482 DIGESTION. 



noid, and digesting fibrin in it. It will be found still to give 

 tin- x:\nthroprotein reaction, though not quite so strongly as 

 the original solution. Wash the precipitate upon the filter 

 several times with distilled water, plug the funnel, and pour on 

 dilute phosphoric acid, so that a part of the Ca H P0 4 is dis- 

 solved, ra II/POJjj, being formed. After several hours remove 

 the plug and' let the fluid run oft'. It will digest fibrin, and has 

 a still weaker xanthoprotein reaction. Wash the precipitate 

 several times with distilled water, plug the funnel again, pour 

 on fresh phosphoric acid, and repeat this several times. At 

 last a tin id is obtained which, although it digests, gives scarcely 

 anv xanthoprotein reaction. To prepare pure pepsin in xuh- 

 tifnin-f. prepare a solution with phosphoric acid and lime-water, 

 as directed above. After precipitating a second time with lime- 

 water, and pressing the precipitate, dissolve it in dilute hy- 

 drochloric acid and filter it into a large flask. Prepare a cold 

 saturated solution of cholesterin in a mixture of 4 parts of alco- 

 hol of 808 sp. gr. and one part of ether. Put a long funnel 

 which will reach to the bottom of the flask into it, and pour in 

 the cholesterin solution in small quantities. It will separate 

 and form a thick scum on the surface of the fluid. After it has 

 attained the thickness of about an inch, take out the funnel, 

 close the mouth of the flask and shake it well, so that as much 

 pepsin as possible may stick to the cholesterin. Filter and 

 wash the precipitate, first with water acidulated with acetic 

 acid, and then with pure water. Continue the washing until 

 the wash-water no longer has an acid reaction, nor gives a pre- 

 cipitate with silver nitrate. Put the moist cholcsterin into a 

 precipitate glass, and shake it with some ether which has been 

 previously agitated with water to free it from alcohol. The 

 ether will dissolve the cholesterin, and the adhering water will 

 separate and form a turbid layer at the bottom of the glass. 

 Pour off the ether and shake the watery solution with new- 

 quantities of ether several times, until a few drops of the ethe- 

 real solution no longer leaves behind crystals of cholesterin 

 when evaporated. Then let the glass stand open, to allow the 

 last thin layer of ether, which cannot be poured off', to evapo- 

 rate. Filter ; a small quantity of a slimy substance remains in 

 the filter, but the filtrate is clear. It is a concentrated solution 

 of pepsin, and the following reactions may be tried with it, or 

 with the solution of pepsin obtained directly from the lime pre- 

 cipitate. 



* 114. Reactions of Pepsin. To show the following reac- 

 tions the solutions referred to in 112 or 113 may be employed. 

 It is not precipitated by 1, concentrated nitric acid ; 2, tannic 

 acid ; .'>, iodine; 4, mercuric chloride. It is precipitated by 1, 

 platinum chloride; 2, lead acetate, both neutral and basic. 



If absolutely pure, it gives no xanthoprotein reaction. When 



