2O6 PHYSIOLOGICAL CHEMISTRY. 



green fluorescence. The acid may be identified also by mixing with a 

 small amount of water and a little cane sugar, and adding then a few 

 drops of strong sulphuric acid. A red color develops which becomes 

 purple. The sulphuric acid must be added in just sufficient quantity to 

 warm the mixture to about 70 or 75 C. This test is the Pettenkofer bile 

 test. 



Fatty Acids Proper. After separating the cholesterol and cholalic acid 

 as just described the true fatty acids are left in the form of insoluble 

 barium soaps. By acidifying with a little hydrochloric acid and shaking 

 with ether these acids go into solution and may be recovered by evapora- 

 tion of the ether. The fatty acids of any lecithin originally present are in- 

 cluded, as the lecithin would be decomposed in the first saponification. The 

 acids of soaps as well as of neutral fats are also included if the original 

 extraction was made, as assumed, on acidified feces. 



Lecithin. The separation of lecithin as such from feces is not practicable 

 but the amount may be estimated from the phosphoric acid separated in 

 the saponification. In the above tests the glycerophosphoric acid would go 

 as barium salt along with cholalic acid into the hot water solution. The 

 phosphate could be recognized or determined in this. But it may be 

 estimated much more accurately by using some of the original crude fat 

 This is mixed with some sodium carbonate and ashed carefully. Then a 

 little saltpeter is added to complete oxidation; the fused mass is dissolved 

 in water. The phosphoric acid may be determined by titration with 

 uranium nitrate, or, better, with molybdic acid by the Pemberton method. 

 In this way it is usually possible to secure enough phosphate to make an 

 accurate titration, in most cases, by starting with a gram of crude fat. 



Soaps. The above tests give the total acids. It may be desirable to 

 measure the amount present in the form of soaps. For this purpose a 

 double extraction is necessary. In one case, the feces are dried and ex- 

 tracted with ether without preliminary acid treatment. The soaps, not 

 being ether soluble, remain behind. Then a second extraction, after acidi- 

 fication, is made; the result gives the total fats and acids and the differ- 

 ence between the two extractions shows the acid due to soaps. 



For these extractions the paper coil method is very satisfactory as 

 sufficient extract may be obtained from about 10 gm. of moist feces for 

 satisfactory weighing. 



CARBOHYDRATES. 



Under this term starch, sugar, gums and cellulose must be 

 included. With a vegetable diet the last named is always 

 present, while on a purely animal diet not one of the group 

 can be found in the feces. 



Starch. This substance is found commonly in feces, under 

 normal as well as pathological conditions, and especially when 

 the diet has contained starch in the form of coarse meal, which 



