RESPIRATION 65 



correspondence; but oxyhaemochromogen, the molecular oxygen 

 compound of haemochromogen, is missing, and it seems that 

 haematin is so readily formed by haemochromogen in the pres- 

 ence of oxygen that oxyhaemochromogen cannot exist. Figure 18 

 shows the positions of the absorption bands in the spectra of NO- 

 haemoglobin and NO-haemochromogen. 



C D E b F 



Figure 18. 



i. Nitric oxide haemoglobin. 2. Oxyhaemoglobin. 3. Carbonic 

 oxide. haemoglobin. 4. Nitric oxide haemochromogen. 5. Obtained 

 by action of nitrous acid on haematin. 



If haemochromogen has been formed from haemoglobin by 

 the action of acids or caustic alkali and heat, a substance possess- 

 ing the spectrum and properties of natural haemoglobin is gradu- 

 ally re-formed on neutralization. 9 As proteins are greatly altered 

 in properties by heating with alkali it would seem from this ob- 

 servation that there may be a number of different haemoglobins, 

 in which, though the haemochromogen part of the molecule is 

 the same in all, the protein part varies. As will be shown later, 

 there is evidence that not only in different species, but also in 

 different individuals of the same species, the protein part of the 

 haemoglobin molecule varies, thus producing slight variations in 

 the properties of the haemoglobin as a carrier of gases, although 

 there is no variation in the oxygen capacity per unit weight of 

 iron present. The haemochromogen part of the molecule seems, 

 on the other hand, to be constant in all the different sorts of haemo- 

 globin, and this brings about the identity of the relations between 

 oxygen capacity, coloring power, and percentage of iron in all 

 the different varieties of haemoglobin, although as regards other 

 properties haemoglobins from different sources vary distinctly. 



9 See Menzies, Journ. of Phystol., XVII, p. 415, 1895, and XLIX, p. 452, 1915- 



