and allowing to stand
for the same length of time.

After the expiration of four hours 20 c.c. of 10 per cent. solution of
potassium iodide and 150 c.c. water are added to the contents of the
bottle, and the excess of iodine titrated with N/10 sodium thiosulphate
solution, the whole being well agitated during the titration, which is
finished with starch paste as indicator. The blank experiment is
titrated in the same manner, and from the amount of thiosulphate
required in the blank experiment is deducted the number of c.c. required
by the unabsorbed iodine in the other bottle; this figure multiplied by
the iodine equivalent of 1 c.c. of the thiosulphate solution and by 100,
dividing the product by the weight of fat taken, gives the "Iodine
Number".

_Example._--1 c.c. of the N/10 sodium thiosulphate solution is found
equal to 0.0126 gramme iodine.

0.3187 gramme of fat taken. Blank requires 48.5 c.c. thiosulphate.

Bottle containing oil requires 40.0 c.c. thiosulphate.

48.5 - 40.0 = 8.5, and the iodine absorption of the fat is--

8.5 x 0.0126 x 100
------------------ = 33.6.
0.3187

Wijs showed that by the employment of a solution of iodine monochloride
in glacial acetic acid reliable iodine figures are obtained in a much
shorter time, thirty minutes being sufficient, and this method is now in
much more general use than the Huebl. Wijs' iodine reagent is made by
dissolving 13 grammes iodine in 1 litre of glacial acetic acid and
passing chlorine into the solution until the iodine is all converted
into iodine monochloride. The process is carried out in exactly the same
way as with the Huebl solution except that the fat is preferably
dissolved in carbon tetrachloride instead of in chloroform.

_Bromine absorption_ has now been almost entirely superseded by the
iodine absorption, although there are several good methods. The
gravimetric method of Hehner (_Analyst_, 1895, 49) was employed by one
of us for many years with very good results, whilst the bromine-thermal
test of Hehner and Mitchell (_Analyst_, 1895, 146) gives rapid and
satisfactory results. More recently MacIlhiney (_Jour. Amer. Chem.
Soc._, 1899, 1084-1089) drew attention to bromine absorption methods and
tried to rewaken interest in them.

The _Refractive index_ is sometimes useful for discriminating between
various oils and fats, and, in conjunction with other physical and
chemical data, affords another means o