326 AGRICULTURAL ANALYSIS 



tube of the pump into the receiving arm of the azotometer just 

 below the surface of the mercury seal, so that the escaping bub- 

 bles will pass into the air and not into the tube, thus avoiding 

 the useless saturation of the caustic potash solution. 



When the flow of carbon dioxid has very nearly or completely 

 ceased, pass the delivery tube down into the receiving arm, so that 

 the bubbles will escape into the azotometer. Light the gas under 

 the 30 centimeter layer of oxid, heat gently for a few moments to 

 drive out any moisture that may be present, and bring to a red 

 heat. Heat gradually the mixture of substance and oxid, lighting 

 one jet at a time. Avoid a too rapid evolution of bubbles, which 

 should be allowed to escape at the rate of about one per second 

 or a little faster. 



When the jets under the mixture have all been turned on, 

 light the gas under the layer of oxid at the end of the tube. 

 When the evolution of gas has ceased, turn out all the lights 

 except those under the metallic copper and anterior layer of 

 cxid, and allow to cool for a few moments. Exhaust with the 

 pump and remove the azotometer before the flow of mercury is 

 stopped. Break the connection of the tube with the pump, stop 

 the flow of mercury, and extinguish the lights. Allow the azo- 

 tometer to stand for at least an hour, or cool with a stream of 

 water until a permanent volume and temperature have been 

 reached. 



Adjust accurately the level of the potassium hydroxid solution 

 in the bulb to that in the azotometer; note the volume of gas, 

 temperature, and height of barometer ; make calculation as xisual, 

 or read results from tables. 



286. Note on Official Volumetric Method. The determina- 

 tion of nitrogen in its gaseous state by combustion with copper 

 oxid, has practically gone out of use as an analytical method 

 The official chemists rarely use it even for control work on sam- 

 ples sent out for comparative analysis. The method recom- 

 mended differs considerably from the process of Jenkins and 

 Johnson, on which it is based. The only source of oxygen in 

 the official method is in the copper oxid. Hence it is necessary 



