THE RED BLOOD-CORPUSCLES 931 



haemoglobin is very similar to that of oxyhaemoglobin, the bands, 

 however, being shifted slightly towards the red end. This solution is 

 of a brighter red than oxyhaemoglobin. Its tint is best observed on 

 diluting the blood to a large extent, when oxyhaemoglobin acquires a 

 yellowish tint, while the pink colour of CO-haemoglobin is retained so 

 long as any colour is visible. The fact that CO-haemoglobin is not 

 altered by reducing agents can be shown by adding ammonium sulphide 

 to CO-haemoglobin and examining with the spectroscope, when no 

 change is observed. 



All these derivatives of haemoglobin, besides their absorption bands in the 

 visible spectrum, have characteristic absorption of light in the ultra-violet 

 spectrum, as has been shown by Gamgee. In the case of oxyhsemoglobin this 

 absorption causes a band (Soret's band) which occupies the greater part of the 

 spectral region between Fraunhofer's lines G and H. In reduced haemoglobin this 

 band is displaced towards the visible part of the spectrum. 



Another compound of haemoglobin with oxygen is methcemoglobin. 

 This substance, although not of normal occurrence in the body, is 

 found in urine and in blood whenever there is a sudden breaking down 

 of red blood-corpuscles with the setting free of haemoglobin in the 

 blood-plasma. It may be prepared by the addition of a ferricyanide, 

 permanganate, or nitrite, or other oxidising or reducing agents to the 

 laked blood or to solutions of oxyhaemoglobin. It is a chocolate-brown 

 substance, crystallisable, and gives a distinct absorption band in the 

 red between Fraunhofer's lines C and D. It is unaltered by exposure 

 to a vacuum. On treatment with reducing agents, however, such as 

 Stokes's fluid, the methaemoglobin is converted into haemoglobin, from 

 which by shaking with air oxyhaemoglobin can be re-formed. The 

 fact that methaemoglobin cannot be reduced by exposure to a vacuum 

 indicates that it is a compound of oxygen with haemoglobin in which 

 the oxygen is in a different state of combination. It has been suggested 

 by Haldane that whereas the formula of oxyhaemoglobin may be 



methaemoglobin would have the more stable composition 



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Hb^ 



The change from oxyhaemoglobin to methaemoglobin is not effected, 

 however, by a simple shifting of the oxygen groups, but must be 

 assumed to involve two distinct events. The whole of the oxygen in 

 loose combination with haemoglobin is given off, and the oxygen in the 



