CHOLIC ACID. 123 



drops of ether, the acid which was previously resinous becomes 

 crystalline, solid, and admits of trituration ; it must be pulverised, 

 washed with water, recrystallised in alcohol, and finally treated 

 with a little ether in order to remove any colouring matter that 

 may be attached to it. 



Tests. Cholic acid even when not perfectly pure may be recog- 

 nised by its reaction with sugar and sulphuric acid. This reaction, 

 which was first discovered by Pettenkofer,* occurs with no other 

 substance than cholic acid ; it is, however, perfectly immaterial 

 whether the cholic acid be already metamorphosed into choloidic 

 acid, or whether it be combined with its adjuncts, as a conjugated 

 acid. Hence we can apply this admirable test to discover generally 

 either the presence of bile or of one of its derivatives. The following 

 is the best method of proceeding. The alcoholic extract of the 

 fluid to be tested for biliary matter must be dissolved in a little 

 water, with which we must then mix a drop of a solution of sugar, 

 (in the proportion of 1 part of sugar to 4 of water) ; and pure 

 English sulphuric acid, free from sulphurous acid, must be added 

 by drops to the mixture ; the fluid now becomes turbid from the 

 separation of the cholic acid, but on the gradual addition of sul- 

 phuric acid the turbidity disappears, and the fluid again becomes 

 perfectly clear ; for the first few moments its colour is yellowish, it 

 very soon however becomes of a pale cherry colour, then of a deep 

 carmine, of a purple, and finally, of an intense violet tint. As, 

 indeed, in all experiments, some practice and attention to certain 

 rules are requisite, without which we may easily fail to apply this 

 test successfully to the detection of bile. For instance, we must 

 avoid the addition of too much sugar, as this is a substance which 

 is easily rendered brown or black by sulphuric acid ; and we must 

 be especially careful, as Pettenkofer himself showed, while adding 

 the concentrated sulphuric acid, not to allow the temperature much 

 to exceed 50 ; but the reaction equally fails when we carry our 

 caution too far, and attempt to avoid any elevation of temperature 

 when the sulphuric acid is added ; indeed, my own experience 

 leads me to believe that an elevation of the temperature nearly to 

 50 is requisite for the success of the experiment. Should the 

 fluid at first assume only a cherry-red or a deep carmine tint, it 

 must be allowed to stand for some time, after which the intense 

 violet colour becomes developed. It is, moreover, immaterial 

 which kind of sugar is used for this test : acetic acid may also be 

 employed in place of sugar. 



* Ann. d. Ch. u. Pharm. Bd. 53, S. 90-96. 



