drawn off by a tap placed at a convenient height for the purpose. The
cover of the tank is generally conical, and is joined to a glass cylinder,
which is cemented to the top of this lead cover, and also to the lead
chimney. In this glass cylinder is a hole into which fits a ground glass
stopper, through which the nitro-glycerine can be drawn off. There will
probably never be more than an inch of nitro-glycerine at the most, and
seldom that. It should be taken to the filter house and treated along with
another charge. The acids themselves may either be run to waste, or better
treated by some denitration plant. This house probably requires more
attention than any other in the danger area, on account of the danger of
the decomposition of the small quantities of nitro-glycerine, which, as it
is mixed with such a large quantity of acids and water, is very apt to
become hot, and decomposition, which sets up in spots where a little
globule of nitro-glycerine is floating, surrounded by acids that gradually
get hot, gives off nitrous fumes, and perhaps explodes, and thus causes
the sudden explosion of the whole. The only way to prevent this is for the
workman in charge to look at the thermometers _frequently_, and at the
colour of the escaping fumes, and if he should notice a rise of
temperature or any appearance of red fumes, to turn on the water and air,
and stir up the mixture, when probably the temperature will suddenly fall,
and the fumes cease to come off.

The cause of explosions in this building is either the non-attention of
the workmen in charge, or the bursting of one of the water pipes, by which
means, of course, the water, finding its way into the acids, causes a
sudden rise of temperature. If the latter of these two causes should
occur, the water should at once be shut off and the air turned on full,
but if it is seen that an explosion is likely to occur, the tank should at
once be emptied by allowing its contents to run away into a drowning tank
placed close outside the house, which should be about 4 feet deep, and