156 ELEMENTARY AGRICULTURAL CHEMISTRY 



contain carbon, hydrogen, oxygen, nitrogen, sulphur, and 

 iron. Haemoglobin contains about 16 to 17 per cent, of 

 nitrogen and about 0-4 to 0-45 per cent, of iron. It is a dark 

 purplish-red coloured substance, which readily combines with 

 oxygen to form the oxy compound, which is bright red in 

 colour. The haemoglobin of the red corpuscles plays an 

 important part in respiration. In the lungs the blood is 

 brought into contact with air, oxygen is absorbed by the 

 haemoglobin, thus causing the purplish red colour of the venous 

 blood to become bright red, almost scarlet. At the same time 

 a considerable quantity of carbon dioxide, most of which is in 

 solution in the blood plasma, probably as bicarbonates, is given 

 up to the air within the lungs. When blood is put under a 

 vacuum, gases are given off. On the average 100 volumes of 

 blood yield : 



A change in the composition of the air is produced by 

 respiration. The average composition of air before and after 

 respiration is as follows: 



The chief changes produced on air by respiration are the 

 removal of oxygen, the addition of carbon dioxide, and its 

 saturation with aqueous vapour. When oxygen combines 



