COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



235. In a clean test-tube place 5 cc. of i per cent, starch paste, 

 freshly prepared with distilled water (see Ex. 135) and 5 cc. of the 

 diluted saliva. Mix well, place a glass tube or pipette in the tube, 

 and place the test-tube in a water bath maintained at about 40 C. 

 Place a series of drops of iodine solution (about 0-02 N.) on a clean, 

 dry, white porcelain or opal glass plate. From time to time allow 

 a drop of the digesting mixture to fall on to one of the drops of 

 iodine, taking care that the iodine is not transferred to the digesting 

 mixture. A blue colour should be produced at first, but with the 

 drops added later the colour becomes blue-violet, red-violet, red- 

 brown, light brown, and finally no increase of colour is obtained. 

 If no increase of colour is obtained with the first drop, repeat the 

 experiment, making the first test immediately after the starch and 

 saliva have been mixed. If the blue colour persists for a long time, 

 try the effect of adding a couple of drops of 5 per cent, sodium 

 chloride to the digesting mixture. 



When a drop of the mixture fails to give a colour with the 

 iodine, boil a few cc. with a little Fehling's solution. A well- 

 marked reduction is obtained, showing that trie enzyme ptyalin has 

 converted the starch into a reducing sugar, which is, however, not 

 glucose, but maltose. 



236. Perform a control test by first boiling and then cooling 

 the saliva before adding it to the starch. The colour with iodine is 

 always blue, and the solution does not reduce Fehling's. 



237. Achromic Point method. Measure 5 cc. of i per 

 cent, soluble starch (see p. 391) into a test-tube. Add 2 cc. of a buffer 

 solution of P H = 6-7 and 2 cc. of 0-5 per cent, sodium chloride. 

 Place the tube in a water bath maintained at 38 to 40 C. for a few 

 minutes. Have ready a series of test-tubes, each containing about 

 3 cc. of distilled water. To each tube add a couple of drops of dilute 

 iodine (about o-oi N.). To the tube containing the warmed starch 

 add 2 cc. of the diluted saliva, mix well, and note the time. At 

 intervals transfer a drop or two of the digesting mixture to one of 

 the tubes containing the diluted iodine by means of a quill tube, and 

 shake. The colour obtained is blue at first. Determine the time 

 when the addition ceases to produce any colour. This point, which 

 is the moment when the last trace of erythro-dextrin is converted 



