154 PHYSIOLOGICAL CHEMISTRY. 



weak base and will show no free acid until the protein, con- 

 sidered as a basic body, is saturated. As digestion proceeds 

 more and more acid must be added to complete the saturation. 

 The amount of acid which may be so added is to some extent 

 a measure of the advancing cleavage. With phenol-phthalein, 

 which is a very weak acid, the whole of the hydrochloric acid 

 behaves as " free " acid. The acid joined to the protein is 

 " combined " acid as far as the dimethylaminoazobenzene is 

 concerned and this indicator may be used to show the excess 

 of free acid in examinations of stomach contents. More will 

 be said about this below. 



As hydrolytic digestion goes on the amount of water com- 

 bined becomes appreciable, and finally may reach three or four 

 per cent, as has been determined by direct experiment. The 

 analysis of the albumose and peptone products shows practi- 

 cally the same thing; these substances are always lower in 

 carbon than are the original proteins since oxygen and hydro- 

 gen have been taken up in the cleavage. These products of 

 diminished molecular weight pass from the stomach in the con- 

 dition of hydrochloride salts into the small intestine, where 

 they undergo a new order of changes. 



THE ISOLATION OF PEPSIN. 



It has been stated already that not one of the enzymes is 

 known in even approximately pure condition. Very strong 

 active extracts of the secretion of the gastric glands of ani- 

 mals may be made by the use of various solvents. Such ex- 

 tracts naturally contain much besides the pepsin, but they are 

 suitable for experimental and other purposes. A good proc- 

 ess originally suggested by Wittich is illustrated by the fol- 

 lowing experiment : 



Ex. Separate the fresh mucous membrane of the hog's stomach from 

 the outer coatings and mince it fine in a meat chopping machine. To 10 

 gm. of the minced membrane add 200 cc. of glycerol to which a little 

 hydrochloric acid has been added. The acid should amount to about o.i 

 per cent of the weight of the glycerol, and may be added in the form of 

 the " normal " volumetric acid of which 5 cc. will be sufficient. Allow the 



