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 tap immediately in front of the measuring tube is momentarily closed, this having been proved to be without ill effect on the progress of the test.  In all experiments done by this test the air correction is subtracted from each reading, and the remainder brought to milligrams of nitrogen with the usual corrections.  As objection has frequently been taken to the test on the ground of difficulty in interpreting the results obtained, Dr Robertson made a series of experiments for the purpose of standardising the test, and at the same time of arriving at the condition under which it could be applied in the most sensitive and efficient manner.  A variety of nitro-celluloses having been tested, there were chosen as typical, of stable and unstable products, service gun-cotton on the one hand, and an experimental gun-cotton, Z, on the other.  The first point brought out by these experiments was the striking uniformity of service gun-cotton, first in regard to the rectilinear nature of the curve of evolution of nitrogen, and secondly in regard to the small range within which a large number of results is included, 15 samples lying between 6.6 and 8.7 mgms. of nitrogen evolved in four hours.  In the case of service gun-cotton, little difference in the rate of evolution of nitrogen evolved is obtained on altering the rate of passage of CO_{2} gas through the wide range of 500 c.c. per hour to 2,500 c.c. per hour.  With Z gun-cotton (see Fig. 52), however, the case is very different.  Operating at a rate of 1,000 c.c. of CO_{2} per hour, a curve of nitrogen evolution is obtained, which is bent and forms a good representation of the inherent instability of the material as proved to exist from other considerations.  Operating at the rate of 1,500 c.c. per hour, as recommended by Dr Will, the evolution of nitrogen is represented by a straight line, steeper, however, than that of service gun-cotton.  The rate of passage of CO_{2} was therefore chosen at 1,000 c.c. per hour, or two-thirds of the rate of Dr Will, and this rate, besides possessing the advantage claimed of rendering diagnostic the manner of nitrogen evolution in Z gun-cotton, has in other cases been useful in bringing out relationships, which the higher rate would have entirely masked.