Chap, ii.] RESPIRATION. 455 



haemoglobin (or more briefly haemoglobin), with a purple col- 

 our and a characteristic onebanded spectrum, we mean haemo- 

 globin which has lost all its loosely associated oxygen. If a 

 quantity of oxyhemoglobin be exposed to an insufficiently low 

 pressure, or to the action of an insufficient quantity of the 

 reducing action, it gives up a part only of its oxygen ; it is 

 only partly reduced. Such a partly reduced solution still shews 

 the two bands of oxyhemoglobin. 



§ 278. When the haemoglobin solution (or crystal) which 

 has lost its oxygen by the action either of the air-pump or of 

 a reducing agent or by the passage of an indifferent gas, is 

 exposed to air containing oxygen, an absorption of oxygen at 

 once takes place. If sufficient oxygen be present, the haemo- 

 globin seizes upon sufficient oxygen to obtain its full complement, 

 each gramme taking up in combination 1-59 c.cm. of oxygen; 

 if there be an insufficient quantity of oxygen the haemoglobin 

 still remains partly reduced; or perhaps we may say that a 

 part only of the haemoglobin gets its allowance while the 

 remainder continues reduced. If the amount of oxygen be 

 sufficient, the solution (or crystal), as it takes up the oxygen, 

 regains its bright scarlet colour and its characteristic absorption 

 spectrum, the single band being replaced by the two. Thus if 

 a solution of oxyhaemoglobin in a test-tube, after being reduced 

 by the action of a drop or two of ammonium sulphide solution 

 and thus shewing the purple colour and the single band, be 

 shaken up with air, the bright scarlet colour at once returns, 

 and when the fluid is placed before the spectroscope, it is seen 

 that the single faint broad band of the reduced haemoglobin 

 has wholly disappeared, and that in its place are the two sharp 

 thinner bands of the oxyhaemoglobin. If left to stand in the 

 test-tube the quantity of reducing agent still present is gener- 

 ally sufficient again to rob the haemoglobin of the oxygen thus 

 newly acquired, and soon the scarlet hue fades back again into 

 the purple, the two bands giving place to the one. Another 

 shake and exposure to air will however again bring back the 

 scarlet hue and the two bands ; and once more these may dis- 

 appear. In fact, a few drops of the reducing fluid will allow 

 this game of haemoglobin taking oxygen from the air and giv- 

 ing it up to the reducer to be played over and over again ; at 

 each turn of the game the colour shifts from scarlet to purple, 

 and from purple to scarlet, while the two bands exchange for 

 the one, and the one for the two. 



§ 279. Colour of Venous and Arterial Blood. Evidently 

 we have in these properties of haemoglobin an explanation of 

 at least one-half of the great respiratory process, and they teach 

 us the meaning of the change of colour which takes place when 

 venous blood becomes arterial or arterial venous. 



In venous blood, as it issues from the right ventricle, the 



