~Separation.~--The nitro-glycerine, together with the mixed acids, flows
from the nitrating house to the separating house, which must be on a lower
level than the former. The separating house contains a large lead-lined
tank, closed in at the top with a wooden lid, into which a lead pipe of
large bore is fixed, and which is carried up through the roof of the
building, and acts as a chimney to carry off any fumes. A little glass
window should be fixed in this pipe in order that the colour of the
escaping fumes may be seen. The conduit conveying the nitro-glycerine
enters the building close under the roof, and discharges its contents into
the tank through the pipe G (Fig. 8). The tank is only about two-thirds
filled by the charge. There is in the side of the tank a small window of
thick plate glass, which enables the workman to see the level of the
charge, and also to observe the progress of the separation, which will
take from thirty minutes to one hour.

The tank should be in connection with a drowning tank, as the charge
sometimes gets very dangerous in this building. It must also be connected
by a conduit with the filter house, and also to the secondary separator by
another conduit. The tank should also be fitted with a compressed air
pipe, bent in the form of a loop. It should lie upon the bottom of the
vat. The object of this is to mix up the charge in case it should get too
hot through decomposition. A thermometer should of course be fixed in the
lid of the tank, and its bulb should reach down to the middle of the
nitro-glycerine (which rests upon the surface of the mixed acids, the
specific gravity of the nitro-glycerine being 1.6, and that of the waste
acids 1.7; the composition of the acids is now 11 per cent. HNO_{3}, 67
per cent. H_{2}SO_{4}, and 22 per cent. water), and the temperature
carefully watched.