PHYSIOLOGICAL CHEMISTEY 347 



blue unabsorbed in CO-haemoglobin than in the former. Hence, com- 

 paring dilute solutions of similar strength in test tubes of the same 

 diameter, the CO-haemoglobin has a distinct bluish tinge, contrasting 

 markedly with the yellowish-red of the oxyhaemoglobin. This dif- 

 ference of end-absorption can be best shown as follows : Take a fairly 

 dilute solution of oxyhaemoglobin showing the two characteristic bands 

 clearly, but not strong enough to produce any intermediate shading 

 Note as carefully as possible where the red and blue are first visible. 

 Pass a stream of coal gas or carbon monoxide through the solution by 

 means of a fine nozzle for two or three minutes. Note the change in 

 -colour produced, and again examine the spectrum. It will now be 

 found that rather more of the blue is visible, whilst the red is un- 

 altered or slightly more absorbed. 



An important difference between oxyhaemoglobin and CO- 

 haemoglobin is seen in the effect of reducing reagents. If CO- 

 haemoglobin be treated with Stokes' fluid or ammonium sulphide, 

 it is unchanged. 



4. The visible Spectrum of Methaemoglobin. To a solution of oxy- 

 haemoglobin, in which the two bands are so wide as to partially 

 overlap, add a few drops of a strong solution of potassium ferricyanide. 

 The colour changes to a chocolate tint. If this be spectroscopically 

 examined, a distinct band is seen on the red side of the D-line, the 

 wave-length of its centre being about A 635 (Spectrum 12). On 

 diluting the solution down, other bauds may be seen one just on 

 the blue side of the D-line (A, 581), another still further towards 

 the blue (A 540), and a fourth may be made out on the bluish-green 

 (A 500) (Spectra 13 and 14). The two middle bands are probably 

 not due to any traces of oxyhaemoglobin, but are characteristic of 

 methaemoglobin. 



If such a solution of methaemoglobin be treated with ammonium 

 sulphide, a transient spectrum of oxyhaemoglobin may be seen, suc- 

 ceeded by a permanent spectrum of reduced haemoglobin. 



If the solution of methaemoglobin be rendered alkaline with 

 ammonia, the colour changes to a more distinct red, and the absorption 

 band in the red disappears and is replaced by a band immediately on 

 the red side of the D-line (Spectrum 15 in Chart). 



By the action of nitric oxide on oxyhaemoglobin, a product is 

 formed called nitric oxide haemoglobin. This is characterised by two 

 bands, which are between the D and E-lines: the band on the red side 

 is somewhat nearer the red end than the corresponding band of oxy- 

 haemoglobin (Spectrum 16). 



If oxyhaemoglobin be treated with a nitrite, as sodium nitrite or 



