THE BLOOD 113 



The crystals also contain a varying amount of water of crystallisa- 

 tion : this may in part explain their different crystalline forms and 

 solubilities. Different observers have analysed haemoglobin. They 

 find carbon, hydrogen, nitrogen, oxygen, sulphur, and iron. The 

 percentage of iron is 0'4. Oxyhaemoglobin may be estimated in the 

 blood (1) by the amount of iron in the ash, or (2) by certain colori- 

 metric methods which are described in the Appendix. 



Haemoglobin is a conjugated protein (see p. 44), and on the addi- 

 tion of an acid or alkali it is broken up into two parts, a protein called 

 globin, and a brown pigment called hcematin, which contains all the 

 iron of the original substance. 



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

 pitable from such solutions * by ammonia. It is a member of the 

 group of proteins called histones (see p. 42), 



Haematin is not crytallisable : according to Nencki and Sieber 

 its formula is C 32 H 3 2N 4 4 Fe. It presents different spectroscopic 

 appearances in acid and alkaline solutions, and yields several products 

 under the influence of certain reagents, w r hich we shall consider in 

 the advanced course. For the present, we shall mention only three 

 of these, haernin, haematoporphyrin, and haemopyrrol. 



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

 in a crystalline form is the best chemical test for blood. Haemin 

 crystals, sometimes called Teich- 



mann's crystals, are prepared for ^r^ 3f^ $C 



microscopic examination by boiling yVj ^-^^ ^.^ ^ \ 



a fragment of dried blood with a '* *ZF ^ J ^^~ 



^ ^ m ^^ Si 



drop of glacial acetic acid on a -^. *jUr ^ Nfc r y ^^ 

 slide ; on cooling, dark - brown *, fl X -" * ^4t 



^^*^- jjjj j* ^^^ar ^ I 



plates and prisms belonging to the M zL=^ ^ ^ fi 



triclinic system, often in star- * ' 



shaped clusters and with rounded ^ ^ < ^ g^ * 



*. l ^ 



angles (fig. 31), separate out. 4, ^ 7 



In the case of an old blood- -rL- -^ 7- 



Stain it 1S necessary to add a Fl - SI- Hsemiii crystals inaguified. (Preyer.) 



crystal of sodium chloride. Fresh 



blood contains sufficient sodium chloride in itself. 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. The 

 haematin unites with the hydrochloric acid, and thus haemin is formed. 

 Nencki has further shown that, when prepared in this way, haemin 

 also contains the acetyl group. 



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



