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 number of bubbles of gas per minute in a small sulphuric acid wash bottle.  A mercury manometer is introduced here, and is useful for detecting a leak in the apparatus.  The rate of flow that gives the most satisfactory results is 1,000 c.c. per hour.  If too rapid it does not become sufficiently preheated in the glass spiral, and if too slow there is a more rapid decomposition of the nitro-cellulose by the oxides of nitrogen which are not removed.

Decomposition Tube.—­This is of the form and dimensions given by Dr Will (15 mm. wide and 10 cm. high), the preheating worm being of the thinnest hydrometer stem tubing.  The ground-in exit tube is kept in position by a small screw clamp with trunnion bearings.

Bath.—­To permit of two experiments being carried on simultaneously, the bath is adapted for two decomposition tubes, and is on the principle of Lothar Meyer’s air bath, that is, the bath proper filled with a high-flashing hydrocarbon oil, and fitted with a lid perforated with two circular holes for the spiral tubes, is surrounded by an asbestos-covered envelope, in the interior of which circulate the products of combustion of numerous small gas jets.  The stirrer, agitated by a water motor, or, better still, a hot-air engine, has a series of helical blades curved to give a thorough mixing to the oil.  Great uniformity and constancy of temperature are thus obtained.  The bath is fitted also with a temperature regulator and thermometer.