202 PHYSIOLOGICAL CHEMISTRY. 



be given. Fine precipitated or floated silica, pow'dered char- 

 coal, carmine and other things have been used in this way. 

 The feces to be examined are collected between the discharges 

 of the inert and insoluble limiting substances. Various de- 

 vices are also used to collect the feces apart from the urine, 

 which is essential for exact tests. 



Reaction. In health the reaction of feces with litmus is 

 practically neutral in most cases, but at times either acid or 

 alkaline reaction may be found. With excessive putrefaction 

 in the lower intestine various aromatic products and ammo- 

 niacal compounds are formed which may show alkaline be- 

 havior with indicators sensitive in this direction. On the 

 other hand free fatty acids may occasionally be present in suf- 

 ficient quantity to give a distinct acid reaction. In speaking 

 of the reaction in the intestine the conditions for the formation 

 of light fatty acids were explained. Putrefaction on the one 

 hand and fermentation on the other are the important factors 

 in this connection; and these depend in turn largely on the 

 diet. Meat diet gives usually neutral or alkaline reaction; 

 with excessive carbohydrates the reaction may turn to acid. 

 With infants on mother's milk the reaction is commonly acid, 

 while with cow's milk it is neutral or alkaline. These tests 

 refer to the behavior with litmus, but with phenol-phthalein, 

 which is not very sensitive with weak alkalies, an acid reac- 

 tion due to carbonic acid is often obtained. This fact should 

 be kept in mind, since the question of reaction is often a ques- 

 tion of indicators. 



In testing for the reaction some of the mixed feces is spread 

 on one side of the test paper by means of a glass rod ; the color 

 effect is looked for on the other side of the paper. If the feces 

 are not quite moist it will be necessary to rub up with a little 

 water. 



Dry Residue or Solids. As explained above, the larger part of the fecal 

 discharge is always water. The amount of solid matter is best obtained 

 by drying a weighed portion, at a relatively low temperature, in a current 

 of hydrogen or air. By evaporating over a water-bath there will be al- 

 ways some loss of volatile substances besides water. It is very difficult 



