THE BLOOD 13 



Haematin, as one of the decomposition products of haemoglobin, 

 has been previously referred to. It will be remembered that it 

 is obtained by decomposing haemoglobin by boiling, or the 

 addition of alkalies, acids, or acid salts ; in either case the haemo- 

 globin splits up into a substance containing the iron, known as 

 haematin, and a proteid substance or substances termed globin. 

 Haematin in the dry state strongly resembles iodine in appearance ; 

 it has a metallic lustre, a blue-black colour, is not crystallisable, 

 and yields, when pulverised, a dark brown powder, which contains 

 8-82 per cent, of iron. It is a remarkably stable substance, and 

 the colouring matter presents a distinctive spectrum both in an 

 acid and alkaline solution (see Fig. 2). Alkaline solutions of 

 haematin can take up and give off oxygen as does haemoglobin. 

 When haematin is treated with glacial acetic acid and common 

 salt, it yields haemin, which, when examined microscopically, is 

 found to consist of prismatic crystals, dark or nearly black in 



>,0 





Fig. 4. — Crystals of H^min Fig. 5. — Crystals of 



(frey). hiematoidin (stewart). 



colour (see Fig. 4). Haemin crystals may be readily produced 

 by warming the dried blood with a drop of glacial acetic acid on 

 a slide ; this is used as a microscopical test. 



When reduced haemoglobin is decomposed by acids or alkalies, 

 oxygen being carefully excluded, it yields haemochromogen, a 

 substance presenting a definite spectrum, and thus a ready means 

 of detecting old blood-stains (see Fig. 2). Haematoporphyrin is 

 obtained by the action of strong sulphuric acid on haematin, 

 which thereby loses its iron ; haematoporphyrin is really haematin 

 from which the iron has been removed ; it is isomeric with 

 bilirubin. The spectrum of this substance in acid and alkaline 

 solution may be seen in Fig. 2. 



Hydrobilirubin is obtained by the action of reducing agents on 

 haematin ; it very closely resembles urobilin, a pigment found in 

 urine. 



Haematoidin (Fig. 5) is found in old blood-clots and in the 

 ovary ; it is a crystalline iron-free product derived from haematin, 



