The chief points to attend to during the progress of the nitration are—­

1.  The temperature registered by the two thermometers.

2.  The colour of the nitrous fumes given off (as seen through the little window in the dome of the apparatus).

3.  The pressure of the compressed air as seen from a gauge fixed upon the air pipe just before it enters the apparatus.

4.  The gauge showing the quantity of glycerine used.  The temperature, as shown by either of the two thermometers, should not be at any time higher than 25 deg.  C.

If it rises much above this point, the glycerine should be at once shut off, and the pressure of air increased for some few minutes until the temperature falls, and no more red fumes are given off.

The nitration being finished, the large earthenware tap at the bottom of the tank is opened, and the charge allowed to flow away down the conduit to the next building, i.e., to the separator.

The nitrating house is best built of wood, and should have a close-boarded floor, which should be kept scrupulously clean, and free from grit and sand.  A wooden pail and a sponge should be kept in the house in order that the workman may at once clean up any mess that may be made, and a small broom should be handy, in order that any sand, &c., may be at once removed.  It is a good plan for the nitrator to keep a book in which he records the time of starting each nitration, the temperature at starting and at the finish, the time occupied, and the date and number of the charge, as this enables the foreman of the danger area at any time to see how many charges have been nitrated, and gives him other useful information conducive to safe working.  Edward Liebert has devised an improvement in the treatment of nitro-glycerine.  He adds ammonium sulphate or ammonium nitrate to the mixed acids during the operation of nitrating, which he claims destroys the nitrous acid formed according to the equation—­

(NH_{4})_{2}SO_{4} + 2HNO_{3} = H_{2}SO_{4} + 2N_{2} + 4H_{2}O.

I am not aware that this modification of the process of nitration is in use at the present time.

The newly made charge of nitro-glycerine, upon leaving the nitrating house, flows away down the conduit, either made of rubber pipes, or better still, of woodwork, lined with lead and covered with lids made of wood (in short lengths), in order that by lifting them at any point the condition of the conduit can be examined, as this is of the greatest importance, and the conduit requires to be frequently washed out and the sulphate of lead removed.  This sulphate always contains nitro-glycerine, and should therefore be burnt in some spot far removed from any danger building or magazine, as it frequently explodes with considerable violence.

[Illustration:  FIG. 6.—­SMALL NITRATOR. N, Tap for Discharging; P, Water Pipes; T, Thermometer; W, Windows; P’, Glycerine Pipe.]