CHAP, ii.] RESPIRATION. 451 



to 4 per cent, of water of crystallization. It will thus be seen 

 that haemoglobin contains, in addition to the other elements 

 usually present in proteid substances, a certain amount of iron ; 

 that is to say, the element iron is a distinct part of the haemo- 

 globin molecule: a fact which of itself renders haemoglobin 

 remarkable among the chemical substances present in the animal 

 body. 



276. The crystals, when seen in a sufficiently thick layer 

 under the microscope, have the same bright scarlet colour as 

 arterial blood has to the naked eye ; when seen in a mass they 

 naturally appear darker. An aqueous solution of haemoglobin, 

 obtained by dissolving purified crystals in distilled water, has 

 also the same bright arterial colour. 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 spectrum 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. 89.) Of these the one 

 towards the red side, sometimes spoken of as the band a, is the 

 thinnest, but the most intense, and in extremely dilute solutions 

 (Fig. 89, 1) is the only one visible ; 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 waves, diminishing from the red end, where 

 the waves are longest, to the blue end, where they are shortest. 

 Thus Fraunhofer's line D corresponds to rays having a wave- 

 length of 589-4 millionths of a millimeter. Using the same 

 unit, the centre of this absorption band a of haemoglobin corre- 

 sponds to the wave-line 578 ; as may be seen in Fig. 89, where 

 however the numbers of the divisions of the scale indicate only 

 100,000 of a millimeter. The other, sometimes called /?, much 

 broader, lies a little to the red side of E, its blueward edge, 

 even in moderately dilute solutions (Fig. 89, 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 thick- 

 ness of 1 c.m. in a solution containing 1 grm. of haemoglobin in 

 10 litres of water), and that when scarcely any of the extreme 

 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 solu- 

 tion 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. 89, 3). This 

 may go on until the two absorption bands become fused together 



