BY DR. LAUDER BRUNTON. 481 



** 112. Preparation of Pure Pepsin from Glycerin 

 Solution. Let the mucous membrane, prepared and cut into 

 pieces, as already directed, lie for 24 hours in absolute alcohol. 

 Filter off the alcohol ; dry the pieces of mucous membrane with 

 a cloth or filtering paper, cover them with glycerin, and let them 

 stand for several days or weeks. Filter the glycerin, first 

 through linen and then through paper. Add a large excess of 

 absolute alcohol to the filtrate and a flocculent precipitate will 

 fall. Filter off the alcohol, pour HC1. of 2 per cent, over the 

 precipitate on the filter, and it will dissolve. Boil a little of 

 the solution with strong nitric acid, and after cooling, add am- 

 monia. It should not give the slightest trace of the xantho- 

 protein reaction. Let a piece of fibrin, either boiled or un- 

 boiled, remain in another portion of the solution for several 

 hours, at 40 C., and it will be digested. Apply the other 

 tests mentioned in 118. Very probably no precipitate may 

 be occasioned by platinum chloride. 



113. Preparation of Pepsin (Brucke's Method). The 

 process by which Briicke separated pepsin, and thus for the 

 first time succeeded in isolating any of the digestive ferments, 

 depends on their being carried down from their solutions along 

 with precipitates produced in them. This has already been 

 mentioned when speaking of saliva, from which Cohnheitn 

 separated ptyalin by Brucke's process. Separate the mucous 

 membrane from the stomachs of two pigs, and cut it up into 

 small pieces, as directed in 109. Digest it at 40 C. with a 

 considerable quantity of dilute phosphoric acid, of the British 

 Pharmacopeia, mixed with its own bulk of water (it thus con- 

 tains 5 per cent, of acid). If necessary, remove the acid, and 

 add fresh portions till the whole of ilie stomach has been dis- 

 solved, with the exception of a slight residue, continuing the 

 process till the liquid which passes through on filtering gives 

 no precipitate with potassium ferrocyanide. Filter the liquid, 

 put a little of the filtrate aside in a test-tube, and add lime- 

 water to the remainder till it turns blue litmus paper slightly 

 violet. Collect the precipitate on a cloth filter, press all the 

 fluid out of it with the aid of a screw-press, and dissolve it while 

 still moist, in water, with the addition of dilute hydrochloric 

 acid (50 cubic centimetres of commercial acid in a litre of 

 water). 



Precipitate the solution a second time with lime-water, col- 

 lect the precipitate on a cloth filter, press out the liquid, pour 

 a little water on it while still moist, and add phosphoric acid 

 to it in small quantities and at long intervals. The pasty tri- 

 basic phosphate Ca^POJ., is thus converted into sandy bibasic 

 phosphate Ca H PO 4 . Filter off the fluid; it contains pepsin 

 still mixed with albuminous substances. Test its digestive 

 power by adding a few drops of it to 0.1 per cent, hydrochloric 

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