CH. XXVI.] DERIVATIVES OF HEMOGLOBIN 431 



Haemochromogen is sometimes called reduced haematin ; it may 

 be formed by adding a reducing agent like ammonium sulphide to an 

 alkaline solution of haematin. Its absorption spectrum shown on the 

 accompanying 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 absorp- 

 tion 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. 365), 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. 



Fio. 365. Haemin crystals. (Frey.) FIG. 36(5. Hae-matoidin crystals. 



(Frey.) 



The action of the acetic acid is (1) to split the haemoglobin into 

 haematin and globin ; and (2) to evolve hydrochloric acid from the 

 sodium chloride. Haemin is usually stated to be a combination of 

 haematin with hydrochloric acid. Haemin may be prepared in other 

 ways, but if prepared with the use of acetic acid, Nencki and Zaleski 

 have shown that it also contains an acetyl group, and ascribe to it the 

 empirical formula, C 34 H 33 4 N 4 ClFe. The chlorine and acetyl are both 

 attached to the iron atom. 



Haematoporphyrin is iron-free haematin ; it may be prepared by 

 mixing blood with strong sulphuric acid ; the iron is taken out as 

 ferrous sulphate. It is also found sometimes in nature ; it occurs in 

 certain invertebrate pigments, and may also be found in certain forms 

 of pathological urine. Even normal urine contains traces of it. It 

 presents different spectroscopic appearances according as it is dis- 

 solved in acid or alkaline media. The absorption spectrum figured 

 (No. 9) is that of acid haematoporphyrin. (See note, p. 444) 



Hsematoidin. This substance is found in the form of yellowish- 

 red crystals (fig. 366) in old blood extravasations, and is derived from 

 the haemoglobin. Its crystalline form and the reaction it gives with 



