152 PHYSIOLOGICAL CHEMISTRY. 



with an excess of alcohol. The crystalline precipitate which, im- 

 mediately forms is filtered as soon as possible, and the taurin now 

 separates, on cooling, into very long needles or prisms. These 

 crystals may be purified by recrystallization from a little warm water. 



Though the taurin shows no positive reactions, it is chiefly 

 identified by its crystalline form, by its solubility in water and in- 

 solubility in alcohol, by its combination with mercuric oxide, by its 

 non-precipitability by metallic salts, and above all by its containing 

 sulphur. 



THE DETECTION OF BILE-ACIDS IN ANIMAL FLUIDS. To 

 obtain the bile-acids pure so that PETTENKOFER'S test can be ap- 

 plied to them, the albumins and fat must first be removed. The 

 albumin is removed by making the liquid first neutral and then 

 adding a great excess of alcohol, so that the mixture contains at least 

 85 vols. per cent of water-free alcohol. Now filter, extract' the pre- 

 cipitated albumin with fresh alcohol, unite all filtrates, distil the 

 alcohol, and evaporate to dryness. The residue is completely ex- 

 hausted with strong alcohol, filtered, and the alcohol entirely evapo- 

 rated from the filtrate. The new residue is dissolved in water, and 

 filtered if necessary, and the solution precipitated by basic lead 

 acetate and ammonia. The washed precipitate is dissolved in boiling 

 alcohol, filtered while warm, and a few drops of soda solution added. 

 Then evaporate to dryness, extract the residue with absolute alcohol, 

 filter, and add an excess of ether. The precipitate now formed may 

 be used for PETTENKOFER'S test. It is not necessary to wait for a 

 crystallization ; but one must not consider the crystals which form 

 in the liquid as being positively-crystallized bile. It is also possible 

 for needles of alkali acetate to'be formed. For the detection of bile- 

 acids in urine see Chapter XIV. 



Bile-pigments. The bile-coloring matters known thus far are 

 relatively numerous, and in all probability there are still more. 

 Most of the known bile-pigments are not found in the normal bile, 

 but occur either in post-mortem bile or, principally, in the bile con- 

 crements. The pigments which occur under physiological condi- 

 tions are the reddish-yellow bilirubin, the green biliverdin, and some- 

 times there is also observed in fresh human bile a pigment closely 

 related to hydrobilirubin. The pigments found in gall-stones are 

 (besides the bilirubin and biliverdin) bilifuscin, biliprasin, 

 bilihumin, bilicyanin (and choletelin ?). Besides these, others have 

 been observed in human and animal bile. The two above-mentioned 

 physiological pigments, bilirubin and biliverdin, are those which 

 serve to give the golden-yellow or orange-yellow or sometimes 



