BLOOD 



487 



salt causes a similar change. This is due to the fact that 

 hannoglohin has an affi^nity for oxygen, which it takes up 

 from the air, forming a definite compound of a bright red 

 colour in which one molecule of haemoglobin links with a 

 molecule of oxygen, HbO,. This is known as oxyhaemoglobin. 



Haemoglobin is closely allied to the proteins, but differs 

 from them in containing 0-42 per cent, of iron in organic 

 combination. 



When light from the sun is allowed to pass through 

 solutions of blood pigments, certain parts of the solar 

 spectrum are absorbed, and when the spectrum is examined, 

 dark bands — the absorp- 

 tion bands — are seen. In 

 a weak solution of oxy- 

 haemoglobin in a thin 

 layer, a dark band is 

 seen in the green and 

 another in the yellow part 

 of the spectrum between 

 Frauenhofer's lines D and 

 E, while the violet end of 

 the spectrum is absorbed 

 (fig. 199). These bands 

 may be broadened or 

 narrowed by strengthening 

 or weakening the solution, 

 or varying the thickness of 

 the layer. In stronger solutions they become broader and 

 finally run together, while more and more of the violet end 

 of the spectrum is absorbed, until, with a solution of 

 sufficient strength, only the red end of the spectrum is 

 visible (fig. 198). 



When the oxygen is taken away and the dark reduced 

 hemoglobin is formed, a single broad band between D and 

 E takes the place of the two bands (fig. 199). If the 

 solution is again shaken up with air, oxygen is taken up and 

 the bands of oxyhgemoglobin reappear {Chemical Physiology). 

 The property of taking oxygen from the air and of again 

 giving it up at a moderate temperature and under a louj 



aCB X> 



Fig. 198. — The parts of the speeirum 

 absorbed by solutions of oxyh£«mo- 

 globin of difierent percentage strengths 

 in a laj'er of 1 cm. thick. 



