64 RESPIRATION 



spectra of oxyhaemoglobin and haemoglobin. To this reduced 

 haematin Hoppe Seyler gave the very suitable name haemo- 

 chromogen, as he believed it to be the parent substance of the color 

 of haemoglobin and its varied derivatives. Thus we can regard 

 haemoglobin as a compound of haemochromogen with a protein, 

 also haematin as an oxygen compound of haemochromogen, while 

 compounds of haemochromogen with carbon monoxide and nitric 

 oxide are also known. 



This conception is confirmed by the fact that the oxygen ca- 

 pacity of haemoglobin varies as its coloring power, and by another 

 still more recently established fact. Till a few years ago it still 

 seemed very doubtful whether there is a fixed and definite rela- 

 tionship between the iron in haemoglobin and its oxygen capacity ; 

 and Bohr''^ thought that he had obtained evidence of the existence 

 of marked variations in the relation between iron and oxygen 

 capacity; and that this relation differs in arterial and venous 

 blood. The doubts on this subject turned round the reliability of 

 the methods of determining iron. But in 191 2 Peters, using a new 

 and very reliable method of iron determination, found that there 

 is a fixed and simple relationship between the oxygen capacity and 

 iron, one molecule of combined oxygen corresponding to one 

 atom of iron.^ 



Still other considerations point in the same direction. When we 

 examine the colors and spectra of the various direct derivatives 

 of haemoglobin and haemochromogen a striking general cor- 

 respondence emerges. Methaemoglobin and haematin have very 

 similar colors and spectra, which differ in a more or less similar 

 manner in acid or alkaline solutions, and give a similar red color 

 and corresponding spectrum on addition of hydrocyanic acid. 

 With carbon monoxide haemochromogen gives the same color 

 and spectrum and takes up the same volume of carbon monoxide 

 as haemoglobin. With the nitric oxide compounds there appears 

 also to be a correspondence. Thus I found that the red color of 

 raw salted meat is due to the presence of NO -haemoglobin, 

 formed by the action on haemoglobin of the reduction product of 

 the niter which is mixed with the salt; and the color is still red 

 after the meat is cooked and the NO-haemoglobin broken up to 

 yield a haemochromogen compound on heating. NO-haemoglobin 

 is also found post mortem in poisoning by nitrites. Between 

 haemoglobin and haemochromogen there is also more or less of 



Bohr, Nagel's Handbuch der Physiologie, I, p. 95, 1905. 

 • Peters. Journ. of Physiol., XLIV, p. 131, 19 12. 



