186 THE BILE. 



or twenty-five minutes. If foreign matters, again, not of a biliary 

 nature, be also present, they are apt to be acted on by the sulphuric 

 acid, and, by becoming discolored, interfere with the clearness and 

 \ brilliancy of the tinges produced. On this account it is indispen- 

 sable, in delicate examinations, to evaporate the suspected fluicLto 

 dryness, extract the dry residue with absolute alcohol, precipitate 

 the alcoholic solution with ether, and dissolve the ether-precipitate 

 in water before applying the test. In this manner, all^ foreign sub- 

 stances which might do harm will be eliminated, and the test will 

 succeed without difficulty. 



It must not be forgotten, furthermore, that the sugar itself is 

 liable to be acted on and discolored by sulphuric acid when added 

 in excess, and may therefore by itself give rise to confusion. A little 

 care and practice, however, will enable the experimenter to avoid 

 all chance of deception from this source. When sulphuric acid is 

 mixed with a watery solution containing cane sugar, after it has 

 been added in considerable excess, a yellowish color begins to show 

 itself, owing to the commencing decomposition of the sugar. This 

 color gradually deepens until it has become a dark, dingy, muddy 

 brown ; but there is never at any time any clear red or purple 

 color,, unless biliary matters be present. If the bile be present in 

 but small quantity, the colors produced by it may be modified and 

 obscured by the dingy yellow and brown of the sugar ; but even 

 this difficulty may be avoided by paying attention to the following 

 precautions. In the first place, only very little sugar should be 

 added, to the suspected fluid. In the second place, the sulphuric 

 acid should be added very gradually, and the mixture closely 

 watched to detect the first changes of color. If bile be present, the 

 red color peculiar to it is always produced before the yellowish 

 tinge which indicates the decomposition of the sugar. When the 

 biliary matters, therefore, are present in small quantity, the addi- 

 tion of sulphuric acid should be stopped at that point, and the 

 colors, though faint, will then remain clear, and give unmistakable 

 evidence of the presence of bile. 



The red color alone is not sufficient as an indication of bile. It 

 is in fact only the commencement of the change 'which indicates the 

 > biliary matters. If these matters be present, the color passes, as 

 we have already mentioned, first into a lake, then into a purple ; 

 and it is this lake and purple color alone which can be regarded as 

 really characteristic of the biliary reaction. 



It is important to observe that Pettenkofer's reaction is produced 



