from entering.  The clip E is now closed, and tube i is put underneath the burette, and the development of NO gas is commenced by heating the contents of the flask f.  When the pressure of the gas in the flask has become greater than the pressure of the atmosphere, the connecting tube begins to swell at i, whereupon clip D is opened, and the boiling continued with frequent shaking of the bulb, until no more nitrous gas bubbles rise up into the soda lye, the distilling over of the HCl causes a crackling noise, the clip D is closed, and E opened.  The burette is again put hermetically on the indiarubber stopper in basin W, and the apparatus is left to cool until the water discharged through P shows the same temperature as the water flowing through (into the cooling jacket) Z. If the level of the soda solution in the tube X is now put on exactly the same level as that in the burette by lowering or elevating the tube X as required, the volume of NO obtained in c.c. can be read off within 1/10 c.c., and the percentage of nitrogen calculated by the usual formula.

[Illustration:  FIG. 45.—­Decomposition Flask for Schultze-Tieman Method.]

The solution of protochloride of iron is obtained by dissolving iron nails, &c., in concentrated HCl, the iron being in excess.  When the development of hydrogen ceases, it is necessary to filter warm through a paper filter, and acidify filtrate with a few drops of HCl.  The soda solution used has a sp. gr. of 1.210 to 1.260; equals 25 deg. to 30 deg.  B. The nitro-cellulose is dried in quantities of 2 grms. at 70 deg.  C. during eight to ten hours, and then three hours in an exiccator over H_{2}SO_{4}.  The results obtained with this apparatus are very accurate.  The reaction is founded upon that of MM.  Champion and Pellet’s method.

The Kjeldahl Method of Determining Nitrogen.—­This method, which has been so largely used by analysts for the determination of nitrogen in organic bodies, more especially perhaps in manures, was proposed by J. Kjeldahl,[A] of the Carlsberg Laboratory of Copenhagen.  It was afterwards modified by Jodlbauer, of Munich,[B] and applied to the analysis of nitro-explosives by M. Chenel, of the Laboratoire Centrale des Poudres, whose method of procedure is as follows:—­0.5 grm. of the finely powdered substance is digested in the cold with a solution of 1.2 grm. of phenol and 0.4 grm. phosphoric anhydride in 30 c.c. of sulphuric acid.  The mixture is kept well shaken until the solution is complete.  From 3 to 4 grms. of zinc-dust is then cautiously and gradually added, the temperature of the mass being kept down until complete reduction has been effected.  Finally, 0.7 grm. of mercury is added, and the process continued in the usual way, according to Kjeldahl; that is, the liquid is distilled until all the ammonia has passed over, and is absorbed in the standard acid.  The distillate is then titrated with standard ammonia.

[Footnote A:  J. Kjeldahl, Zeitschrift Anal.  Chem., 1883, xxii., p. 366.]