HAEMOGLOBIN, CARBON MONOXIDE AND OXYGEN 163 



between 4 x 1-4 and 4 x 2-fold, but in point of fact the velocity is 

 only increased 2-3-fold. Again an increase of hydrogen-ion concentra- 

 tion from pH 10 to pH 6 should increase the velocity not less than 

 6 X 1-5 times, but in reaUty the effect of increased hydrogen-ion 

 concentration is almost negligible (2), (3). 



There is another difficulty also, namely, that the reaction 



HbOo -I- CO 



HbCO + Oo 



proceeds on the average at more than twice the velocity calculated 

 from the individual velocity coefficients of the two stages. 



The study of that reaction (observed first by Haldane(2) and con- 

 firmed by Hartridge(3)) formed the starting-point of Hartridge and 

 Roughton's work. Their method briefly was to dissociate carboxy- 

 haemoglobin by hght in the presence of oxygen and then to observe 

 the rate at which the COHb was re-formed. 



The effect of light upon carboxyhaemoglobin is no haphazard affair. 

 In a system which consists of O2, CO, oxy- and carboxyhaemoglobins 

 an equihbrium is always being maintained between these four sub- 

 stances, which behaves quantitatively as though it were expressed 

 by the equation 



HbCO + 0^^^ HbOa 4- CO . 



When Hght of definite brightness shines on a solution containing 

 the above substances, a fresh equilibrium is estabUshed, so that 

 more HbOg and less HbCO is in the solution. When the bright 

 light is turned off the CO which has been displaced reunites with 

 the haemoglobin, the rate of this reunion can be measured, and there- 

 fore the velocity constant of the reaction 



HbOa + CO --^ HbCO + O2 

 can be calculated. 



