430 THE BLOOD. [CH. xxvi. 



reaction it gives with fuming nitric acid shows it to be closely 

 allied to Bilirubin, the chief colouring matter of the Bile, and on 

 analysis it is found to be identical with it. 



Like haematoporphyrin, haematoidin is free from iron. These 

 two substances are not identical (e.g., hsematoidin shows no 

 spectroscopic bands) ; they are probably isomeric. 



Compounds of Haemoglobin. 



Haemoglobin forms at least four compounds with gases : 



With oxveen / ' Oxyhasmoglobin. 



J I 2 - Methaemoglobin. 



With carbonic oxide . . . 3. Carbonic oxide haemoglobin. 



With nitric oxide . . . 4. Nitric oxide haemoglobin. 



These compounds have similar crystalline forms; they each 

 probably consist of a molecule of haemoglobin combined with one 

 of the gas in question. They part with the combined gas 

 somewhat readily ; they are arranged in order of stability in the 

 above list, the least stable first. 



Oxyhaemoglobin is the compound that exists in arterial 

 blood. Many of its properties have been already mentioned 

 The oxygen linked to the haemoglobin, which is removed by the 

 tissues through which the blood circulates, may be called the 

 respiratory oxygen of haemoglobin. The processes that occur in 

 the lungs and tissues, resulting in the oxygenation and de-oxy- 

 genation respectively of the haemoglobin, may be imitated outside 

 the body using either blood or pure solutions of haemoglobin. 

 The respiratory oxygen can be removed, for example, in the 

 Torricellian vacuum of a mercurial air-pump, or by passing a 

 neutral gas like hydrogen through the blood, or by the use of 

 reducing agents like ammonium sulphide or Stokes' reagent.* 

 i gramme of haemoglobin will combine with 1*34 c.c. of oxygen. 



If any of these methods for reducing oxy haemoglobin is used, 

 the bright red (arterial) colour of oxyhaemoglobin changes to the 

 purplish (venous) tint of haemoglobin. On once more allowing 

 oxygen to come into contact with the haemoglobin, as by shaking 

 the solution with the air, the bright arterial colour returns. 



These colour-changes may be more accurately studied with the 

 spectroscope, and the constant position of the absorption bands 



* Stokes' reagent must always be freshly prepared ; it is a solution of 

 ferrous sulphate to which a little tartaric acid has been added, and then 

 ammonia till the reaction is alkaline. 



