RESPIRATION 167 



with oxygen. By dividing the molecular heat of combination 

 by the heat produced by one gram of haemoglobin uniting 

 with oxygen, the molecular weight was found to be 15,200.* 

 Thus a result based on the assumption that oxygen unites 

 chemically with haemoglobin gives practically the same 

 molecular weight that is given by other methods of measure- 

 ment, thus indicating that the union is probably chemical. 



Another criterion of chemical combination is that the 

 properties of the compound must be different from a mixture 

 of the original substances. In some cases the difference may 

 be comparatively slight, e.g. sulphurous acid uniting with 

 oxygen to form sulphuric acid. In the case of haemoglobin 

 we find that there is a marked difference between the spectral 

 appearances of oxyhaemoglobin and haemoglobin, such a 

 spectral difference being greatly in favour of a chemical 

 change when haemoglobin combines with oxygen. 



The various compounds and derivations of haemoglobin 

 show absorption in different portions of the visible spectrum, 

 and we must also remember that lines and bands occur in 

 the ultra violet and infra red portions of the spectrum. 



The various compounds and derivatives of haemoglobin and 

 some of the corresponding spectra are given in diagrammatic 

 form. The diagram shows that haemoglobin can unite with 

 oxygen, carbon monoxide or nitric oxide. The compound of 

 oxygen with haemoglobin is least stable and that with nitric 

 oxide most stable, the compound with carbon monoxide being 

 intermediate. Oxygen is easily removed from oxyhaemoglobin 

 by reducing agents or by the tissues, but carbon monoxide 

 is so firmly united that the haemoglobin is rendered useless as 

 an oxygen carrier. The relative combining powers of oxygen 

 and carbon monoxide with haemoglobin are as i : 224 f and 

 i : 290 for two samples of human blood when the carbon 

 dioxide pressure and salts are the same. 



Methaemoglobin is formed from oxyhaemoglobin in a variety of 

 ways, amongst which are the action of ferricyanides, nitrates, 

 iodine, excess of neutral salts. It contains the same amount 

 of oxygen as oxyhaemoglobin, but united in a different way so 

 that the oxygen is not given off to the tissues. By rendering 

 methaemoglobin alkaline it passes into alkaline methaemoglobin, 

 and on treating this solution with ammonium sulphide 

 oxyhaemoglobin is formed which is then reduced to haemoglobin. 



* J. Barcroft and A. V. Hill, Journ. Physiol., 1909, vol. 39, p. 411. 

 f C. Douglas, J. S. Haldane and J. B. S. Haldane., Journ. Physiol., 

 1912, vol. 44, p. 274. 



