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 been as completely separated as possible, are run
down the conduit to the secondary separator, in order to recover the last
traces of nitro-glycerine that they contain. The composition of the waste
acids is generally somewhat as follows:--Specific gravity, 1.7075 at 15°
C.; sulphuric acid, 67.2 per cent.; nitric acid, 11.05 per cent.; and
water, 21.7 per cent., with perhaps as much as 2 per cent. of nitric
oxide, and of course varying quantities of nitro-glycerine, which must be
separated, as it is impossible to run this liquid away (unless it can be
run into the sea) or to recover the acids by distillation as long as it
contains this substance. The mixture, therefore, is generally run into
large circular lead-lined tanks, covered in, and very much like the
nitrating apparatus in construction, that is, they contain worms coiled
round inside, to allow of water being run through to keep the mixture
cool, and a compressed air pipe, in order to agitate the mixture if
necessary. The top also should contain a window, in order to allow of the
interior being seen, and should have a leaden chimney to carry off the
fumes which may arise from decomposition. It is also useful to have a
glass tube of 3 or 4 inches in diameter substituted for about a foot of
the lead chimney, in order that the man on duty can at any time see the
colour of the fumes arising from the liquid. There should also be two
thermometers, one long one reaching to the bottom of the tank, and one to
just a few inches below the surface of the liquid.

The nitro-glycerine, of course, collects upon the surface, and can be