66 RESPIRATION 



The original ferricyanide method for determining the oxygen 

 capacity of haemoglobin was very accurate, but required a good 

 deal of blood, and was also slow on account of the time needed 

 for exact equalization of temperature and gas pressures. Mr. 

 Barcroft was then beginning his important series of investiga- 

 tions on the metabolism of the salivary glands and other organs. 

 As he required a blood-gas method suitable for very small volumes 

 of blood he asked me whether the ferricyanide method could be 

 adapted for the purpose, and I designed an apparatus which we 

 jointly tested and described, and which turned out so successfully 

 that, in one form or another, it has now almost displaced the 

 mercurial blood pump. 10 In this apparatus the oxygen combined 

 in the haemoglobin of the very small quantity of blood required 

 is liberated by ferricyanide, and the CO 2 by acid. The amount of 

 gas liberated in either case is determined, not from the increase 

 in volume which its liberation causes, but from the increase of 

 pressure when the total volume of gas is kept rigorously constant. 

 I adopted this principle as the result of much previous experience 

 in the measurement of small differences in gas volumes. Certain 

 causes of difficulty are eliminated by the pressure method, and by 

 the adoption, as in the original ferricyanide method, of a control 

 arrangement by which the effects of changes in temperature and 

 barometric pressure during the experiment are eliminated. Vari- 

 ous improvements in the technique of collecting and sampling 

 blood drawn directly from blood vessels were also introduced by 

 Mr. Barcroft. 



This apparatus has been modified in various ways by different 

 investigators, and some of the modifications are improvements. 

 Others, however, seem to me to be the reverse. In the Appendix 

 there is a description of a new and much more exact method in 

 which the volumes of oxygen and CO 2 are measured directly. 



Besides the oxygen chemically combined with haemoglobin, 

 the blood contains a certain small amount of oxygen in simple 

 solution. In accordance with Henry's law of solution of gases in 

 liquids this amount varies with the partial pressure of oxygen in 

 the atmosphere with which the blood is saturated, which in the 

 case of arterial blood in the living body is (with certain reserva- 

 tions discussed in Chapters VII and VIII), the alveolar air. The 

 amount of oxygen in free solution can be measured directly when 

 the haemoglobin is by one means or another put out of action in 

 respect to its power of entering into molecular combination with 



10 Barcroft and Haldane, Journ. of Physiol., XXVIII, p. 232, 1902. 



