which a vacuum has been formed by boiling, and then closing the taps.  The solution is then heated, the taps on the delivery tube opened, and the end placed under the collecting tube, and the NO evolved collected.  The NO gas is not evolved until the solution has become somewhat concentrated.  Eder substituted a solution of ferrous sulphate in HCl for ferrous chloride.  Care must be taken that the flask used is strong enough to stand the pressure, or it will burst.

The same chemists (Compt.  Rendus, lxxxiii. 707) also devised the following method for determining the NO_{2} in nitro-glycerine:—­A known quantity of a solution of ferrous sulphate of previously ascertained reducing power is placed in a flask, acidified with hydrochloric acid, and its surface covered with a layer of petroleum oil.  About .5 grm. of the nitro-glycerine is then introduced, and the flask heated on the water bath.  When the sample is completely decomposed, the liquid is heated to boiling to remove nitric oxide, and the excess of ferrous sulphate ascertained by titration with standard permanganate; 56 of iron (Fe) oxidised by the sample correspond to 23 of NO_{2} in the sample of nitro-glycerine.

The Schultze-Tieman Method for determining nitrogen in nitro-explosives, especially nitro-cellulose and nitro-glycerine.—­The figure (No. 44) shows the general arrangement of the apparatus.  I am indebted for the following description of the method of working it to my friend, Mr William Bate, of Hayle.  To fill the apparatus with the soda solution, the gas burette is put on the indiarubber stopper of basin W, and firmly clamped down.  Then the taps A and C are opened, and B closed.  When the burette is filled with soda solution half-way up the funnel Y, A and C are closed, and B opened.  The arrows show the inlet and outlet for the cooling water that is kept running through the water jacket round the nitrometer tube.  To collect the gas, raise the nitrometer off the rubber stopper, and place the gas tube from the decomposition apparatus in the glass dish W and under the opening of the nitrometer.

[Illustration:  Fig. 44.  SCHULTZE-TIEMAN APPARATUS.]

For the estimation of nitrogen in nitro-cellulose take .5 to .65 grm., and place in the decomposition flask f (Fig. 45), washing in with about 25 c.c. of water by alternately opening clips D and E. The air in the flask is driven out by boiling, whilst the air is shut off by the tube i dipping into the basin W, which is filled with the soda lye, and tube K is placed in the test tube R, which contains a few c.c. of water.  As soon as all the air is completely driven out, clips D and E are closed, and the gas jet is taken away. (This flask must be a strong one, or it will burst.) Into test tube R, 25 c.c. of concentrated solution of protochloride of iron and 10 to 15 c.c. concentrated hydrochloric acid are poured, which are sucked up into the developing flask f by opening clip E, air being carefully kept