with the undiluted saliva, repeat, 

 but use saliva diluted two or three times with water. 



While we are eating, the food obviously stays too short 

 a time in the mouth to allow the conversion of any large 

 amount of its starch into sugar before it is swallowed. 

 Whatever actual work the saliva may do in bringing about 

 this chemical change must evidently be done chiefly in the 

 stomach, and this will be studied in the next section. 



We have dwelt at length upon the enzyme action of saliva 

 not merely for its own sake but rather because the behavior 



