Apparatus and Methods 77 



Since the changes in the level of the mercury in pipette A are slight 

 (these changes corresponding to the carbon-dioxide percentages of normal 

 air) the leveling bulb F is not usually moved. On the other hand, the 

 leveling bulb E is hung alternately on the upper and lower hooks in order 

 to expel air from pipette B into the solution in either C or Z), or out into 

 the air at the completion of the analysis. Minor changes in level of the 

 mercury in the two graduated pipettes are produced by pressure on the 

 rubber tubing with the delicate screw-cocks P and N. By shutting the 

 glass stop-cock / below and screwing in the cock P, pressure can be pro- 

 duced against the rubber tubing so as arbitrarily to adjust at will the 

 level of mercury in the capillary portion of A. A similar adjustment of the 

 mercury level in pipette B may also be made by means of the screw-cock 

 N. In the manometer M a small drop of light petroleum oil serves as an 

 index. When the stop-cocks c and d are removed so that there is atmos- 

 pheric pressure on each side, this globule should stand in the exact cen- 

 ter of the manometer if the apparatus is properly leveled. All stop-cocks 

 are well ground, perfect in fit, and lubricated by a thin layer of mutton 

 tallow. The two calibrated pipettes A and B may be designated respect- 

 ively the compensating and the measuring pipettes, although measure- 

 ments are actually made in both. 



Compensating pipette. The compensating pipette is used not only 

 for the final adjustment of the volume of gas in the measuring pipette B, 

 but also for reading directly in its lower graduated portion the percentage 

 of carbon dioxide. The volume of this pipette from the zero mark to the 

 glass stop-cock is GO cubic centimeters. The graduations demand a spe- 

 cial discussion. In all measurements made with this pipette, the gas in 

 both A and B is under definite, though slight, decrease in tension. If 

 the volume of gas in the measuring pipette has been decreased one-thou- 

 sandth by the absorption of carbon dioxide by the potassium hydroxide, 

 to adjust the air in the compensating pipette to the same tension, the vol- 

 ume of air must be expanded one-thousandth by lowering the mercury 

 a certain amount in the graduated portion of the tube. In the graduation 

 of this pipette, therefore, due cognizance has been taken of the altera- 

 tion in pressure in the pipette B. The graduations are so arranged as to 

 make the instrument direct reading, the level of the mercury in pipette 

 A indicating the percentage of carbon dioxide in the air sample. 



This instrument was primarily designed to study the percentage of 

 oxygen in the air-content or the ventilating current of a respiration cham- 

 ber. Under these conditions the carbon dioxide may at times be nearly 

 1 per cent of the air and the deficiency in oxygen approximately the same. 

 The pipette A is so constructed that percentages of carbon dioxide as 

 great as 1 per cent can be measured. Each small division indicates 0.01 

 per cent and tenths of divisions are readily estimated by the eye, so that 

 records may be obtained with three significant figures when the percent- 

 age of carbon dioxide is greater than in normal air. 



