wering the pressure tube, and the
whole apparatus allowed to stand for twenty minutes, in order to allow the
gas evolved to assume the temperature of the room. A thermometer should be
hung up close to the bulb of the measuring tube. At the end of the twenty
minutes, the levels of the mercury in the pressure and measuring tubes are
equalised, and the final adjustment obtained by slightly opening the tap
on the measuring tube (very slightly), after first adding a little
sulphuric acid to the cup, and observing whether the acid runs in or moves
up. This must be done with very great care. When accurately adjusted, it
should move neither way. Now read off the volume of the NO gas in cubic
centimetres from the measuring tube. Read also the thermometer suspended
near the bulb, and take the height of the barometer in millimetres. The
calculation is very simple.

EXAMPLE--COLLODION-COTTON.

0.6[A] grm. taken. Reading on measuring tube = 114.6 c.c. NO. Barometer--
758 mm. Temperature--15 deg. C.

[Footnote A: 0.5 grm. is enough in the case of gun-cotton.]

Since 1 c.c. NO = 0.6272 milligramme N, and correcting for temperature and
pressure by the formula

760 x (1 + _d_^{2}) (_d_ = .003665), for temperature 15 deg. = 801.78,[A]

then

(114.6 x 100 x 750 x .6272)/(801.7 x. 6) = 11.22 per cent. nitrogen.

[Footnote A: See Table, page 244.]

The nitrogen in nitro-glycerine may of course be determined by the
nitrometer, but in this case it is better to take a much smaller quantity
of the substance. From 0.1 to 0.2 grm. is quite sufficient. This will give
from 30 to 60 c.c. of gas, and therefore a measuring tube without a 100
c.c. bulb must be used.

EXAMPLE.

0.1048 grm. nitroglycerine taken gave 32.5 c.c. NO. Barometer, 761 mm.
Temperature, 15 deg. C.

Therefore,

(3.25 x 100 x 761 x .6272)/(801.78 x.1048) = 18.46 per cent. N. Theory =
18.50 per cent.

Professor Lunge has devised another form of nitrometer (Fig. 42), very
useful in the nitrogen determination in explosives. It consists of a
measuring tube, which is widened out in the middle to a bulb, and is
graduated above and below into 1/10 c.c. The capacity of the whole
apparatus is 130 c.c.; that of each portion of the tube being 30 c.c., and
of the bulb 70 c.c. The upper portion of the graduated tube serves to
measure small volumes of gas, whilst larger volumes are read off on the
lower part.

[Illustration: FIG. 42. FIG. 43. SOME NEW FORMS OF NITROMETER.]