THE RESPIRATORY CHANGES IN THE BLOOD. 353 



water, has also the same bright arterial color. A tolerably dilute solution 

 placed before the spectroscope is found to absorb certain rays of light in a 

 peculiar and characteristic manner. A portion of the red end of the spec- 

 trum is absorbed, as is also a much larger portion of the blue end ; but what 

 is most striking is the presence of two strongly marked absorption bands, 

 lying between the solar lines D and E. (See Fig. 97.) Of these the one 

 toward the red side, sometimes spoken of as the band (a), is the thinnest, but the 

 most intense, and in extremely dilute solutions (Fig. 97, 1) is the only one vis- 

 ible ; its middle lies at some little distance to the blue side of D. Its position 

 may be more exactly defined by expressing it in wave-lengths. As is well 

 known, the rays of light which make up the spectrum differ in the length of 

 their ways, diminishing from the red end, where the waves are longest, to the 

 blue end, where they are shortest. Thus, Frauenhofer's line D corresponds to 

 rays having a wave-length of 589.4 millionths of a millimetre. Using the same 

 unit, the centre of this absorption band, a, of hemoglobin corresponds to the 

 wave-length 578 ; as may be seen in Fig. 97, where, however, the numbers 

 of the divisions of the scale indicate only 100,000ths of a millimetre. The 

 other, sometimes called b, much broader, lies a little to the red side of E, 

 its blueward edge, even in moderately dilute solutions (Fig. 97, 2), coming 

 close up to that line ; its centre corresponds to about wave-length 539. Each 

 band is thickest in the middle and gradually thins away at the edges. 

 These two absorption bands are extremely characteristic of a solution of 

 haemoglobin. Even in very dilute solutions both bands are visible (they 

 may be seen in a thickness of 1 cm. in a solution containing 1 grm. of 

 haemoglobin in 10 litres of water), and that when scarcely any of the ex- 

 treme red end and very little of the blue end is cut off. They then appear 

 not only faint but narrow. As the strength of the solution is increased the 

 bands broaden and become more intense ; at the same time both the red end, 

 and still more the blue end, of the whole spectrum are encroached upon (Fig. 

 97, 3). This may go on until the two absorption bands become fused together 

 into one broad band (Fig. 97, 4). The only rays of light which 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 ad- 

 vancing 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 color of the solution as seen by transmitted light. 

 Exactly the same appearances are seen when crystals of haemoglobin are 

 examined with a micro-spectroscope. 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 micro-spectroscope. In fact, the spec- 

 trum of haemoglobin is the spectrum of normal arterial blood. 



289. 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 color. 

 The quantity of oxygen given off is definite, 1 grm. of the crystals giving off 

 1.59 c.c. of oxygen measured at 760 mm. Hg. and C. In other words, 

 the crystals of haemoglobin, over and above the oxygen which enters inti- 

 mately 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 



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