Comparative Air-Analyses 85 



level is set exactly at zero by the fine adjusting screw N. The stop- 

 cock c is then turned so that the compensating pipette A communicates 

 directly with the room air, and by means of the finely threaded adjust- 

 ment screw P, the level of the mercury in A is also brought to the zero 

 point. Under these conditions, therefore, the gas in both pipettes is at 

 the same temperature and pressure. To test this the stop-cocks c and d 

 are simultaneously and cautiously turned and the two reservoirs A and B 

 placed in communication with the manometer; there should be no move- 

 ment of the oil-drop on the scale, thus indicating constancy in tempera- 

 ture and barometric conditions. The stop-cocks c and d are again turned 

 so as to cut off the manometer, and the sample is ready for analysis. 



The gas is first sent into the potassium hydroxide to absorb the carbon 

 dioxide. This is done by turning the stop-cock a 180 degrees and subse- 

 quently turning stop-cock b so as to allow the air to enter the capillary 

 leading to C. It is next slowly passed into the potassium hydroxide 

 twice by raising and lowering the mercury leveling-bulb E. The air is 

 then drawn back into B, the level of the reagent is again set, and the level 

 of the mercury in the pipette B is brought down to the original zero-mark. 

 Under these conditions there is a slightly diminished pressure in the pi- 

 pette B, due to the decrease in volume it has sustained by the absorption 

 of the carbon dioxide; consequently, before connecting pipette A to the 

 manometer, the pressure of the air inside A, which is now acting as the 

 compensating vessel, must be somewhat decreased. This decrease in 

 pressure is obtained by lowering the mercury column by an amount 

 which is estimated as being approximately the percentage of carbon 

 dioxide in the air. For ordinary outdoor air this is not far from 0.03 per 

 cent. Under these conditions, therefore, when the stop-cocks c and d 

 communicating with the manometer are turned there will be a slight de- 

 flection of the oil globule on the scale to the right or to the left, if the 

 pressure is greater in either A or B. For example, if the movement of the 

 petroleum globule is toward the right, it signifies that the pressure in the 

 pipette A is greater ; accordingly, it is necessary to lower the mercury in A 

 until the petroleum drop is exactly in the center. When the manometer 

 shows that the temperature and pressure are the same in both pipettes, 

 a direct reading of the percentage of carbon dioxide may be obtained by 

 noting the fall of the mercury level in A. 



To obtain a second reading, the manometer stop-cocks are again turned 

 90 degrees, and then, by properly turning the stop-cocks a and b, the air 

 is once more sent into the potassium-hydroxide solution. The level of 

 the reagent is again set and after communicating with the manometer the 

 reading is taken. The second reading is usually identical with the first, 

 as the amount of carbon dioxide to be absorbed is extremely small. 



At this point the carbon dioxide has been completely removed from the 

 gas in the apparatus, but the capillary tube between the stop-cock 6 and 

 the level of the potassium hydroxide now contains air instead of nitrogen. 



