CHAP, ii.] RESPIRATION. 563 



corpuscles remain in as natural a condition as possible) is examined 

 with the spectroscope, as well as when a drop of blood, which from 

 the necessary exposure to air is always arterial, is examined with 

 the micro-spectroscope. In fact, the spectrum of haemoglobin is 

 the spectrum of normal arterial blood. 



346. 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 T59 c.cm. of oxygen 

 measured at 760 mm. Hg and C. In other words, the crystals 

 of haemoglobin, over and above the oxygen which enters intimately 

 into the composition of the molecule (and which alone is given in the 

 elementary composition previously stated), 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 pres- 

 sure. 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 thicker ridges are purple, the thin edges appear greenish. 



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 when subjected in the air-pump to 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; the 

 stream of hydrogen acts like an oxygen-vacuum to the haemo- 

 globin and thus dissociation is effected. Carbonic acid gas is 

 unsuitable for this purpose, since, as we shall see, being an 

 acid it acts in another way on the haemoglobin. The oxygen may 

 also be removed from the haemoglobin not only by physical but 

 also by chemical means, as 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 greenish 

 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 hcemoglobin, as we may now call it, offers a spectrum 

 (Fig. 75, 5) very different from that of the unreduced solution. 



362 



