484 THE GASTRIC JUICE [CH. XXXI. 



position. It is clear, colourless, has a specific gravity of 1003 1006, 

 and is feebly dextro-rotatory. It contains 0*4 to 0'6 per cent, of 

 hydrochloric acid. It is strongly proteolytic, and inverts cane sugar. 

 When cooled to C. it deposits a precipitate of pepsin, and this 

 carries down with it the acid in loose combination, especially in the 

 layers first deposited. Its percentage composition is very similar 

 to that of a proteid, only it contains chlorine in addition to the usual 

 elements. The numbers agree closely with those obtained by Kuhne, 

 who used ammonium sulphate as the precipitant. The following are 

 the analytical figures : 



Pepsin precipitated Precipitated by 



by cold. Am 2 SO 4 . 



Per cent. Per cent. 



Carbon 50 '73 50 '37 



Hydrogen 



Chlorine 



Sulphur 



Nitrogen 



Oxygen 



7-23 

 1-01 to 1-17 



0-98 



Not estimated 

 The remainder 



6-88 

 0-89 

 1-34 



14 -55 to 15-0 

 The remainder. 



Pepsin stands apart from nearly all other ferments by requiring 

 an acid medium in order that it may act. Probably a compound of 

 the two substances, called pepsin-hydrochloric acid) is the really active 

 agent. Other acids may take the place of hydrochloric acid, but none 

 act so well. Lactic acid is often found in gastric juice: this is 

 derived by fermentative processes from the food. 



The digestive powers of the acids are proportional to their dissociation and the 

 number of H ions liberated. The anions, however, modify this by having different 

 powers of retarding the action. The greater suitability of hydrochloric over lactic 

 acid, for instance, in gastric digestion is due to the fact that the former acid more 

 readily undergoes dissociation. 



Hydrochloric acid is absent in some diseases of the stomach ; the best colour 

 tests for it are the following : 



(a) Gunsberg's reagent consists of 2 parts of phloroglucinol, 1 part of vanillin, 

 and 30 parts of rectified spirit A drop of filtered gastric juice is evaporated with 

 an equal quantity of the reagent. Red crystals form, or if much peptone is present, 

 there will be a red paste. The reaction takes place with one part of hydrochloric 

 acid in 10,000. The organic acids do not give the reaction. 



(6) Tropaeolin test. Drops of a saturated solution of tropaeolin -00 in 94 per 

 cent, methylated spirit are allowed to dry on a porcelain slab at 40 C. A drop of 

 the fluid to be tested is placed on the tropaeolin drop, still at 40 C. ; and if hydro- 

 chloric acid is present, a violet spot is left when the fluid has evaporated. A drop 

 of *006 per cent, hydrochloric acid leaves a distinct mark. 



(c) Topfer's test. A drop of dimethyl-amido-azo-benzol is spread in a thin film 

 on a white plate. A drop of dilute hydrochloric acid (up to 1 in 10,000) strikes with 

 this in the cold a bright red colour. 



Lactic acid is soluble in ether, and is generally detected by making an ethereal 

 extract of the stomach contents, and evaporating the ether. If lactic acid is present 

 in the residue it may be identified by the following way : 



A solution of dilute ferric chloride and carbolic acid is made as follows : 



10 c.c. of a 4-per-cent solution of carbolic acid. 



20 c.c. of distilled water. 



1 drop of the liquor ferri perchloridi of the British Pharmacopoeia. 



On mixing a solution containing a mere trace (up to 1 part in 1 0.000) of lactic 



