of the nitrating house, and should have a pipe let in flush with the
bottom, and going through the dome of the nitrating apparatus. It must of
course be provided with a tap or stop-cock, which should be placed just
above the point where the pipe goes through the lead dome.

Some method of measuring the quantity of glycerine used must be adopted. A
gauge-tube graduated in inches is a very good plan, but it is essential
that the graduations should be clearly visible to the operator upon the
platform in front of the apparatus. A large tap made of earthenware (and
covered with lead) is fixed in the side of the nitrating tank just above
the bottom, to run off the charge after nitration. This should be so
arranged that the charge may be at option run down the conduit to the next
house or discharged into a drowning tank, which may sometimes be necessary
in cases of decomposition. The drowning tank is generally some 3 or 4
yards long and several feet deep, lined with cement, and placed close
outside the building.

The apparatus having received a charge of mixed acids, the water is
started running through the pipes coiled inside the tank, and a slight
pressure of compressed air is turned on,[A] to mix the acids up well
before starting. The nitration should not be commenced until the two
thermometers register a temperature of 18° C. The glycerine tap is then
partially opened, and the glycerine slowly admitted, and the compressed
air turned on full, until the contents of the apparatus are in a state of
very brisk agitation. A pressure of about 40 lbs. is about the minimum (if
247 lbs. of glycerine and 16 cwt. of acids are in the tank). If the
glycerine tube is fitted with an injector, it may be turned on almost at
once. The nitration will take about thirty minutes to complete, but the
compressed air and water should be kept on for an additional ten minutes
after this, to give time for all the glycerine to nitrate. The temperature