c.c. of the oil is measured into a
Hirschsohn flask, about 30 c.c. of a hot saturated solution of sodium
bisulphite added, and the flask immersed in a boiling water bath, and
thoroughly shaken at frequent intervals. Further quantities of the
bisulphite solution are gradually added, until, after about one hour,
the unabsorbed oil rises into the neck of the flask, where, after
cooling, its volume is read off, and the percentage of absorbed oil, or
aldehydes, calculated.

In the case of lemon oil, where the proportion of aldehydes, though of
great importance, is relatively very small, it is necessary to first
concentrate the aldehydes before determining them. For this purpose, 100
c.c. of the oil is placed in a Ladenburg fractional distillation flask,
and 90 c.c. distilled off under a pressure of not more than 40 mm., and
the residue steam distilled. The oil so obtained is separated from the
condensed water, measured, dried, and 5 c.c. assayed for aldehydes
either by the process already described, or by the following process
devised by Burgess (_Analyst_, 1904, 78):--

Five c.c. of the oil are placed in the Hirschsohn flask, about 20 c.c.
of a saturated solution of neutral sodium sulphite added, together with
a few drops of rosolic acid solution as indicator, and the flask placed
in a boiling water-bath and continually agitated. The contents of the
flask soon become red owing to the liberation of free alkali by the
combination of the aldehyde with part of the sodium sulphite, and this
coloration is just discharged by the addition of sufficient 10 per
cent. acetic-acid solution. The flask is again placed in the water-bath,
the shaking continued, and any further alkali liberated neutralised by
more acetic acid, the process being continued in this way until no
further red colour is produced. The flask is then filled with the sodium
sulphite solution, the volume of the cooled unabsorbed oil read off, and
the percentage of aldehydes calculated as before.

_Solidifying Point, or Congealing Point._--This is of some importance in
the examination of anise and fennel oils, and is also useful in the
examination of otto of rose. A suitable apparatus may be made by
obtaining three test tubes, of different sizes, which will fit one
inside the other, and fixing them together in this way through corks.
The innermost tube is then filled with the oil, and a sensitive
thermometer, similar to that described under the Titre test for fats,
s