90 THE GASTRIC JUICE. 



Butyric acid can be distinguished by its odor, which is 

 that of rancid butter. It can be separated from its solution 

 by shaking with ether. The acid is more readily soluble in 

 ether than in water, and hence remains in the ether, and is 

 perceptible when the latter evaporates. It can also be re- 

 moved by distillation, as it passes off with the steam. The 

 acetic acid is also volatile, and if present it distills with the 

 butyric acid. If the steam is condensed the quantity of 

 these volatile acids can be determined by the use of a stand- 

 ard solution of sodium hydrate. Since they both have the 

 same significance, indicating fermentation, it is usually 

 unnecessary to separate them. 



207. Shake 10 cubic centimeters of dilute butyric 

 acid in a test-tube with about 4 cubic centimeters of ether. 

 Pour off the ether and repeat the operation. Allow the 

 ether to evaporate, away from lights and fires, and notice 

 the odor of the acid which remains. 



The presence of pepsin in a solution like the gastric 

 juice is best detected by trying its digestive power on fibrin. 

 Unless the liquid contains a proper amount of acid it must 

 be acidified so as to contain about 0.2 per cent, of hydro- 

 chloric acid. The rapidity of digestion can be best per- 

 ceived by coloring the fibrin dark red with a solution of 

 carmin in ammonia. This coloring matter is insoluble in 

 water, but is set free, coloring the liquid, as the fibrin dis- 

 solves. The depth of color denotes the amount digested. 

 The colored fibrin may be kept on hand for any length of 

 time by pressing out most of the water, then preserving in 

 glycerin or ether. 



208. Stain to a deep red some shreds of washed fibrin 

 with a solution of carmin dissolved in ammonia. After 

 washing with water place these in several test-tubes; add 



