OXYH^EMOGLOBIN. 241 



air-pump, or by exposing the solution to a mixture of nitrogen 

 and hydrogen. Various reducing agents rob the oxy haemoglobin 

 of its oxygen ; and if blood or a solution of oxyhaemoglobin be 

 sealed in a glass tube so as to exclude the air, the loose oxygen 

 is taken up by some of the other constituents of the blood, and the 

 oxyhsemoglobin becomes gradually reduced to haemoglobin. This 

 depends on the putrefactive changes in the proteids, and may be 

 prevented by careful aseptic precautions. If the reduced haemoglo- 

 bin be shaken for a few moments with air, the bright color charac- 

 teristic of oxyhaemoglobin soon reappears, and if the reducing 

 agent be not injurious to the blood, the reduction and reoxidation 

 may be repeated several times, the haemoglobin going through the 

 changes which take place in it during normal respiration. 



The union of oxygen with haemoglobin solutions is not mere 

 absorption of the oxygen by the liquid, but a definite chemical 

 combination. This is seen from the following facts: (1) When 

 the pressure is removed, the oxygen does not come away from the 

 liquid in accordance with the law which governs the escape of 

 absorbed gas (vide p. 246). (2) The two substances give a dif- 

 ferent result when examined with the spectroscope. The reduced 

 haemoglobin gives one wide diffuse band, which lies between the 

 D and E lines of the solar spectrum, and much of the violet end 

 is cut off. The spectrum, which is characteristic of reduced 

 haemoglobin, is replaced by two bands when the haemoglobin com- 

 bines with oxygen one broad band in the green near E, and a 

 narrow one, more clearly defined, in the yellow close to the D 

 line ; both bands lie between D and E. With strong solutions 

 the spectrum is darkened at either extremity, and the two bauds 

 become wider and tend to fuse into one. (3) Further, the oxy- 

 gen may be replaced by other substances which unite with the 

 haemoglobin. One of the most important of these is carbonic 

 oxide, which forms a much more stable compound with haemo- 

 globin than oxygen. It is of a bright cherry-red color, and has 

 two absorption bands in the spectrum very like those of oxyhae- 

 moglobin ; that in the yellow is, however, removed a greater dis- 

 tance from the D line toward the violet end. 



It is this compound which is formed in poisoning with car- 

 21 



