276 EXCRETION. 



biliary salts which produce similar reactions, these are not 

 met with in the animal fluids, and consequently are not 

 liable to produce confusion. If a considerable quantity of 

 bile be present in any fluid, and if there be not a large ad- 

 mixture of animal matters, the test may be employed with- 

 out any preparation ; but in delicate examinations, it is 

 best to evaporate the suspected liquid, extract the residue 

 with absolute alcohol, precipitate with ether, and dissolve 

 the ether-precipitate in distilled water. By this means a 

 clear solution is obtained, which will react distinctly, even 

 when the biliary salts exist in very small quantity. Petten- 

 kofer's test is applicable to any of the biliary salts, whatever 

 6e their form, and the reaction is dependent upon the pres- 

 ence of cholic acid, which enters into the composition of all 

 the varieties of the biliary acids. 



The following is one of the most common methods of 

 employing Pettenkofer's test : To the suspected solution we 

 add a few drops of a strong solution of cane-sugar in water. 

 Sulphuric acid is then slowly added, to the extent of about 

 two-thirds of the bulk of the liquid. It is recommended to 

 add the acid slowly, so that the temperature shall be but 

 little raised. If a large quantity of the biliary salts be 

 present, a red color shows itself almost immediately at the 

 bottom of the test-tube, and soon extends through the en- 

 tire liquid, rapidly deepening until it becomes of a dark lake 

 or purple. If the biliary matters exist in very small pro- 

 portion, it may be several minutes before any red color 

 makes its appearance, and the change to a purple is corre- 

 spondingly slow, the whole process occupying from fifteen to 

 twenty minutes. Many organic matters may be rendered 

 dark by the action of the acid, and the sugar itself will be 

 acted upon, even if no bile be present, but the color due to 

 the sugar alone is yellow. The peculiar play of colors above 

 described can easily be recognized after a little practice, and 

 is observed only in the presence of the biliary salts. 



The ordinary modifications in the application of this test 



