APPENDIX. 403 



ence of starch. Pour a second portion into another tube, add 

 sodium hydrate and copper sulphate, and boil. If the yellow 

 color appears it indicates that some of the starch has already 

 been digested by the saliva i. e., has been changed to grape- 

 sugar, which remains dissolved in the fluid in the test-tube. If 

 the yellow color does not appear on the first trial, make another 

 after an interval of a few minutes. 



(4) To show that digested starch is capable of absorption, while 

 undigested starch is not. 



Prepare two dialyzers. The parchment, or parchment paper, 

 which in each dialyzer separates the contents of the inner from 

 the contents of the outer jar may be considered to represent 

 roughly the membrane lining the alimentary canal, through 

 which membrane substances are absorbed into the system. 

 Into the inner jar of one dialyzer put a solution of grape-sugar ; 

 into the inner jar of the other put some thin starch -paste. 

 After an hour or two test the water in the outer jar of the first 

 dialyzer for the presence of grape-sugar ; that in the outer jar 

 of the other dialyzer for starch. It will be found that grape- 

 sugar i. e., digested starch dialyzes, while undigested starch 

 does not. In other words, undigested starch cannot be ab- 

 sorbed. The experiment may be varied by putting both grape- 

 sugar and starch-paste into the same dialyzer. Or, a mixture of 

 starch-paste and saliva may be put into the one, while starch- 

 paste alone is put into the other dialyzer. 



GASTRIC DIGESTION. 



(1) Some of the chemical reactions of undigested albuminous 

 substances (proteids). 



Into a bowl or beaker break the white of an egg, cut it to 

 pieces with a pair of scissors, add fifteen or twenty times its 

 bulk of water, mix thoroughly by stirring, but do not beat it, 

 then strain through muslin to remove the fine flakes of coagu- 

 lated matter. 



(a) Fill a test-tube one-fourth full of the mixture and boil. 

 The albumen coagulates. 



(b) Prepare another tube and add a few drops of nitric acid. 

 The albumen coagulates. Boil. The coagulated mass turns 

 yellow. Cool the tube and add ammonia. The color deepens 

 to orange. 



