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.

~Reduction Tube~--This is of copper, and consists of two parts, the outer
tube and an inner reaching to nearly the bottom of the former. Into the
inner tube fits a spiral of reduced copper gauze, and into the annular
space between the tubes is fitted a tightly packed reduced copper spiral.
At the bottom the inlet tube dips into a layer of copper oxide asbestos,
on the top of which is a layer of reduced copper asbestos. Through the
indiarubber cork passes a glass tube, which leads the CO_{2} and nitrogen
out of the reduction tube. As the portion of the tube containing the
spirals is heated to redness, water jackets are provided on both inner and
outer tubes to protect the indiarubber cork.

~Nitrogen Measuring Apparatus.~--The measuring tube with zigzag
arrangement is used, having been found very economical in potash. It is
most convenient to take readings by counterbalancing the column of potash
solution and reading off the volume of gas at atmospheric pressure. For
this purpose the