THE BLOOD 97 



oxygen as oxyhaemoglobin, only combined differently. The oxygen 

 is not removable by the air-pump, nor by a stream of a neutral gas 

 like hydrogen. It can, however, by reducing agents like ammonium 

 sulphide, be made to yield haemoglobin. Methaemoglobin is of a 

 brownish-red colour, and gives a characteristic absorption band in 

 the red between the C and D lines (fig. 37, spectrum 5). 



The ferricyanide of potassium or sodium not only causes the 

 conversion of oxyhaemoglobin into methaemoglobin, but if the reagent 

 is added to blood which has been pre%"iously laked by the addition of 

 twice its volume of water there is an evolution of oxygen. If a small 

 amount of sodium carbonate or ammonia is added as well to prevent 

 the evolution of any carbonic acid, and the oxygen is collected and 

 measured, it is found that all the oxygen previously combined in 

 oxyhaemoglobin is discharged. This is at first sight puzzling, because, 

 as just stated, methaemoglobin contains the same amount of oxygen 

 that is present in oxyhaemoglobin. What occurs is that after the 

 oxygen is discharged from oxyhaemoglobin, an equal quantity of 

 oxygen takes its place from the reagents added. The oxygen atoms 

 of the methaemoglobin must be attached to a different part of the 

 haematin group from the oxygen atoms of the oxyhaemoglobin, so 

 that the haematin group when thus altered loses its power of com- 

 bining with oxygen and carbonic oxide to form compounds which 

 are dissociable in a vacuum. 



Dr. Haldane, to whom we owe these interesting results, gives the 

 following provisional equation to represent what occurs : — 



HbOa -|-4Na3Cy6FeH-4NaHC03=Hb02 + 4Na4Cy6Fe. 



[oxyhsemo- [sodium ferri- [sofiium bicar- [meth«mo- [sodium ferro- 



globin] cyanide] bonate] globin] cyanide] 



+ 4CO2 + 2H.2O + O2. 



[carbonic [water] [oxygen] 

 acid] 



Carbonic Oxide Haemoglobin may be readily prepared by passing 

 a stream of carbonic oxide or coal gas through blood or through a 

 solution of oxyhaemoglobin. It has a peculiar cherry-red colour. Its 

 absorption spectrum is very like that of oxyhaemoglobin, but the two 

 bands ai-e slightly nearer the violet end of the spectrum (fig. 37, 

 spectrum 4). Reducing agents, hke ammonium sulphide, do not 

 change it; the gas is more firmly combined than the oxygen in 

 oxyhaemoglobin. CO-haemoglobin forms crystals like those of 

 oxyhaemoglobin : it resists putrefaction for a very long time. 



Carbonic oxide is given off during the imperfect combustion of 

 carbon such as occurs in charcoal stoves : this acts as a powerful 



H 



