THE BLOOD. 



159 



iron-containing substance, haematin. The molecule of haemoglobin 

 is a very large one, and its formula is given by one authority as 

 ^758Hi203^i95S 3 Fe0 218 . Although it is a colloid, haemoglobin crystallises 

 fairly readily. The crystals vary in shape in different animals, but in 

 all cases they belong to the rhombic system. In man they are rhombic 

 prisms, in guinea-pigs they form tetrahedra (fig. 50). Haemoglobin is 

 purple in colour, is soluble in water, and its solutions, examined with 

 the spectroscope, show a broad absorption band in the green between 

 Frauenhofer's lines D and E (fig. 51, sp. 2). 



G 



BC D 



Eb 



Sp. 1 



2 



FIG. 51. Spectra of haemoglobin and its derivatives. 

 Spectrum Analysis. ) 



Oxyhaenioglobin. 



Haemoglobin. 



Methaemoglobin. 



Alkaline 

 methsemoglobin. 



Acid haematin 

 in ether. 



Alkaline haematin 

 in rect. apt. 



Reduced haematin 

 (haemochromogen). 



Acid 

 heematoporphyrin. 



Alkaline 

 haematoporphyrin. 



(From Mac Munn's 



The most important property of haemoglobin is its affinity for 

 oxygen, each molecule combining with two atoms of oxygen to form 

 oxyhaemoglobin. The combination is a loose one, for the attached 

 oxygen is given up if the solution containing the compound be exposed 

 to a vacuum, or if a reducing agent, such as ammonium sulphide, be 

 added to it, HbO., (oxyhaamoglobin) again becoming Hb (haemoglobin). 

 In the living body the same reduction takes place as the blood circulates 

 through the capillaries of the active tissues. Oxyhaemoglobin is charac- 

 terised by a scarlet colour, and its spectrum exhibits two absorption 

 bands in the green, between the D and E lines (fig. 51). 



The oxygen-carrying power of haemoglobin depends upon hsematin 



