72 The Estimation of Nitrogen [107 



with a pipette and run it into the flask a, washing it in with 20 c.c. 

 of warm water. Next weigh out roughly a gram of pure ferrous 

 sulphate and dissolve it in 20 c.c. of hot water to which a drop 

 of sulphuric acid has been added. Whilst it is dissolving, close 

 the stopcock of the funnel, see that the end of the evolution 

 tube dips under the mercury, and boil the liquid in the flask 

 until it is reduced to about half its bulk. This will drive all 

 air out of the apparatus, and on allowing it to cool the mer- 

 cury will rise in the side tube nearly to the barometric height. 

 If the mercury does not rise freely, then there is some leak in the 

 apparatus. Should all go well, heat up again to boiling and pour 

 the ferrous sulphate solution into b. Turn on the tap cautiously, 

 and allow nearly all the liquid to run into the flask. Now fill 

 the measuring tube, k, with mercury, and invert it over the end 

 of the evolution tube. Pour 40 c.c. of strong sulphuric acid into 

 b, and turn on the tap so that the acid may drop slowly into 

 the flask. Nitric oxide will at once come off. When nearly all 

 the acid has been added (only so much being left as will pre- 

 vent the access of air to the tap), turn off the tap and heat 

 the flask cautiously. After all the gas has come off from the 

 apparatus — i.e., when it ceases to collect in k — boil briskly for 

 half-a-minute, and allow to cool. Remove the measuring tube 

 to the deepest part of the trough, and allow to stand for an 

 hour to cool. When cold, sink the tube until the mercury 

 stands at the same level inside and outside. Now read off the 

 volume of the gas. Usually a little water collects over the 

 mercury inside the tube. In adjusting the levels of the mer- 

 cury this is neglected, but the volume of the water must be 

 noted. Note also the height of the barometer and the 

 temperature of the air in the room. 



Nitric oxide is slightly soluble in water, and therefore an 

 allowance must be made. Experiment has shown that under 

 the conditions of this experiment the water which collects in 



