atty acids the free fatty acid is deducted, and the quantity of
combined fatty acids thus obtained.

~Impurities.~ The following impurities may be found in bad samples of
glycerine:--Lead, arsenic, lime, chlorine, sulphuric acid, thio-sulphates,
sulphides, cyanogen compounds, organic acids (especially oleic acid and
fatty acids[A]), rosin products, and other organic bodies. It is also said
to be adulterated with sugar and glucose dextrine. Traces of sulphuric
acid and arsenic may be allowed, also very small traces indeed of lime and
chlorine.

[Footnote A: These substances often cause trouble in nitrating, white
flocculent matter being formed during the process of washing.]

The organic acids, formic and butyric acids may be detected by heating a
sample of the glycerine in a test tube with alcohol and sulphuric acid,
when, if present, compound ethers, such as ethylic formate and butyrate,
the former smelling like peaches and the latter of pine-apple, will be
formed.

~Oleic Acid~, if present in large quantity, will come down upon diluting
the sample with water, but smaller quantities may be detected by passing a
current of nitrogen peroxide, N_{2}O_{4} (obtained by heating lead
nitrate), through the diluted sample, when a white flocculent precipitate
of elaidic acid, which is less soluble than oleic acid, will be thrown
down. By agitating glycerol with chloroform, fatty acids, rosin oil, and
some other impurities are dissolved, while certain others form a turbid
layer between the chloroform and the supernatant liquid. On separating the
chloroform and evaporating it to dryness, a residue is obtained which may
be further examined.

~Sodium Chloride~ can be determined in 100 c.c. of the glycerine by adding
a little water, neutralised with sodium carbonate, and then titrated with
a deci-normal solution of silver nitrate, using potassium chromate as
indicator.

~Organic Impurities~ of various kinds occur in crude glycerine, and are
mostly objectionable. Their sum may be determined with fair accuracy by
Sulman and Berry's method: 50 grms. of the sample are diluted with twice
its measure of water, carefully neutralised with acetic acid, and warmed
to expel carbonic acid; when cold, a solution of basic lead acetate is
added in slight but distinct excess, and the mixture well agitated. The
formation of an abundant precipitate, which rapidly subsides, is an
indication of considerable impurity in the sample. To ascertain its