CHAP, ii.] RESPIRATION. 597 



Products of the decomposition of Hcemoglobin. 



350. Although a crystalline body, haemoglobin diffuses with 

 great difficulty. This arises from the fact that it is in part a 

 proteid body ; it consists of a colourless proteid, associated with a 

 coloured substance, which may be separated out from the haemo- 

 globin, though not in the exact condition in which it naturally 

 exists in the compound; this substance when separated out 

 appears as a brownish-red body known as hcematin. All the iron 

 belonging to the haemoglobin is in reality attached to the haematin. 

 A solution of haemoglobin, when heated, coagulates, the exact 

 temperature at which the coagulation takes place depending on 

 the amount of dilution ; at the same time it turns brown from the 

 setting free of the haematin. If a strong solution of haemoglobin 

 be treated with acetic (or other) acid, the same brown colour, from 

 the appearance of haematin, is observed. The proteid constituent 

 however is not coagulated, but by the action of the acid passes 

 into the state of acid-albumin. On adding ether to the mixture, 

 and shaking, the haematin is dissolved in the supernatant acid 

 ether, which it colours a dark red, and which, examined with the 

 spectroscope, is found to possess a well-marked spectrum, the 

 spectrum of the so-called acid haematin of Stokes (Fig. 76, 6). 

 The proteid in the water below the ether appears in a coagulated 

 form owing to the action of the ether. In a somewhat similar 

 manner alkalis split up haemoglobin into a proteid constituent 

 and haematin. 



The exact nature of the proteid constituent of haemoglobin 

 has not as yet been clearly determined. It was supposed to 

 be globulin (hence the name haematoglobulin, contracted into 

 haemoglobin), but though belonging to the globulin family, has 

 characters of its own ; it is possibly a mixture of two or more 

 distinct proteids. It has been provisionally named globin and is 

 said to be free from ash. 



351. Haematin when separated from its proteid fellow, and 

 purified, appears as a dark-brown amorphous powder, or as a scaly 

 mass with a metallic lustre, having the probable composition of 

 634, H 35 , N 4 , Fe, O 5 . It is fairly soluble in dilute acid or alkaline 

 solutions, and then gives characteristic spectra (Fig. 76, 1, 2, 5). 



An interesting feature in haematin is that its alkaline solution 

 is capable of being reduced by reducing agents, the spectrum 

 changing at the same time (Fig. 76, 3), and that the reduced 

 solution will, like the haemoglobin, take up oxygen again on 

 being brought into contact with air or oxygen. This would 

 seem to indicate that the oxygen- holding power of haemoglobin 

 is connected exclusively with its haematin constituent. 



By the action of strong sulphuric acid haematin may be robbed 

 of all its iron. It still retains the feature of possessing colour, the 



