PRACTICAL EXERCISES 423 



blue by the union of the starch with iodine set free from the iodic 

 acid by the action of the sulphocyanic 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 o- 2 per 

 cent., a proportion 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 C i.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 erythrodextrin 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. 422) in 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. 317). 



(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., into D 0-9 c.c., into E 0*6 c.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 con- 

 tains starch in uniform concentration, and ferment in varying con- 

 centration. 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. 



* As it filters slowly, unfiltered saliva may be used for Experiments 

 (e). (/), and (i). 



