76 THE GASTRIC JUICE. 



membrane by means of this dilute acid. To obtain a strong 

 solution of pepsin it is best to let it stand for twenty-four 

 hours in a cool place, though it will be found in. the solu- 

 tion in a very short time. The rennin, which is dissolved 

 at the same time, is destroyed by the pepsin on standing. 

 Filter the liquid through muslin, then, if necessary, 

 through paper. This solution can be used directly for 

 digestive experiments or for preparing the purified pepsin. 



180. Purified pepsin can be obtained by precipitating it with 

 some substance which is thrown down as a finely-divided precipi- 

 tate. To effect this, neutralize the hydrochloric acid solution of 

 pepsin in the preceding experiment and acidify with phosphoric 

 acid, or extract the pepsin from the membrane with water acidified 

 slightly with phosphoric acid. Precipitate the phosphoric acid 

 solution by adding lime-water or milk of lime until the liquid is 

 alkaline. Filter off the precipitate, which contains the pepsin mixed 

 with calcium phosphate. Dissolve in water with the addition of 

 hydrochloric acid. Remove the salts by dialysis. The pepsin does 

 not pass through the membrane. A purer form is obtained by 

 repeating the solution in acid and precipitation with lime-water 

 or by precipitating with alcohol before dialyzing. If the dry sub- 

 stance is desired, the drying must be at a low temperature. This 

 may be done over sulphuric acid in a vacuum. 



181. PROOF OF THE EXISTENCE OF ZYMOGENS. This is 

 based upon the facts that dilute acids convert the zymogen into 

 the enzyme and that dilute alkalies destroy the latter, but not the 

 former. 



Dilute 5 cubic centimeters of the glycerin extract of the gas- 

 tric mucosa from a recently killed animal in a test-tube (A) with 

 an equal volume of 0.2-per-cent. hydrochloric acid. In another 

 tube (B) dilute the glycerin extract with as much water. Warm 

 both for 15 minutes at 38; then add to each one-half of its volume 

 of 1-per-cent sodium carbonate and warm at 38 for half an hour. 

 Carefully neutralize them and add enough hydrochloric acid to 

 make 0.2 per cent. Test for the presence of pepsin by a shred of 

 fibrin. It is found in B, but not in A. 



