432 THE BLOOD [CH. XXVI. 



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., haematoidin shows no spectroscopic 

 bands) ; they are probably isomeric. 



Compounds of Haemoglobin. 



Haemoglobin forms at least four compounds with gases : 



With oxve-en *" L Oxyhaemoglobin. 



\2. Methsemoglobin. 



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-oxygenation 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.* One gramme of haemoglobin will combine with T34 c.c. 

 of oxygen. 



If any of these methods for reducing oxyhaemoglobin 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 seen 

 constitutes an important test for blood pigment. It will be first 

 necessary to describe briefly the instrument used. 



The Spectroscope. When a ray of white light is passed through 



* 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. 



