=155. Volumetric Estimation by Combustion with Copper Oxid.=—This classical method of analysis is based on the supposition that by the combustion of a substance containing nitrogen in copper oxid and conducting the products of the oxidation over red-hot copper oxid and metallic copper, all of the nitrogen present in whatever form will be obtained in a free state and can subsequently be measured as a gas. The air originally present in all parts of the apparatus must first be removed either by a mercury pump or by carbon dioxid or by both together, the residual carbon dioxid being absorbed by a solution of caustic alkali. Great delicacy of manipulation is necessary to secure a perfect vacuum and as a rule a small quantity of gas may be measured other than nitrogen so that the results of the analyses are often a trifle too high. The presence of another element associated with nitrogen, or the possible allotropic existence of that element, may also prove to be a disturbing factor in this long-practiced analytical process. For instance, if nitrogen be contaminated with another element, _e. g._, argon, of a greater density the commonly accepted weight of a liter of nitrogen is too great and tables of calculation based on that weight would give results too high. First will be given the official method for this process, followed by a few simple variations thereof, as practiced in this laboratory. =156. The Official Volumetric Method.=—This process may be used for nitrogen in any form of combination.[133] The apparatus and reagents needed are as follows: _Combustion tube_ of best hard Bohemian glass, about sixty-six centimeters long and 12.7 millimeters internal diameter. _Azotometer_ of at least 100 cubic centimeters capacity, accurately calibrated. _Sprengel mercury air-pump._ _Small paper scoop_, easily made from stiff writing paper. _Coarse cupric oxid._—To be ignited and cooled before using. _Fine cupric oxid._—Prepared by pounding ordinary cupric oxid in a mortar. _Metallic copper._—Granulated copper, or fine copper gauze, reduced and cooled in a current of hydrogen. _Sodium bicarbonate._—Free from organic matter. _Caustic potash solution._—Make a supersaturated solution of caustic potash in hot water. When absorption of carbon dioxid, during the combustion, ceases to be prompt, the solution must be discarded.