142 METHODS FOR THE ANALYSIS OF BLOOD 



the lower part of y to be drawn back into x. (Any diffusion with 

 the outside air at the top of y will not reach to the bottom of the 

 tube owing to its length.) At the end of five minutes the cock & 

 is turned so as to connect y with c, thus closing x, which then 

 contains exactly 100 c.c. of alveolar air at atmospheric pressure 

 and at the temperature of the water in which the apparatus is 

 immersed, this temperature remaining constant throughout the 

 determination. The apparatus is removed from the water, the 

 tube c is placed beneath the surface of some 10 per cent sodium 

 hydroxide solution, some of the alkali is drawn up into y, the 

 apparatus is held in such a position that y is rather depressed in 

 order to prevent the escape of small bubbles of gas from x, the 

 cock is turned so as to connect y with x, and some of the alkali 

 is forced into x. The cock b is at once turned, closing x and con- 

 necting y with c through which the remainder of the alkali is 

 allowed to flow. The apparatus is inverted several times during 

 the course of half a minute which is sufficient time for the absorp- 

 tion of all the carbon dioxide. It is then returned to the water 

 which rises through c into y, after which 6 is turned to connect y 

 and x and the whole is allowed to remain for five minutes to again 

 equalize the temperature. It is then raised rapidly until the 

 water in the graduated portion of is at the same level as the 

 water outside the apparatus, i.e., when the gas within the tube x 

 is under atmospheric pressure. 



Calculation. The reading of the bottom of the meniscus of 

 the fluid in x is taken and represents, without any further calcu- 

 lation or correction, the percentage of carbon dioxide in the alveolar 

 air. If it is desired to express this percentage as the partial pressure 

 of COo in millimeters of mercury it is multiplied by a figure 40 

 mm. less than the prevailing barometric pressure; e.g., if the read- 

 ing of the apparatus is 5.5 then the calculation will be as follows : 

 5.5 per cent CO 2 or 0.055 X (760-40). This 40 mm. is the tension 

 of water vapor in the lungs at body temperature. It is sufficient 

 for clinical purposes to use the mean barometric pressure of the 

 locality, neglecting the daily variations. 



