8o THE CHEMISTRY OF CATTLE FEEDING 



compounds, and they give much the same dissociation products 

 plus those that are derived from the nucleic acid radicle. 



Haemoglobin, the substance of which the red corpuscles 

 of blood are mainly composed, is perhaps the most important 

 example of the chromo-proteins. In this case the chromo- 

 genic substance is a compound of iron, but little is known 

 regarding the way in which the iron is combined with the other 

 atoms in the molecule, except that it is non- ionic. 



Haemoglobin has the peculiar property of forming com- 

 pounds with oxygen and with carbon-monoxide. The oxygen 

 compound is formed in the normal course of animal respira- 

 tion, i.e. when the blood is exposed to the action of air in the 

 lungs. It is in this way that oxygen is carried to all the 

 tissues of the body to which the haemoglobin readily yields it 

 up. The deoxidised haemoglobin again returns to the lungs, 

 and the process is repeated continuously during life. The 

 carbon-monoxide compound is more stable, and animals in- 

 haling air contaminated with that gas are gradually asphyxi- 

 ated, as the process of oxidation is thus arrested. Both 

 compounds are of a bright red colour, whereas the deoxidised 

 haemoglobin is bluish. In normal circumstances, therefore, 

 the arterial blood of animals is red and the venus blood is 

 blue ; but in animals which have been poisoned with carbon- 

 monoxide gas, both the arterial and the venus blood are red. 

 An analogous compound called haemocyanin, in which the 

 iron of haemoglobin is replaced by copper, has been found in 

 the blood of crayfish and other cephalopods in which it 

 apparently performs similar functions. 



Apart from their physiological importance, the compounds 

 of haemoglobin with oxygen and with carbon-monoxide are 

 of considerable interest from another point of view. From the 

 proportion in which it unites with these substances, it is 

 possible to determine the molecular weight of haemoglobin, 

 and the result corresponds closely with that calculated from the 

 percentage composition on the assumption that the molecule 

 contains only one atom of iron. According to this datum the 

 simplest formula for haemoglobin is C 7 5 8 H 120 3N 195 O2 

 which corresponds to a molecular weight of 16,669. 



