THE BLOOD. l6l 



globin the same oxygen is combined as two separate atoms, Hb< . The 



addition of potassium ferricyanide to a solution of oxyhsemoglobin 

 probably leads to the removal of the dissociable oxygen, and then re- 

 oxidises the haemoglobin into the more stable form of methsemoglobin. 



When ammonium sulphide is added to a solution of methsemoglobin, 

 the colour changes to red and later to purple. The intermediate red 

 stage gives a spectrum corresponding with that of oxyhsemoglobin, the 

 purple product is haemoglobin, and this yields oxyhsemoglobin when 

 it is shaken with air. By means of hydrazine hydrate it can be shown 

 that methsemoglobin contains less dissociable oxygen than oxyhsemo- 

 globin. The latter compound, when treated with hydrazine, gives off a 

 volume of nitrogen corresponding with the volume of dissociable oxygen. 



Hb0 2 + N 2 H 4 . H,0 = N 2 + 3H 2 + Hb. 



When methsemoglobin is treated in the same way it never yields more 

 than half the amount of gas given off from a similar weight of oxy- 

 hsemoglobin (Buckmaster). The explanation of this fact is still obscure. 



If a solution of haemoglobin (or oxyhsemoglobin) is warmed with an 

 acid or an alkali, the giobin is converted into metaprotein, and hoematin 

 is set free. In the pure condition hsematin is a dark brown or black 

 amorphous substance, insoluble in water, soluble in acids or alkalies. 

 In acid solution its spectrum shows, besides other absorption bands, a- 

 characteristic band in the red, nearer the red end of the spectrum than 

 that given by metheemoglobm (fig. 51). The spectrum of the alkaline 

 solution exhibits a rather faint band just to the red side of the D line. 

 On the addition of ammonium sulphide, alkali hsematin is converted 

 into reduced alkali hsematin or hcemochromogen, the spectrum of which 

 shows two absorption bands in the green, some distance to the violet 

 side of the D line, the band nearer D being the more distinct of the two 

 (fig. 51). As these bands can be seen in extremely dilute solutions, 

 the formation of hsemochromogen constitutes a delicate spectroscopic 

 test for blood pigment. 



As has already been said, hsematin contains the iron of the Hb 

 molecule, and it has had the formula C 34 H 34 N 4 Fe0 5 assigned to it. It 

 forms a compound with hydrochloric acid, hydrochloride of hsematin 

 or hcemin, which is easily obtained by heating blood with glacial acetic 

 acid in the presence of sodium chloride. Hsemin occurs in dark brown 

 rhombic crystals (fig. 52), and its formation is utilised as a medico- 

 legal test for blood. In the reduced condition, hsematin will recombirie 

 with globin to form hsemoglobin or a substance indistinguishable from 



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