76 PRACTICAL PHYSIOLOGY. [IX. 



place the tube in a water-bath at 40 C. Clark's commercial rennet 

 will do. Rennet is obtained from the fourth stomach of the calf. 

 The milk becomes solid in a few minutes, forming a curd, and 

 by-and-by the curd of casein contracts and squeezes out a fluid 

 the whey. 



(b.) Repeat the experiment, but previously boil the rennet. No 

 such result is obtained as in (a.), because the rennet ferment or 

 rennin is destroyed by heat. 



9. Comparison of Mineral and Organic Acids. 



(a.) Take two test-tubes A and B. Place in A 10 cc. of a 0.2 

 per cent, solution of hydrochloric acid, and in B 10 cc. of a 2 per 

 cent, solution of acetic acid. To both add a few drops of 

 oo-Tropseolin dissolved in alcohol. The very dilute mineral acid 

 in A renders it rose-pink, while the organic acid does not affect its 

 colour. Or, what is perhaps a better method, allow a drop of a 

 saturated alcoholic solution to evaporate on a white porcelain slab at 

 40 C., and while at this temperature add a drop of the acid solution. 

 On evaporation a violet tint indicates an inorganic acid, .005 HC1 

 can be thus detected (Langfay). It is stated not to be quite a 

 reliable test in the presence of certain organic matters. 



(b.) Repeat (.), but add to the acids a dilute watery solution of 

 methyl- violet, and note the change of colour produced by the 

 mineral acid. It becomes blue and then green. If a strong solution of 

 acid be used, the colour is discharged, but this is never the case 

 with the percentage of acid in the stomach. The peptones in 

 gastric juice may be precipitated by the previous addition of 10 per 

 cent, tannic acid, and then the test can be applied. In the presence 

 of proteids in gastric juice it does not give absolutely reliable results. 



(c.) Repeat (a.) with the same acids, but use paper stained with 

 congo-red, and observe the change of colour to blackish-blue or 

 intense blue produced by the hydrochloric acid. Wash in ether ; 

 if the red colour reappears the acid is organic, if not, mineral. 

 Organic acids make it violet, not blue. 



(?.) Phloro-Glucin and Vanillin (Gunzburg]. Dissolve 2 grams of phloro- 

 glucin and i gram of vanillin in 100 cc. alcohol. Mix equal quantities of this 

 with the fluid to be tested, and evaporate the mixture in a watch-glass on a 

 water-bath. Do not allow the fluid to boil. The presence of HC1 is shown 

 by the formation of a delicate rose-red tinge or stain, or, if there be much 

 hydrochloric acid, of red crystals. This reaction will detect .06 per cent. 

 HC1, and is said not to be impeded by organic acids, albumin, or peptone. 

 The test is an expensive one. 



(e.) Benzo-Purpurin 6 B. Use blotting-papers soaked in a saturated watery 

 solution of this fluid and dried. HC1 (.4 grm. in 100 cc.) makes them dark 

 blue, while organic acids make them brownish-violet. If both HC1 and 

 organic acids be present, the stain is brownish black ; but if the stain be 

 suspected to be partly due to HC1, wash the paper in a test-tube with 

 sulphuric ether, which removes the stain due to the organic acid, leaving that 



