This section is from the book "Diseases Of The Intestines", by Max Einhorn. Also available from Amazon: Diseases Of The Intestines A Text-Book For Practitioners And Students Of Medicine.
Whenever biliary pigments appear in the dejecta, biliary acids, as a rule, accompany them. The presence of biliary acids is best revealed by Pettenkofer's test, and is made as follows: A small quantity of fecal matter is thoroughly treated with alcohol, which is then evaporated. To the residue a weak watery solution of bicarbonate of sodium is added, and to this mixture a small quantity of cane sugar and a few drops of sulphuric acid. When biliary acids are present a characteristic red or pink color arises.
Normally the biliary pigment within the in-testiual tract becomes changed into urobilin, which is the principal factor of the characteristic brownish color of the faeces. The best test for the presence of urobilin is Fleischer's 1 procedure which is as follows: A small quantity of faeces is put into a test tube and a small amount of alcohol with a few drops of hydrochloric or acetic acid added; the mixture is then left undisturbed for a short time. The presence of urobilin produces a yellow or brown color, the latter, if present in large amount. If the alcohol is now poured out and a few drops of sodium hydrate added, as well as a small quantity of a chloride-of-zinc solution, there appears, according to the amount of urobilin, a more or less greenish fluorescence in direct rays of light, while in transmitted light the fluid appears pink or yellowish-red. If the watery extract of faeces to which some ammonia has been added is filtered and chloride of zinc added, the presence of urobilin produces a pinkish-red precipitate.
If this precipitate is filtered under addition of alcohol containing some ammonia there appears a more or less greenish fluorescence (Schmidt's 2 test). A small piece of fecal matter is treated with a concentrated watery solution of sublimate and thoroughly mixed with a glass rod. The presence of urobilin gives rise either immediately or a little later to a pinkish-red color, while biliver-din, if present, produces a greenish color.
Normally urobilin is present in the faeces. Its absence is observed only in pathological conditions.
The acholic stool presents a grayish-white, ashy gray, or clay color. It is usually of a soft salvelike consistency. It occurs (1) in conditions in which there is a total absence of bile in the intestine, and (2) whenever the absorption of fat is greatly impaired. Until very recently the grayish-white color has been generally ascribed to the absence of biliary pigments and their modifications (urobilin), but Fleischer and Bunge 1 have conclusively shown that the whitish color may be observed in faeces containing urobilin, the color being due to the presence of large amounts of fat. In the latter instance the stool, after being treated with large amounts of ether, thus separating the contained fat, assumes a brownish color. This I can confirm also from my own experience.
1 R. Fleischer: "Krankheiten des Darms, "p. 1160, Wiesbaden, 1896. 2 A. Schmidt. Verhandlungen des Congresses f. Innere Med., 1895.