204 COMPOSITION OF THE DIGESTIVE JUICES. [CH. VIII. 



248. The detection of pepsin, Obtain some fibrin that has 

 been stained with carmine (see Note below). Treat the ferment 

 solution with the same volume of 0-4 per cent. HC1. Divide this 

 into two equal portions and label them A and B. Boil B for a 

 minute, and cool the tube. To each tube add a few flakes of the 

 stained fibrin. Place them on the warm bath for ten minutes. 

 Shake and observe the colour of the fluid. In A it will be red. In 

 B it will be almost or quite colourless. 



NOTE. The carmine solution for staining fibrin is prepared by dissolving 

 i gram, of carmine in about i cc. of ammonia and adding 400 cc. of water. 

 The solution is kept in a loosely-stoppered bottle till the smell of ammonia has 

 become faint. Fresh washed fibrin is chopped finely, placed in the carmine 

 solution for twenty-four hours, strained off and washed in running water till the 

 washings are colourless. If not required immediately, it should be kept under 

 ether and washed with water before use. It cannot be used for testing for 

 trypsin, owing to the solubility of the dye in alkalies. 



249. Destruction of pepsin by alkalies. To 5 cc. of the pepsin 

 solution add i per cent, caustic soda drop by drop until the reaction 

 is just faintly alkaline to litmus paper. Place in the warm bath 

 for 10 minutes. Make the reaction just acid to litmus by the 

 addition of 0-4 per cent, hydrochloric acid, and then add to the 

 mixture its own volume of the same acid. Add some carmine fibrin 

 and place the tube in the warm bath. The fibrin does not dissolve 

 owing to the destruction of the pepsin by the alkali. 



NOTE. In light of the above experiment, it is unnecessary to test an 

 alkaline solution for pepsin. 



250. The estimation o! Pepsin by Fuld's method. 



Principle. An acid solution of edestin (the protein 

 of hemp seeds) is precipitated by sodium chloride : the 

 peptic digestion products are not precipitated. 



Solutions required. 



1. Hydrochloric acid. Dilute 30 cc. of N/io HC1 to 100 cc. with distilled 

 water. 



2. o- 1 per cent, solution of edestin. Dissolve o-i gram, of pure edestin 

 in 100 cc. of the hydrochloric acid at boiling point. Cool and make up to 

 100 cc. with the hydrochloric acid. If the solution is not clear it must be 

 filtered. 



3. Saturated (33 per cent.) solution of sodium chloride. 



Methoi. Number a series of clean tubes from i to 10. Into 



