CHAP, ii.] RESPIRATION. 453 



into one broad band (Fig. 89, 4). The only rays of light which 

 then pass through the haemoglobin solution are those in the 

 green between the blueward edge of the united bands and the 

 general absorption which is now rapidly advancing from the blue 

 end, and those in the red between the united bands and the 

 general absorption at the red end. If the solution be still 

 further increased in strength, the interval on the blue side of 

 the united bands becomes absorbed also, so that the only rays 

 which pass through are the red rays lying to the red side of D ; 

 these are the last to disappear, and hence the natural red colour 

 of the solution as seen by transmitted light. Exactly the same 

 appearances are seen when crystals of haemoglobin are examined 

 with a microspectroscope. They are also seen when arterial 

 blood itself (diluted with saline solutions so that the 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 microspectroscope. In fact, the spectrum of haemoglobin is 

 the spectrum of normal arterial blood. 



277. 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. In other words, the crys- 

 tals 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 combina- 

 tion only, and which may be dissociated from them by subject- 

 ing 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 

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

 quantity of oxygen given off is said to be definite ; thus 1 grm. 

 of the crystals of dog's blood gives off 1-59 c.cm. of oxygen 

 measured at 760 mm. Hg and C. ; but there are some reasons 

 for thinking that even in the same blood the quantity may vary. 



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 



