CII. XXIX.] 



DERIVATIVES OF HEMOGLOBIN 



463 



iron is 04. On adding an acid or alkali to haemoglobin, it is broken 

 up into two parts a brown pigment called hcematin, which contains 

 all the iron of the original substance, and a protein called globin. 



FIG. 323. Oxyhsemoglobin crystals tetrahedral, 

 from blood of the guinea-pig. 



FIG. 329. Hexagonal oxyhaemoglobin crystals, 

 from blood of squirrel. (After Funke.) 



Haematin is not crystallisable ; it has the formula C 34 H 33 5 N 4 Fe 

 or C 34 H 35 5 N 4 Fe ; its constitutional formula is, however, not known. 

 Haematin presents different spectroscopic appearances in acid and 

 alkaline solutions (see accompanying plate). On decomposition it 

 yields pyrrol derivatives (see small print, p. 464). 



Globin is coagulable by heat, soluble in dilute acids, and pre- 

 cipitable from such solutions by ammonia. It belongs to the class of 

 proteins called histones (see p. 427). 



Haemochromogen is sometimes called reduced haematin ; it may 

 be formed by adding a reducing agent such as ammonium sulphide to 

 an alkaline solution of haematin, and has recently been obtained in 

 crystalline form. Its absorption spectrum, shown on the accompany- 

 ing plate (No. 8), forms the best spectroscopic test for blood pigment ; 

 the suspected pigment is dissolved in potash, and ammonium sulphide 

 added. Very dilute specimens show the absorption bands, especially 

 the one midway between D and E. 



Haemin is of great importance, as the obtaining of this substance 

 forms the best chemical test for blood. Haemin crystals may be pre- 

 pared for microscopical examination by boiling a fragment of dried 

 blood with a drop of glacial acetic acid on a slide ; on cooling, triclinic 

 plates and prisms of a dark brown colour, often in star-shaped 

 clusters and with rounded angles (fig. 330), separate out. In the 

 case of an old blood-stain it is necessary to add a crystal of sodium 

 chloride. Fresh blood contains sufficient sodium chloride in itself. 



The action of the acetic acid is to split the haemoglobin into 



