148 
ON COD-LIVER OIL. 
But though we may thus readily prove that the suspect- 
ed oil contains no artificially added iodine, the iodine which 
is naturally contained in, and more intimately combined 
with the oil, may be frequently recognized by another pro- 
cess. Marchand gives the following directions for detecting 
it : Saponify the oil with soda, carbonize the soap thus ob- 
tained, digest the coal in distilled water, add a drop of 
starch paste, and subject the mixture to the action of a vol- 
taic battery, the positive pole being placed in contact with 
the starch paste, the negative pole with the solution. If 
iodine be present the starch becomes blue. Marchand states 
that by this test, the iodine can be detected in the urine of 
a patient soon after he has taken the oil. This, however, 
is certainly not always correct, for I submitted the urine of 
a young gentleman, who, for several weeks had taken with 
great benefit a table-spoonful of cod-liver oil thrice daily, 
to the action of a galvanic battery of fifty pairs of plates for 
several hours, without obtaining the slightest evidence of 
the presence of iodine. 
Sulphuric acid has been employed as a test for cod-liver 
oil. If a drop of concentrated sulphuric acid be added to 
fresh cod-liver oil, the latter assumes a fine violet colour, 
which soon passes into yellowish or brownish-red. Some 
samples of oil produce at once the red colour, without the 
preliminary violet tint. Gobley, who noticed this reaction 
in the case of oil of the liver of the ray, says, that oil which 
has been prepared by ebullition in water, does not possess 
this property, but yields with sulphuric acid a clear red co- 
lour. This, however, is an error, at least with respect to 
cod-liver oil. It has been erroneously supposed by some 
persons that this violet colour was due to the evolution of 
iodine by the action of the acid on an alkaline iodide con- 
tained in the oil. If that where the case, the presence of a 
little starch-paste would be sufficient to convert the violet 
into an intense blue colour ; which is not the case c The 
