46 AGRICULTURAL ANALYSIS 



At the point b there is sometimes formed a little bubble of hydro- 

 chloric acid in the state of gas, which by heating the flask A 

 completely disappears. 



The flask A is next warmed gently until the rubber tubes at 

 the pinch-cocks begin to assume their normal condition. The 

 pinch-cock at g is now replaced by the thumb and finger, and as 

 soon as the pressure within the flask A is somewhat stronger, 

 caused by the nitric oxid gas evolved from the mixture, it is 

 allowed to pass through the tube e f g h and escape into the meas- 

 uring cylinder C. By a manipulation of the finger and thumb 

 at g, it is possible to prevent regurgitation of the sodium hydroxid 

 into A, and at the same time to relieve the pressure of the nitric 

 oxid in A, which would be difficult to do by means of the pinch- 

 cock alone. 



The boiling of the liquid is continued until there is no longer 

 any increase of the volume of gas in the measuring cylinder C. 

 After the end of the operation the tube g h is removed from the 

 dish B and the measuring tube C is closed by means of the 

 thumb while its end is still beneath the surface of the soda-lye, 

 and it is shaken until all traces of any hydrochloric acid which 

 may have escaped absorption are removed. It is then placed 

 in a large glass cylinder filled with water at the temperature 

 at which the volume of gas is to be read. After being kept at 

 this constant temperature for about half an hour the volume of 

 the nitric oxid can be read. For this purpose the measuring 

 cylinder C is sunk into the water of the large cylinder until 

 the level of the liquids within and without the tube is the same. 

 The usual correction for pressure of the atmosphere, as deter- 

 mined by the barometer, and for the tension of the aqueous 

 vapor at the temperature at which the reading is made, is ap- 

 plied. The correction is made by means of the following formula : 



v , as V X 273 X (B -/) 

 (273 + t) X 760 



In this formula V denotes the volume of the gas at the tem- 

 perature of zero and at 760 millimeters barometric pressure; V, 

 the volume of the gas as read at the barometric pressure observed, 



