[Footnote A:  Thus if 92 lbs. glycerine give 227 lbs. nitro-glycerine, (277 x 1)/92 = 2.47 lbs.]

[Illustration:  FIG. 5.—­TOP OF NITRATOR. A, Fume Pipe; B, Water Pipes for Cooling; C, Acid Mixture Pipe; E, Compressed Air; G, Glycerine Pipe and Funnel; T, Thermometer; W, Window.]

Before the apparatus is ready for use, it requires to have two thermometers fixed, one long one to reach to the bottom of the tank, and one short one just long enough to dip under the surface of the acids.  When the tank contains its charge, the former gives the temperature of the bottom, and the latter of the top of the mixture.  The glycerine should be contained in a small cistern, fixed in some convenient spot upon the wall 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 deg.  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 should be kept as low as possible (not above 18 deg.  C.).

[Footnote A:  At the Halton Factory, Germany, cylinders of compressed carbon dioxide are connected with the air pipes so that in the event of a failure of the air supply the stirring can be continued with this gas if necessary.]