To prepare nitrogen gas, it is only necessary to place a little dry phosphorus in a Berlin porcelain cup on a wine glass, and to stand them in a soup plate containing water. The phosphorus is set on fire with a hot wire, and a gas jar or cylindrical jar is then carefully placed over it, so that the welt of the jar stands in the water in the soup plate. At first, expansion takes place in consequence of the heat, but this effect is soon reversed, as the oxygen is converted into a solid by union with the phosphorus, forming a white smoke, which gradually disappears. (Fig. 106.) [Illustration: Fig. 106. A. Cylindrical glass vessel, open at one end, and inverted over B, the wine-glass, supporting C, the cup containing the burning phosphorus, and the whole standing in a soup-plate, D D, containing water.] Supposing two grains of phosphorus had been placed in a platinum tube, and just enough atmospheric air passed over it to convert the whole into phosphoric acid, the weight of the phosphorus would be increased to 4½ grains by the addition of 2½ grains of oxygen; [Page 104] now one cubic inch of oxygen weighs 0.3419, or about 1/3rd of a grain, hence 7.3 cubic inches of oxygen disappear, which weigh as nearly as possible 2½ grains, so that as 36.5 cubic inches of air contain 7.3 cubic inches of oxygen, that quantity of air must have passed over the 2 grains of phosphorus to convert it into 4½ grains of phosphoric acid. For very delicate purposes, nitrogen is best prepared by passing air over finely-divided metallic copper heated to redness; this metal absorbs the whole of the oxygen and leaves the nitrogen. The finely-divided copper is procured by passing hydrogen gas over pure black oxide of copper. _Second Experiment._ [Illustration: Fig. 107. A. Glass jar, with collar of leather, through which the stamper, C, works. B B. The tube containing the finely-divided lead, part of which falls out, and is ignited, and retained by the little tray just below, being part of the iron stand, D D, with crutches supporting the ends of the glass tube, and the whole stands in the dish of water, E E.]