DIGESTION 185 



acid. In most cases the rapidity of action <is directly proportional to the 

 amount of ferment present. 



3. The Acid of Gastric Juice. The digestive powers of the acids are pro- 

 portional 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 or diminished in some diseases of the 

 stomach, especially in cancer ; this is true for cancer in general even when 

 the stomach is not involved ; 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. 



(b) Tropaeoliii test. Drops of a saturated solution of tropseolin-OO 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 hydrochloric acid is present a violet spot is left when the fluid has 

 evaporated. A drop of 0'006-per-cent. hydrochloric acid leaves a distinct 

 mark. . 



(c) Topfer's test. A drop of diniethyl-amino-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 sometimes present in the gastric contents, being derived by 

 fermentative processes from the food. It is soluble in ether, and is generally 

 detected by making an ethereal extract of the stomach contents, and evapo- 

 rating the ether. If lactic acid is present in the residue it may be identified 

 by Uffelmann's reaction in 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 10,000) of 

 lactic acid with this violet solution, it is instantly turned yellow. Larger 

 percentages of other acids for instance, more than 0'2 per cent, of hydro- 

 chloric acid are necessary to decolorise the test solution. 



The reaction is not absolutely convincing, since other acids (though in 

 larger percentages) decolorise the solution, but the characteristic yellow colour 

 given even by dilute lactic acid is not developed. Note the decolorisation 

 which occurs when 0*2 hydrochloric acid is added to Uffelmann's reagent. 



Hopkins's reaction for Lactic Acid. Place 3 drops of a 1-per-cent. alco- 

 holic solution of lactic acid in a clean, dry test-tube, add 5 c.c. of concen- 

 trated sulphuric acid and 3 drops of a saturated solution of copper sulphate. 

 Mix thoroughly and place the test-tube in a beaker of boiling water for five 

 minutes. Then cool thoroughly under the tap, and add 2 drops of a 0'2-per- 



