RESPIRATION 



409 



help of the oxygen liberated from a titrated solution of hydrogen perox- 

 ide. But this is a rather serious complication, and even if the calibration is 

 correctly made it can only apply correctly at a certain barometric pressure 

 and would not be quite valid over the variations of barometric pressure 

 ordinarily met with. I cannot, therefore, regard this plan as satisfactory 

 for some kinds of exact work. On the other hand this objection 

 does not apply where the empirical calibration is not needed, as in 

 determinations of the percentage saturation of haemoglobin with oxy- 

 gen — for instance in investigating dissociation curves of oxyhaemo- 

 globin. Barcroft has also devised a small model, for which only o.i cc. 

 of blood is required. 



A very different form of the ferricyanide method has recently been 

 introduced by Yandell Henderson and Smith.^^^ The blood (i cc.) is 

 introduced (under ammonia solution without contact with air, just as 

 in the Barcroft-Haldane method) into the bottom of a diffusion tube 

 of about 12 cc. capacity. This tube is provided with a 3-way tap at the 

 bottom end and a thin rubber stopper at the top, and is graduated for a 

 short distance from the top. A fine hypodermic needle is then thrust 

 through the rubber to equalize the pressure inside and outside of the 

 tube, the needle withdrawn, and the blood and ammonia solution mixed 

 so as to lake the blood. Ferricyanide solution is then injected through the 

 stopper, and the tube rotated for five minutes so that the whole excess 

 of free oxygen diffuses out into the air of the tube. The tube is then 

 inverted and the stopper removed under water so that the pressure inside 

 and outside the tube is equalized. The volume of gas in the tube is read 

 off ; and finally nearly the whole of this gas is drawn into a Haldane gas- 

 analysis apparatus, and the oxygen percentage determined. From the 

 increased oxygen percentage of this gas as compared with air, and the 

 volume of gas in the tube, the oxygen given off by the blood can easily 

 be calculated. The CO2 in the blood is estimated similarly; and both 

 oxygen and CO2 can be estimated in the same sample of blood. This 

 method seems to be about as accurate as the Barcroft-Haldane method, 

 and to be easier for those familiar with accurate gas analysis. It appears 

 to be specially suitable for comparisons of the arterial and venous blood 

 in animals ; and evidently any CO in blood can be estimated conveniently 

 by this method, which also has the advantage that corrections for physical 

 solution of gases are greatly reduced. 



Still another method is to use the Van Slyke vacuum apparatus in 

 connection with ferricyanide. ^^ This, however, involves the various 



"Yandell Henderson and Smith, Journ. of Biol. Chem., XXXIII, p. 39, 19 18. 

 "Van Slyke, Journ. of Biol. Chem., XXX, p. 347, 1917 ; and XXXIII, p. 127. 

 1918. 



