N/2 hydrochloric acid added, when it
should give, with two or three drops at most, a blue colour with the Congo
red, or pink with the methyl orange, &c. The object of this test is to
show that the nitro-glycerine is free from any excess of soda, i.e., that
the soda has been properly washed out, otherwise the heat test will show
the sample to be better than it is. The heat test must also be applied.

[Footnote A: A. Leroux, _Bul. Soc. Chim. de Bel._, xix., August 1905,
contends that experience does not warrant the assumption that free acid is
a source of danger in nitro-glycerine or nitro-cellulose; free alkali, he
states, promotes their decomposition.]

Upon leaving the filter house, where it has been washed and filtered, and
has satisfactorily passed the heat test, it is drawn off from the lowest
tank in indiarubber buckets, and poured down the conduit leading to the
precipitating house, where it is allowed to stand for a day, or sometimes
longer, in order to allow the little water it still contains to rise to
the surface. In order to accomplish this, it is sufficient to allow it to
stand in covered-in tanks of a conical form, and about 3 or 4 feet high.
In many works it is previously filtered through common salt, which of
course absorbs the last traces of water. It is then of a pale yellow
colour, and should be quite clear, and can be drawn off by means of a tap
(of vulcanite), fixed at the bottom of the tanks, into rubber buckets, and
is ready for use in the preparation of dynamite, or any of the various
forms of gelatine compounds, smokeless powders, &c., such as cordite,
ballistite, and many others.

Mikolajezak (_Chem. Zeit._, 1904, Rep. 174) states that he has prepared
mono- and di-nitro-glycerine, and believes that the latter compound will
form a valuable basis for explosives, as it is unfreezable. It is stated
to be an odourless, unfreezable oil, less sensitive to percussion,
friction, and increase of temperature, and to possess a greater solvent
power for collodion-cotton than ordinary nitro-glycerine. It can thus be
used for the preparation of explosives of high stability, which will
maintain their plastic nature even in winter. The di-nitro-glycerine is a
solvent for tri-nitro-glycerine, it can therefore be mixed with this
substance, in the various gelatine explosives in order to lower the
freezing point.

~The Waste Acids.~--The waste acids from the separating house, from which
the nitro-glycerine has b