PRACTICAL EXERCISES 455 



starch with iodine set free from the iodic acid by the action of the sulpho- 

 cyanic acid. 



(e) Take some boiled starch mucilage, and test it for reducing sugar 

 by Trommer's test (p. 10). If no sugar is found, take three test- 

 tubes, label them A, B, C, and nearly half fill each with the boiled 

 starch. To A add a little saliva,* to B some saliva which has been 

 boiled, to C a little saliva which has been neutralized, and as much 

 0-4 per cent, hydrochloric acid as has been taken of the mucilage, so as 

 to make the strength of the acid in the mixture 0-2 per cent., a propor- 

 tion well below that of the gastric juice . Put the test-tubes into a water- 

 bath at 40 C. In a few minutes test the contents for reducing sugar. 

 Abundance will be found in A, none in B or C. In B the ferment 

 ptyalin has been destroyed by boiling; in C its action has been inhibited 

 by the acid. If the test-tubes have been left long enough in the bath, 

 no blue colour will be given by A on the addition of iodine, but a strong 

 blue colour by B and Ci.e., the starch will have completely disappeared 

 from A. 



(/) Put some starch in a test-tube, add a little saliva, and hold in the 

 hand or place in a bath at 40 C. On a porcelain slab place several 

 separate drops of dilute iodine solution. With a glass rod add a drop 

 of the mixture in the test-tube to one of the drops of iodine at intervals 

 as digestion goes on. At first only the blue colour given by starch will 

 be seen; a little later a violet colour, due to the presence of erythro- 

 dextrin in addition to some unaltered starch. A little later the colour 

 will be reddish, the starch having disappeared and the erythrodextrin 

 reaction being no longer obscured. Later still no colour reaction will 

 be obtained, the erythrodextrin having undergone further changes, and 

 only sugar (maltose, isomaltose, and perhaps a trace of dextrose) and 

 achroodextrin a kind of dextrin which gives no colour with iodine 

 being present. 



(g) Put two pieces of glass tube filled with tinted starch paste (p. 454) 

 into separate test-tubes. Cover one with 3 c.c. and the other with 

 6 c.c. of saliva. The saliva must all be taken from the same stock, and 

 must not be collected separately. Put in a bath at 38 C., and when a 

 fair amount of digestion has taken place in each, measure the length 

 of the column digested, and determine the relation between the amount 

 digested in the two tubes (p. 342). 



(h) Dilute 2 c.c. of saliva with distilled water up to 20 c.c., and filter. 

 Take six test-tubes of the same width, and label them A, B, C, etc. 

 Measure into A 3 c.c. of the diluted saliva, into B 2 c.c., into C 1-3 c.c., 

 intoDo'9C.c.,into Eo'6c.c., and into F 0-4 c.c. Thus a series is obtained 

 in which each tube contains (approximately) two-thirds as much ferment 

 as the one it follows. Add distilled water to tubes B to F, sufficient to 

 make up the volume in each to 3 c.c. Place the tubes in a beaker of 

 iced water; add to each 10 c.c. of a I per cent, solution of boiled starch 

 previously cooled in iced water, and shake so as to mix the contents. 

 Each tube now contains starch in uniform concentration, and ferment 

 in varying concentration. The low temperature prevents digestion till 

 all the tubes are ready. Now put the tubes simultaneously into a water- 

 bath at 40 C. for half an hour, and then back again into iced water 

 to prevent further digestion. Move them about in the iced water to 

 cool rapidly. Fill up the tubes with distilled water nearly to the top. 

 add a drop or two of iodine solution to each, and mix uniformly. The 

 tubes to which the smallest amounts of saliva were added will probably 



* As it filters slowly, unriltered saliva may be used for Experiments (e), 



