36 



GAS ANALYSIS. 



[CH. II 



mercury, and a drop or two (0-2— 03 c.c.) of water is 

 placed above the mercury column in the 

 calibrated tube to protect the leaf from 

 mercury fumes. By applying suction at 

 jB the mercury column is raised- to a 

 desired height. The suction is best ap- 

 plied through a washing bottle contain- 

 ing water, so that the breath of the 

 operator may not come directly in com- 

 munication with the air in the gas-tube. 

 An india-rubber tube fitting over B serves 

 to connect with the washing bottle, and 

 also to close the tube when desired. 

 When the mercury column is at a suffi- 

 cient height, the tube is temporarily 

 closed with a clip and afterwards more 

 securely by a bit of glass rod R, whose 

 lower surface is ground flat and greased, 

 so that when pushed home it fits close 

 against the ground surface of B. 



The volume of the air contained in 

 the tube is now read off on the calibrated 

 tube, and at the same time the height of 

 the little column of water above it is re- c^fed^'fro^^Pfefier 

 corded. Readings of the barometer and ^c- «^*- 



thermometer are also taken. From 8 to 10 c.c. -of COj 

 which has been washed in NaHCOj to fi:'ee it from HCl 

 is now passed into the tube, and the readings are again 

 taken. Before introducing the COj its purity should be 

 tested by ascertaining that it is entirely absorbed over 



