RESPIRATION 417 



similarly for each degree below 13. The glass tubing passing through 

 the cork is 4 mm. in bore in order to give room for the CO 2 given off 

 without its coming in contact appreciably with the rubber connecting 

 tubing. For determining CO 2 in blood it is better to use an ordinary 

 cork than a rubber stopper in the blood flask, as the rubber leads to a 

 slow absorption of CO 2 . 



A further negative correction is required for any CO 2 present in the 

 solutions used, or absorbed from the air in the flask ; also for the small 

 error in the opposite direction owing to disappearance of ammonia 

 vapor from the air of the flask. The joint correction, which ought to be 

 very small, and may be either positive or negative, can be ascertained 

 by a blank experiment in which boiled distilled water in place of blood 

 is used in the flask. Or if the capacities of the two bottles are nearly 

 equal the blank experiment may be performed in one flask along with the 

 blood experiment in the other. In this way the correction is eliminated. 



As shown by this method by Meakins and Davies, arterial blood gives 

 slightly more CO 2 than denbrinated blood at the same partial pressure 

 of CO 2 , as found in the experiments of Christiansen, Douglas, and 

 myself. 16 



The following example illustrates the mode of calculation. The volume 

 of CO 2 given off from i.oo cc. of human arterial blood was 0.482 cc. as 

 read from the burette. Reduced to dry gas at o and 760, and calcu- 

 lated per 100 cc. of blood, this was 45.9 cc. The correction at 13 for the 

 CO 2 left in solution was 16.5 per cent, but as the temperature of the bath 

 was 15 the proper correction was 15.3 per cent. Hence the CO 2 con- 

 tained in 100 cc. of blood was 52.9 cc. A blank control experiment was 

 made simultaneously in the other flask, so there was no further correction. 



In any cases where both the oxygen and CO 2 in a sample of blood are 

 required, it is better and quicker to make the determinations simul- 

 taneously in two different apparatus. 



For the proper working of the apparatus it is essential that all the 

 joints, including the cork, should be absolutely tight. There is no 

 difficulty about this if the rubber tubing used is smooth and clean. To 

 test for tightness the burette should be read after the gauges are steady. 

 Positive or negative pressure is then produced for some time in the 

 apparatus by raising or lowering the leveling tubes. On readjusting the 

 gauges, the reading should be exactly the same as before, if the apparatus 

 is tight. If a leak exists it can soon be localized by putting pressure on 

 one part after another of the connections. 



The apparatus can be put together without very much trouble, and if 



16 Christiansen, Douglas, and Haldane, Journ. of PAysiol., XLVIII, p. 272, 

 1914. 



