378 PRACTICAL PHYSIOLOGY 



the liquid to be tested. Slowly evaporate to dryness. With dilute 

 hydrochloric acid a red colour develops, with lactic acid no red colour. 



EXPERIMENT III. The Tropaeolin Test. Place a drop of a saturated 

 solution of Tropaeolin-00 l in 95 per cent, alcohol in a dry evaporating 

 dish, and dry it with moderate heat. To the yellow stain which 

 results apply a drop of 0'2 per cent, hydrochloric acid. A purple colour 

 is produced. Repeat with the other acid solutions provided. 



EXPERIMENT IV. Di-methyl-amino-azo-benzol 



N(CH 3 ) 2 -C 6 H 4 -N = N-C 6 H 5 



(Topfer's Test). 2 Add 1-2 drops of the reagent to some of the solution 

 to be tested. If this contain free mineral acid a pinkish red colour 

 develops. Organic acids, even when quite dilute, will also give a faint 

 red colour with this reagent. 



It will be found, as a result of these experiments, that the reactions 

 obtained with the hydrochloric acid solutions are more or less simulated 

 by those of the stronger lactic acid solution, except in the case of 

 Giinzberg's reaction. On the other hand, this reagent gives a positive 

 result with hydrochloric acid diluted to 1 in 10,000 parts. The 

 tropaeolin and the azo-benzol reactions with 0*2 HC1 are also quite 

 distinguishable from those given by lactic acid solutions of the above 

 strengths, but in greater dilutions of HC1 the distinction is by no 

 means so definite. 



If the contents of the stomach (removed by a stomach tube or 

 through a fistula) be tested with any of the above reagents some three 

 hours after an ordinary meal, results like those obtained with the HC1 

 solutions will be observed. This is taken as evidence of the presence 

 of free hydrochloric acid. Absolute proof that it is this, and not some 

 other mineral acid, that is the cause of the positive result has been 

 furnished by comparing the total amount of bases with the total 

 amount of acids in the ash of gastric juice (see p. 381): it has been 

 found that there is an excess of chlorine over that required to combine 

 with the bases to form chlorides. This chlorine must exist in the juice 

 as hydrochloric acid. 



The Functions of the Acid. These are generally stated as being 

 two in number. 



1. To assist in the action of the proteolytic ferment, pepsin. Evi- 

 dence of this will be furnished when we study the action of pepsin. 

 For the present it is important to note that the acid combines with 

 the protein, and that the amount of acid which thus combines increases 



1 00 represents a trade brand. 



2 Dissolve O'o gr. di-methyl-amino-azo-benzol in 100 c.c. 95 per cent, alcohol. 



