120 ESSENTIALS OF CHEMICAL riTVSTOLOO-Y 



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. 



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

 following provisional equation to represent what occurs : 



Hb0 2 + 4Na 3 Cy G Fe + 4NaHCO :J =Hb0 2 + 4Na 4 Cy G Fe 



[oxyhaemo- [sodium ferri- [sodium bicar- [metlianno- [sodium ferro- 

 giobin] cyanide] bonate] globin] cyanide] 



[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 oxybaemoglobin. It has a peculiar cherry-red colour. Its 

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

 bands are slightly nearer the violet end of the spectrum (fig. 37, 

 spectrum 4). Reducing agents, such as ammonium sulphide, do not 

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

 oxyhaemoglobin. CO-haemoglobin forms crystals like those of oxy- 

 haemoglobin : 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 ; it is a powerful poison 

 combining with the haemoglobin of the blood, and thus interfering 

 with normal respiratory processes. The colour of the blood and its 

 resistance to reducing agents are in such cases characteristic. . 



Nitric Oxide Haemoglobin. When ammonia is added to blood, and 

 then a stream of nitric oxide is passed through it, this compound is 

 formed. It may be obtained in crystals isomorphous with oxy- and 

 CO-haemoglobin. It also has a similar spectrum. It is even more 

 stable than CO-haemoglobin ; it has little practical interest, but is of 

 theoretical importance as completing the series. 



TESTS FOR BLOOD 



These may be gathered from preceding descriptions. Briefly, 

 they are microscopic, spectroscopic, and chemical. The best chemical 

 test is the formation of haemin crystals. The old test with tincture 



