186 THE BLOOD 



in air or pure oxygen, it gradually assumes a much lighter color, because its hemo- 

 globin is^thereby converted into oxyhemoglobin. These changes may be considera- 

 bly hastened by warming the blood. The conversion of oxyhemoglobin into 

 hemoglobin may also be attained by adding a reducing agent to the blood. Such 

 agents as ammonium sulphid, an ammoniacal solution of ferrous tartrate or hy- 

 drazin, are commonly employed. 1 



The power of hemoglobin to combine with oxygen seems to depend upon the iron 

 which it contains. The figures given above show that the amount of iron varies 

 only very slightly, and hence, the quantity of hemoglobin may be ascertained by 

 simply determining the iron content of the blood. One atom of iron corresponds 

 to about two atoms or one molecule of oxygen. 



Methemoglobin. 2 This body is a compound of hemoglobin and 

 oxygen which does not occur normally in the body. It appears 

 whenever large amounts of hemoglobin are set free in consequence of 

 an increased destruction of red cells. The administration of such sub- 

 stances as acetanilid, antifebrin and the nitrites is said to effect its 

 formation in the circulating blood. It is also found in the urine and 

 in the contents of cysts and old extravasates. It may be prepared by 

 permitting blood or a solution of oxyhemoglobin to stand for a long 

 time in the air, or by mixing a sample of blood with different oxi- 

 dizing or reducing substances, such as ozone, potassium permanganate, 

 ferricyanid or chlorate. Most observers agree that methemoglobin 

 is a compound of hemoglobin with oxygen in which this gas is held in 

 a different state of combustion. The compound is thereby rendered 

 more stable, a change which is clearly betrayed by its greater resist- 

 ance to vacuum. Not being able to unload its oxygen freely in the 

 tissues, it is useless as a respiratory agent. 



Methemoglobin exhibits a brownish tint and crystallizes in needles. Haldane 

 suggests for oxyhemoglobin the formula : Hb\ and for methemoglobin, the for- 



,0 

 mula: Hb^T . The conversion of the former into the latter is not accomplished 



directly by a mere shifting of the oxygen, but in an indirect manner, i.e., by first 

 dissolving all the oxygen and uniting any molecule of this gas that may be available, 

 with the radicle. 



Other Compounds of Hemoglobin. If blood is freely exposed to 

 carbon monoxid, a compound is formed between this gas and the 

 hemoglobin which is known as carbon monoxid hemoglobin (CO Hb). 1 

 One molecule of the gas combines with one molecule of hemoglobin, 

 thus effecting a very stable union which strongly resists the action 



1 Stokes's solution consists of : 



Ferrous sulphate 2.0 per cent. 



Tartaric acid 3.0 per cent. 



When about to use this solution, add ammonium hydrate until the precipitate 

 formed at first is redissolved. 



2 Discovered by Hoppe-Seyler, Handb. d. physiol. chem. Analyse, 1865, 205. 



3 Attention was first called to this fact by Cl. Bernard, in 1857. 



