SALIVARY DIGESTION. 59 



contents of the fourth tube add two drops of boiled saliva. Test 

 frequently by the iodine test, using the test-tablet, and note in which 

 tube the most rapid digestion occurs. Explain the results. 



1 6. Influence of Dilution. Take a series of 6 test-tubes each 

 containing 9 c.c. of water. Add i c.c. of saliva to tube i and shake 

 thoroughly. Remove i c.c. of the solution from tube i to tube 

 2 and after mixing thoroughly remove i c.c. from tube 2 to tube 3. 

 Continue in this manner until you have 6 saliva solutions of gradu- 

 ally decreasing strength. Now add starch paste in equal amounts 

 to each tube, mix very thoroughly and place on the water-bath at 

 40 C. After 10-20 minutes test by both the iodine and Feh- 

 ling's tests. In how great dilution does your saliva act? 



17. Influence of Acids and Alkalis. (a) Influence of Free 

 Acid. Prepare a series of six tubes in each of which is placed 4 

 c.c. of one of the following strengths of free HC1 : 0.2 per cent, 

 o.i per cent, 0.05 per cent, 0.025 per cent, 0.0125 per cent and 

 0.006 per cent. Now add 2 c.c. of starch paste to each tube and 

 shake them thoroughly. Complete the solutions by adding 2 c.c. 

 of saliva to each and repeat the shaking. The total acidity of this 

 series would be as follows: o.i per cent, 0.05 per cent, 0.025 per 

 cent, 0.0125 per cent, 0.006 per cent and 0.003 per cent. Place 

 these tubes on the water-bath at 40 C. for 10-20 minutes. Divide 

 the contents of each tube into two parts, testing one part by the 

 iodine test and testing the other, after neutralization, by Fehling's 

 test. What do you find? 



(b) Influence of Combined Acid. Repeat the first three experi- 

 ments of the above series using combined hydrochloric acid (see 

 page 119) instead of the free acid. How does the action of the 

 combined acid differ from that of the free acid? 



(c) Influence of Alkali. Repeat the first four experiments under 

 (a) replacing the HC1 by 2 per cent, i per cent, 0.5 per cent and 

 0.25 per cent Na 2 CO 3 . Neutralize the alkalinity before trying the 

 iodine test (see Starch, 5, page 44). 



(d) Nature of the Action of Acid and Alkali. Place 2 c.c. of 

 saliva and 2 c.c. of 0.2 per cent HC1 in a test-tube and leave for 

 15 minutes. Neutralize the solution, add 4 c.c. of starch paste 

 and place the tube on the water-bath at 40 C. In 10 minutes 

 test by the iodine and Fehling's tests and explain the result. Repeat 

 the experiment, replacing the 0.2 per cent HC1 by 2' per cent 

 Na 2 CO 3 . What do you deduce from these two experiments? 



1 8. Influence of Metallic Salts, etc. In each of a series of 



