CHAP ii.] RESPIRATION. 337 



When crystals of haemoglobin, prepared in the way described 

 above, are subjected to the vacuum of the mercurial air-pump, they 

 give off a certain quantity of oxygen, and at the same time they 

 change in colour. The quantity of oxygen given off is definite, 

 1 grm. of the crystals giving off 1'59 c.cm. of oxygen. In other 

 words, the crystals of haemoglobin over and above the oxygen 

 which enters intimately into their composition, (and which alone 

 is given in the elementary composition stated on p. 335), contain 

 another quantity of oxygen, which is in loose combination only, 

 and which may be dissociated from them by subjecting them to a 

 sufficiently low pressure. The change of colour which ensues when 

 this loosely combined oxygen is removed, is characteristic; the 

 crystals become darker and more of a purple hue, and at the same 

 time dichroic, so that while the thin edges appear green, the 

 thicker ridges are purple. 



An ordinary solution of haemoglobin, like the crystals from 

 which it is formed, contains a definite quantity of oxygen in 

 a similarly peculiar loose combination; this oxygen it also gives up 

 at a sufficiently low pressure, becoming at the same time of 

 a purplish hue. This loosely combined oxygen may also be 

 removed by passing a stream of hydrogen or other indifferent gas 

 through the solution, whereby dissociation is effected. It may 

 also be got rid of by the use of reducing agents. Thus if a few 

 drops of ammonium sulphide or of an alkaline solution of ferrous 

 sulphate, kept from precipitation by the presence of tartaric acid, 

 be added to a solution of haemoglobin, or even to an unpurified 

 solution of blood corpuscles such as is afforded by the washings 

 from a blood clot, the oxygen in loose combination with the 

 haemoglobin is immediately seized upon by the reducing agent. This 

 may be recognised at once, by the characteristic change of colour ; 

 from a bright scarlet the solution becomes of a purplish claret colour, 

 when seen in any thickness, but green when sufficiently thin : the 

 colour of the reduced solution is exactly like that of the crystals 

 from which the loose oxygen has been removed by the air-pump. 



Examined by the spectroscope, this reduced solution, or solution 

 of reduced haemoglobin as we may now call it, offers a spectrum 

 (Fig. 58. 5) entirely different from that of the unreduced solution. 

 The two absorption bands have disappeared, and in their place 

 there is seen a single, much broader, but at the same time much 

 fainter band whose middle occupies a position about midway be- 

 tween the two absorption bands of the unreduced solution, though 

 the red-ward edge of the band shades away rather farther towards 

 the red than does the other edge towards the blue; its centre 

 corresponds to about wave length 555. At the same time the 

 general absorption of the spectrum is different from that of the 

 unreduced solution ; less of the blue end is absorbed. Even when 

 the solutions become tolerably concentrated, many of the bluish- 

 green rays to the blue side of the single band still pass through. 

 F. 22 



