DIGESTION EXPERIMENTS 117 



filled with water and kept at a constant temperature of about 

 99 F. will answer) ; thermometer with scale on stem or in 

 glass tube; wire test tube racks, each with a capacity of 3 doz. 

 test tubes; one wooden test tube rack with holes and pins for 

 each pair of students; dialyzer, which may be made from a 

 lamp chimney, the large end being covered with bladder, in- 

 testine, or moist parchment paper ; funnels 2 in. in diameter for 

 students' use; funnels 6 or 8 in. in diameter for preparing 

 material; six 6 in. test tubes for each student; nests of 3 

 beakers of from 4 to 6 oz. capacity; filter papers, 1 package 

 -31/2 in. in diameter, a few, 6 or 8 in. ; litmus paper. 



Experiments l : 



1) Penetration of membranes by crystalloids (sugar, salt) 

 and by peptone: 



a) Place dialyzer obliquely in a glass, so that the membrane- 

 covered end does not rest on the bottom. Carefully pour the 

 solution to be tested (1 teaspoonful sugar, or salt, to ^ cup 

 water) into open end of dialyzer so that no particle of it is 

 spilled. Pour pure water into the glass to exactly the same 

 level as solution in dialyzer and allow to stand over night. 

 Test the water in glass by taste or chemically. 



b) Test peptone and other solutions similarly. 



1 Use a test tube for each experiment beginning with 4. 

 Identify them by slipping bits of paper containing names of 

 students and contents into the open ends. Place the test tube for 

 all digestion experiments in that portion of the water bath 

 reserved for each student. It is best to use small amounts of 

 substances for digestion, to fill the test tubes about two-thirds 

 full of the solution, and to shake frequently. Two or three drops 

 of the solutions of pepsin and of pancreatic extract are sufficient. 



The digestion ordinarily requires some time, and it may be 

 necessary in certain cases to prepare the solutions, put them in 

 the water bath, and leave them until the next day for examina- 

 tion. If the test tubes are shaken every few minutes, the digestion 

 of starch and of fibrin (or egg albumin) is so rapid that it may 

 be possible to get the proper reactions in half an hour. If the 

 laboratory section extends for two hours, the digestions will be 

 sufficiently completed at the end of that time for all characteristic 

 tests. It is best to have the pupils prepare the digestion test tubes 

 first, then to have the demonstrations and experiments which can 

 be accomplished immediately, and to leave the examination- of 

 digestion products until the last. 



