152 HEMOGLOBIN 



been natural therefore to expect that the velocity with which carbon 



monoxide united with haemoglobin would have been greater than 



that with which the pigment united with oxygen, but that is not 



the case. On the other hand, haemoglobin unites with oxygen ten to 



twelve times as fast as mth carbon monoxide. What is the solution? 



It is as follows. The pressure of gas at which the haemoglobin half- 



saturates itself, and maintains an equihbrium in that condition, 



depends upon two things, namely, k the velocity with which the 



haemoglobin unites with the gas, and k-^ the velocity with which it 



dissociates from it, and is proportional to the quotient of the two, 



k 

 i.e. to ,^. When therefore I said that the affinity of haemoglobin 



for carbon monoxide was 245 times its affinity for oxygen, I meant 



that , was 245 times smaller than -,-— . The statement therefore 

 ^co ^02 



involved nothing either about the absolute values of kcQ and ^o,> or 

 even about their relative values. It means, however, that if Ajqo is 

 only one-tenth of ko^, k^ co wiU be but 2450 of k^ Og- I^i other words, 

 the apparently high affinity of haemoglobin for carbon monoxide 

 would be due, not to a high velocity constant for the union of CO 

 with haemoglobin, but to the extreme slowness with which the two 

 separate once they are brought together. 



Considerable experimental difficulties attend the measurements both 

 of the association and dissociation velocities. These I touched upon 

 rather hghtly a few pages back, because I wished to make clear the 

 theory of what was taking place, without comphcating the dis- 

 cussion by the introduction of details of technique. I therefore passed 

 over the method of spectroscopic measurement by saying that the 

 density of the CO-haemoglobin bands was measured from point to 

 point as the fluid passed along the tube; and that starting from 

 observations taken immediately after the mixed fluid left the mixing 

 chamber, at which point the CO-bands were invisible, the bands 

 became denser as the fluid progressed. Strictly speaking the Hartridge 

 reversion spectroscope does not measure the density of the bands. 

 It measures the shifting of the bands in mixtures of oxy- and carboxy- 

 haemoglobin according to the relative quantities of the two. But here 

 there is no oxy haemoglobin. It is possible to attain the object in 

 view, of that of determining the amount of CO-haemoglobin, by placing 

 a trough of oxyhaemoglobin in the path of the light, so that the 

 observer looks through the optical mixture of a known quantity of 



