[25] The cholesterol was obtained by extracting the dry puer with ether, saponifying with alcoholic potash. Shaking out the unsaponifiables with ether, evaporating to dryness. The residue was taken up with absolute alcohol, decolorized with animal charcoal. On cooling, characteristic crystals of cholesterol separated. Hoppe-Seyler finds from 1 per cent. to 4 per cent. cholesterol in fæces. See also Gaultier, Coprologie Clinique, p. 160. The colouring matters of the puer are nearly all derived from bile products. Most of them are found in the petroleum extract, which contains far more of the colouring matters than the chloroform extract. The following bile colouring matters have been prepared by Merck, of Darmstadt: bilihumin, biliprasin, bilirubin, bilifuscin, biliverdin.[26] So far as has been ascertained by the author, their effect in the bating is nil, but, in so far as they impart colour to the pelt, they are detrimental. [26] For qualitative tests for the colouring matters, see Gaultier, Précis de Coprologie Clinique, p. 159. Experiments on the action of bile are given in the paper reprinted in Chapter VI. Recently Eberle[27] has proposed to add a proportion of gall juice (bile) to a bate containing pancreatic enzymes, with a view of energizing the latter. This part of the subject will be dealt with in Chapter V. [27] English Patent, 21202, 1909. See also Chapter VIII. *The Reactions of the Puer.*--The organic acids are present principally in the form of the sodium or calcium salts. The intestinal juices contain sodium carbonate, but this is neutralized by lactic acid found in the intestines, so that the excreta usually have an acid reaction. The reaction of a fresh puer liquor is faintly acid to litmus, but this acid is immediately neutralized by the introduction of the skins, so that the rest of the bating process takes place in a neutral or alkaline medium, and the quantity of lime removed by the free acids is probably small. The acidity of a filtered puer liquor made up freshly and titrated with N/5 soda, and phenolphthalein as indicator, was found to equal 10 c.c. N/1 acid per 1000 c.c. of bate. In the volume of liquor used, namely, 900 litres, this would neutralize only 250 grm. of lime (CaO). The determination of the exact acidity or alkalinity of puer liquors by ordinary methods with indicators is not easy, as the end reactions are not sharp. The liquor above described was alkaline to methylorange and litmus, although acid to phenolphthalein, and the results also vary somewhat, according to the quantity of bate used for titration, and according to the strength of the acid or alkali used. The method used, both at the Trent Bridge laboratory and Messrs. Doerr and Reinhart’s laboratory at Worms, is as follows:--