6o RESPIRATION 



This substance, which crystallizes in a similar form to oxyhaemo- 

 globin but has a dull brown color in acid solution and a brownish 

 red color in alkaline solution, was found by Hiifner to take up in 

 its formation from haemoglobin just as much oxygen as oxy- 

 haemoglobin ; but the oxygen is not given off in a vacuum. On the 

 other hand it yields its oxygen much more rapidly to a reducing 

 agent than oxyhaemoglobin or free oxygen does, and is thus an 

 oxidizing agent of some activity. Thus if a drop of ammonium 

 sulphide solution is mixed with a solution of methaemoglobin in 

 the absence of free oxygen the methaemoglobin is instantly re- 

 duced to haemoglobin, as shown by the change of color and spec- 

 trum. But if free oxygen is present the color and spectrum of 

 oxyhaemoglobin appear, since the ammonium sulphide combines 

 far more slowly with free oxygen, or with the combined oxygen 

 of oxyhaemoglobin, so that the haemoglobin formed instantly 

 from the methaemoglobin is able to combine with the free oxygen 

 and remain for a long time as oxyhaemoglobin. 



While investigating the action of poisons which form met- 

 haemoglobin in the living body I noticed that when ferricyanide 

 and certain other reagents act on oxyhaemoglobin to form methae- 

 moglobin fine bubbles are liberated, and on further investigation 

 the liberated gas was found to be oxygen.^ I then measured ac- 

 curately the liberated oxygen, and found that the volume of oxy- 

 gen liberated by ferricyanide from blood agrees exactly with the 

 volume liberated by the mercurial pump from combination in the 

 blood. Ferricyanide also liberates carbon monoxide from its com- 

 bination with haemoglobin, and the volume liberated corresponds 

 with the volume of oxygen liberated by a corresponding quantity 

 of oxyhaemoglobin. The following figures were obtained. 



From their behavior, it appears that oxyhaemoglobin and CO- 

 haemoglobin are molecular compounds in which the molecules of 



* Haldane, Journ. of Physiol., XXII, p. 298, 1898. 



