652 
COD-LIVER OIL. 
very well if we dissolve a little extract of ox-bile in water, and 
test the solution with sugar and oil of vitriol. The colour deve¬ 
loped agrees with that produced by the addition of oil of vitriol to 
cod-liver oil, which De Jongh has shewn contains the essential 
constituents of the bile. 
Pettenkofer remarks, that the presence of a very great excess of 
chlorides will change the violet-red colour into a brownish-red. 
This fact is deserving of notice, because it may aid in accounting 
for the fact that some specimens of cod-liver oil strike a brownish- 
red, not a violet-red colour, with oil of vitriol. 
Strecker* confirms Platner’s observation that both cholic and 
paracholic acids produce the same colour with sugar and oil of 
vitriol as bile does; so that Pettenkofer’s test doubtless acts on 
one or both of these acids. Now De Jongh has shewn that cho¬ 
lic acid is contained in cod-liver oil, and we have, therefore, good 
reason for believing that it is in part by the action of oil of vitriol 
on this acid that the violet-red colour is produced in cod-liver oil. 
But it is well known that for the development of this colour in 
bile it is necessary to use, besides oil of vitriol, a third agent 
(sugar). Pettenkofer observes that for cane-sugar we may substi¬ 
tute grape-sugar or starch; in fact, any substance which can by 
the action of oil of vitriol be converted into grape-sugar. No such 
substance has hitherto been detected in cod-liver oil, and, therefore, 
it may be said the necessary ingredient to produce this character¬ 
istic re-action of oil of vitriol on cholic acid is wanting. Strecker 
has recently supplied the wanting link. In his valuable paper on 
ox-bile, to which I have already referred, he observes that acetic 
acid may be substituted for sugar. To the liquid supposed to con¬ 
tain bile add a few drops of acetic acid, and then concentrated sul¬ 
phuric acid, when a magnificent purple-red colour is developed. 
If the quantity of bile be small, it may be necessary to use heat. 
Now, as cod-liver oil contains acetic acid, we have the requisite 
agent to enable the oil of vitriol to act on the cholic acid, and the 
development of the purple or violet-red colour is then readily ac¬ 
counted for. 
I have already noticed the red colour produced by the action of 
oil of vitriol on gaduin (supposed by Berzelius to be derived from 
the bile). Here, then, is another source for the red colour caused 
by the action of sulphuric acid on cod-liver oil. 
It follows, therefore, from what has been now stated, that oil of 
vitriol is a test for liver oils. It does not distinguish one liver oil 
from another, for it re-acts equally with the oil of the liver of the 
* Ann. der Chemie und Phcirmacie, Bd. lxv, s. 15 . 1848 . 
