124 HEMOGLOBIN 



Hill's theory regards the haemoglobin molecule which contains one 

 atom of iron and which has a molecular weight of about 17,000, 

 as being the unit, which may be represented by the symbol Hb. 

 These units he regarded as capable of combining into aggregates. 

 Any particular aggregate, which now becomes a molecule, was de- 

 noted as (Hb)^. 



In a solution of haemoglobin — I use the word "solution " in its 

 strict sense — Hill supposed that there were aggregates of all sorts of 

 sizes, so that m had all sorts of values but the average value of m 

 was n. On this point HjU's theory has been much misunderstood. 

 When he used the symbol (Hb)„ he did not conceive that each 

 aggregate consisted of n molecular luiits but that n was the average 

 value of m, where m represented the number of units in each aggregate, 

 the values of m in any particular solution being very various. Granting 

 the above assumption, and another, namely, that all the haemoglobin 

 aggregates are either oxyhaemoglobin or reduced haemoglobin, the 

 equation for the formation of oxyhaemoglobin would be as follows : 



Hb„ + n02:i^Hb„02„. 



A word concerning the second assumption. It means that inter- 

 mediate substances between reduced haemoglobin and oxyhaemo- 

 globin cannot exist in a stable condition. Consider a case, for instance, 

 of a molecule in which m = 2. One might suppose, following the usual 

 chemical analogies, that the oxygen might be added in two stages : 



(1) Hbg-f 02 = Hb202. 



(2) Hb202+02=Hb204. 



Hill's assumption did not preclude such a mode of formation. It 

 stipulated, however, that should two molecules of Hb202 exist they 

 should at once break up thus : 



2Hb202 = Hb2-f-Hb204. 



On the reasoning of chemical analogies, the assumption was never 

 quite an easy one. So far as the facts are concerned, no substance 

 intermediate between reduced haemoglobin and oxyhaemoglobin is 

 known to exist. 



The smallest value which m can have is unity, and the simplest 

 haemoglobin solution of which one can conceive on Hill's theory is 

 one in which there was no aggregation and in which, therefore, 

 w = 1. In that case the equation would be 



Hb + Oa^iSiHbOg, 



