CH. xxvi.] BLOOD CRYSTALS. 429 



best spectroscopic test for blood pigment; the suspected pigment 

 is dissolved m 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 sub- 

 stance forms the best chemical test for blood. Hsemin crystals, 

 sometimes called after their discoverer Teichmann's crystals, are 

 composed of hydrochloride of hsematin. They may be prepared 

 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. 362), separate 

 out. 



In the case of an old blood stain it is necessary to add a 



1,J 







, 



> * 



I 



Fig. /'j. Hit-lain crystals. (Frey.) Fig. 363. Haematoidin crystals. 



(Frey.) 



crystal of sodium chloride. Fresh blood contains sufficient sodium 

 chloride in itself. 



The action of the acetic acid is, (i) to split the haemoglobin 

 into haematin and globin ; and (2) to evolve hydrochloric acid 

 from the sodium chloride. The hasmatiu unites with the hydro- 

 chloric acid and thus haemin is formed. The formula for haemin, like 

 that of haamatin, is variously given. Morner gives C 36 H 36 N 4 FeC10 4 . 



Hsematoporphyrin is iron-free haematin ; it may be prepared 

 by mixing blood with strong sulphuric acid ; the iron is taken 

 out as ferrous sulphate. This substance 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 spectro- 

 scopic appearances according as it is dissolved in acid, neutral or 

 alkaline media. The absorption spectrum figured (No. 9) is that 

 of acid haematoporphyrin. 



Haematoidin. This substance is found in the form of 

 yellowish red crystals (fig. 363) in old blood extravasations, and 

 is derived from the haemoglobin. Its crystalline form and the 



