THE GASTRIC JUICE. 123 



of phosphoric acid, and freed from carbon dioxide by momentary 

 ebullition. On cooling, it is extracted with 100 c.c. of ether by 

 shaking for about thirty minutes. The ethereal extract is poured 

 off, the ether distilled, and the residue taken up with 45 c.c. of 

 water. After filtering, the solution is then treated with 5 c.c. of 

 concentrated sulphuric acid and a pinch of manganese dioxide, and 

 carefully heated to boiling. Should lactic acid be present, this is 

 now decomposed, and acetic aldehyde liberated, which can be demon- 

 strated by passing the vapor into a test-tube containing Nessler's 

 reagent or an alkaline solution of iodopotassic iodide. In the first 

 instance, yellowish-red aldehyde of mercury is formed, while iodoform 

 results in the latter, and can be readily recognized from its odor, 

 which becomes marked when the solution is heated. 



Tests for Acetic Acid and Butyric Acid. These acids can 

 usually be recognized by their odor. Chemically they can be 

 demonstrated as follows : 



Test for Acetic Acid. Ten c.c. of the filtered stomach-contents 

 are extracted with ether as above. The ether is distilled off, the 

 residue taken up with a few drops of water and accurately neutral- 

 ized with sodium hydrate. To this solution a drop or two of a very 

 dilute solution of ferric chloride is added, when in the presence of 

 acetic acid a dark-red color develops. With nitrate of silver, on the 

 other hand, a precipitate is obtained which is soluble in hot water. 



Test for Butyric Acid. The ethereal extract of 10 c.c. of the 

 stomach-contents is freed from ether by distillation, the residue is 

 dissolved in a few cubic centimeters of water, and treated with a trace 

 of calcium chloride in substance. In the presence of butyric acid 

 small oil droplets separate out, the nature of which is readily recog- 

 nized from the pungent odor. If, in the place of calcium chloride, 

 a slight excess of baryta-water is used, highly refractive rhombic 

 platelets or granular, wart-like masses are obtained on evaporation, 

 which consist of barium butyrate. 



Butyric acid can also be recognized by the peculiar odor of pine- 

 apple which develops when the dry residue of the ethereal solution 

 is treated with a little sulphuric acid and alcohol. The reaction is 

 due to the formation of butyl ethylate, C 4 H 7 O 2 .C 2 H 5 . 



Quantitative Estimation of Lactic Acid. This is best accom- 

 plished by means of Boas' method: The decomposition of the lactic 

 acid is effected as described above. After the addition of the sul- 

 phuric acid and manganese dioxide the flask is closed with a doubly 

 perforated stopper. Through one aperture a bent tube passes to the 

 condenser, while a straight tube passes through the other opening, 

 and is provided at its free end with a small piece of rubber tubing 

 that is clamped ; this tube should dip well into the liquid, and serves 

 for passing a current of air through the solution when the distilla- 

 tion is completed. The mixture is then distilled until about four- 

 fifths of the contents have passed over, excessive heat being carefully 

 avoided, so as to prevent decomposition of the aldehyde. The dis- 



