tap in order to allow of the escape of any air bubbles, and clean the surface of the mercury and the inside of the cup with a small piece of filter paper.  Now close the tap, and pour the solution of the nitro-cotton into the cup.  Rinse out the bottle with 15 c.c. of sulphuric acid, contained in a pipette, pouring a little of the acid over the stopper of the weighing bottle in case some of the solution may be on it.  Now lower the pressure tube a little, just enough to cause the solution to flow into the bulb of the measuring tube, when the tap is slightly opened.  When the solution has run in almost to the end, turn off the tap, wash down the sides of the bottle, and add to the cup of the nitrometer; allow it to flow in as before, and then wash down the sides of the cup with 10 c.c. of sulphuric acid, adding little by little, and allowing each portion added to flow into the bulb of the nitrometer before adding the next portion.  Great care is necessary to prevent air bubbles obtaining admission, and if the pressure tube is lowered too far, the acid will run with a rush and carry air along with it.

[Illustration:  FIG. 41.—­ORDINARY FORM OF LUNGE NITROMETER.]

The solution being all in the measuring tube, the pressure tube is again slightly raised, and the tube containing the nitro-cotton solution shaken for ten minutes with considerable violence.  It is then replaced in the clamp, and the pressure relieved by lowering the pressure tube, and the whole apparatus allowed to stand for twenty minutes, in order to allow the gas evolved to assume the temperature of the room.  A thermometer should be hung up close to the bulb of the measuring tube.  At the end of the twenty minutes, the levels of the mercury in the pressure and measuring tubes are equalised, and the final adjustment obtained by slightly opening the tap on the measuring tube (very slightly), after first adding a little sulphuric acid to the cup, and observing whether the acid runs in or moves up.  This must be done with very great care.  When accurately adjusted, it should move neither way.  Now read off the volume of the NO gas in cubic centimetres from the measuring tube.  Read also the thermometer suspended near the bulb, and take the height of the barometer in millimetres.  The calculation is very simple.

EXAMPLE—­COLLODION-COTTON.

0.6[A] grm. taken.  Reading on measuring tube = 114.6 c.c.  NO.  Barometer—­ 758 mm.  Temperature—­15 deg.  C.

[Footnote A:  0.5 grm. is enough in the case of gun-cotton.]

Since 1 c.c.  NO = 0.6272 milligramme N, and correcting for temperature and pressure by the formula

760 x (1 + d^{2}) (d = .003665), for temperature 15 deg. = 801.78,[A]

then

(114.6 x 100 x 750 x .6272)/(801.7 x. 6) = 11.22 per cent. nitrogen.

[Footnote A:  See Table, page 244.]

The nitrogen in nitro-glycerine may of course be determined by the nitrometer, but in this case it is better to take a much smaller quantity of the substance.  From 0.1 to 0.2 grm. is quite sufficient.  This will give from 30 to 60 c.c. of gas, and therefore a measuring tube without a 100 c.c. bulb must be used.