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132 HEMOGLOBIN 



was quite uninfluenced by the others, then one might expect a rect- 

 angular hyperbola as the dissociation curve. 



Now the hypothesis that the molecules of iron are so far from one 

 another that their activities can be regarded as independent is not 

 unreasonable, and the existence of intermediate oxides, Hb402, 

 Hb404 and Hb406 , is what might be expected on chemical analogies. 

 So far, so good. The reader may ask with reason. Where is the difficulty ? 

 Firstly, there is the fact that these intermediate oxides have never 

 been observed either spectroscopically or otherwise. If you present 

 haemoglobin with, say, one-quarter the amount of oxygen necessary for 

 its saturation, there is no evidence of Hb^Og . Now this difficulty might 

 conceivably be met by the assumption that the oxides are imstable. 

 That explanation cannot be accepted ; in theory it is the same thing 

 as saying that they do not exist, and is in fact the assumption made 

 by Hill as the basis of his equation, which would be 



y Kx^ 



Too" H- Kx^' 



Secondly, it makes no provision for the curve ever being other 

 than a hyperbola. 



The best appreciation of the situation which can be given at present 

 seems to be the following : 



1. Fundamentally we must start with weak solutions. 



2. The molecular weight of haemoglobin in such is 67,000, or (16,700)4. 



3. The reaction with oxygen is 



Hb4-f402=Hb408. 



4. The dissociation curve of the reaction with O2 or CO in weak 

 solutions is very far from being expressed by the equation 



y ^ Kx^ 

 100" 1 + Kx*' 



6. The dissociation curve is very near to being 



y _ Kx 

 Too" 1 + Kx' 



6. The facts contained in paragraphs 1-5 (above) can be reconciled 

 by assuming the existence of intermediate oxides, which exist for 

 a finite time, but which have never been seen. 



7. They leave unexplained the more famihar dissociation curve 

 of blood 



100 ~ 1 -f Kx"'^' 



