GASTRIC DIGESTION. 55 



(c.) To 5 cc. of milk in a test-tube add a few drops of 

 Benger's liquor pepticus, and place in a water-bath. Observe 

 how the casein first clots, and is then partially dissolved to 

 form a yellowish coloured fluid, with a bitter taste and 

 peculiar odour. There generally remains a very con- 

 siderable clot of casein ; and, in fact, the gastric digestion 

 of milk is slow, especially if compared with its tryptic 

 digestion (Lesson VIII., 9). Test the fluid for peptones 

 with the biuret test, and observe the beautiful light pink 

 colour obtained. The bitter taste renders milk "peptonised" 

 by gastric juice unsuitable for feeding purposes. 



7. Action of Rennet on Milk. 



(a.) Place some milk in a test-tube, label it A, add a 

 drop or two of rennet, shake it up, and place the tube in a 

 water-bath at 40 0. Observe that 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 same experiment, but previously boil the 

 rennet. No such result is obtained as in (a.) 



8. 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 5 per cent, solution of acetic acid. To both add a few 

 drops of o-o-Tropseolin dissolved in alcohol. The very 

 dilute mineral acid in A renders it rose-pink, while the far 

 stronger organic acid does not affect its colour. 



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

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

 the mineral acid. It becomes blue. 



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

 stained with congO-red, and observe the change of colour 

 to blue produced by the hydrochloric acid. 



(d.) Instead of threads stained with congo-red, use papers 

 similarly stained, or a watery solution of congo-red. 



(e.) For lactic acid. Prepare a fresh solution by mixing 

 10 cc. of a 4 per cent, solution of carbolic acid, with 20 cc. 



