(2.5 grms.) is packed into the decomposition tube
15 mm. wide and 10 cm. high, and heated by an oil bath to a constant
temperature, the oxides so produced are forced over ignited copper, where
they are reduced, and the nitrogen retained in the measuring tubes. Care
must be taken that the acid decomposition products do not condense in any
portion of the apparatus. The air in the whole apparatus is first
displaced by a stream of carbon dioxide issuing from a carbon dioxide
generator, or gas-holder, and passing through scrubbers, and this stream
of gas is maintained throughout the whole of the experiment, the gas being
absorbed at the end of the system by strong solution of caustic potash. To
guard against the danger of explosions, which occasionally occur, the
decomposition tube and oil bath are surrounded by a large casing with
walls composed of iron plate and strong glass.

Dr Will's apparatus has been modified by Dr Robertson,[A] of the Royal
Gunpowder Factory, Waltham Abbey. The form of the apparatus used by him is
shown in Fig. 51.

~CO_{2} Holders.~--Although objection has been taken to the use of
compressed CO_{2} in steel cylinders on account of the alleged large and
variable amount of air present, it has, nevertheless, been found possible
to obtain this gas with as little as 0.02 per cent. of air. Frequent
estimations of the air present in the CO_{2} of a cylinder show that even
with the commercial article, after the bulk of the CO_{2} has been
removed, the residual gas contains only a very small amount of air, which
decreases in a gradual and perfectly regular manner. For example, one
cylinder which gave 0.03 per cent. of air by volume, after three months'
constant use gave 0.02 per cent. The advantage of using CO_{2} from this
source is obvious when compared with the difficulty of evolving a stream
of gas of constant composition from a Kipps or Finkener apparatus. A
micrometer screw, in addition to the main valve of the CO_{2} cylinder, is
useful for governing the rate of flow. A blank experiment should be made
to ascertain the amount of air in the CO_{2} and the correction made in
the readings afterwards.

[Footnote A: _Jour. Soc. Chem. Ind._, June 30, 1902, p. 819.]

[Illustration: Fig 51.--Will's Apparatus for Testing Nitro-cellulose]

~Measurement of Pressure and Rate of Flow.~--Great attention is paid to
the measurement of the rate of flow of gas, which is arrived at by
counting with a stop-watch the