78 Composition of the Atmosphere 



For the purpose of this discussion, however, we have to deal only with 

 small percentages of carbon dioxide never over 0.08 and the special 

 extended graduation of this pipette is only of incidental interest. The 

 variations usually found in the carbon-dioxide content of outdoor air are 

 so small that all adjustments of the mercury-level can be made by the 

 adjusting screw P without disturbing the leveling-bulb F. 



Measuring pipette. The measuring pipette B has two bulbs, the zero 

 mark being placed slightly below the lower bulb. Between the two bulbs 

 is a constricted portion which is graduated and represents that part of 

 the pipette corresponding to from 19.5 to 21.0 per cent of the total volume. 

 This constriction favors the accurate determination of oxygen, since after 

 the gas is absorbed from a sample of air and is again drawn into the pipette, 

 the mercury-level must be raised a sufficient amount to correspond to the 

 volume of oxygen absorbed. This is usually between 19.5 to 21.0 per 

 cent in experiments with the respiration chamber, and for outdoor un- 

 contaminated air is generally not far from 20.940 per cent. The gradua- 

 tions are such that each scale division corresponds to 0.01 per cent, and 

 hence direct readings may be accurately made to 0.001 per cent. To avoid 

 errors in parallax, the graduation marks on both pipettes completely 

 circle the glass. The total content of pipette B is 60 c. c. A few drops of 

 water are placed in both pipettes A and B and the air is thus continually 

 saturated with water-vapor. 



The upper end of each pipette is connected by capillary glass tubing 

 to the various stop-cocks, while the lower end passes through a hole in 

 the bottom of the glass reservoir and is there connected with the adjusting 

 screws P and N, the stop-cocks / and e, and the leveling-bulbs F and E, 

 respectively. Special water-tight closures are necessary where the end 

 of the pipette passes through the glass. These are shown in detail in 

 fig.l. 



After the absorption of oxygen, the level of the mercury in pipette B 

 indicates the percentage of oxygen in carbon-dioxide-free air. When 

 ordinary outdoor air with a carbon-dioxide content of but 0.030 per cent 

 is to be considered, it matters but little, so far as the expression of the 

 percentage of oxygen is concerned, whether the air is carbon-dioxide-free 

 or not, since in one case the proportion would be 20.93 c. c. of oxygen in 

 100 c. c. of air and in the other 20.93 c. c. in 100 c. c-0.03 c. c, or 99.97 

 c. c. of carbon-dioxide-free air; thus the percentage would not be measur- 

 ably affected. When, however, there may be 0.8 per cent of carbon di- 

 oxide in air taken from a respiration chamber, it becomes a matter of some 

 moment whether the percentage is of carbon-dioxide-free or of carbon- 

 dioxide-containing air. 



After the absorption of the carbon dioxide, the mercury-level in the 

 compensating pipette is so adjusted that the air in this vessel is under 

 slightly decreased tension the decrease in tension being equivalent to 

 the volume of the carbon dioxide absorbed. The final adjustment of the 



