THE RESPIRATORY CHANGES IN THE BLOOD. 355 



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

 time dichroic, so that while the thicker parts 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 

 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 haemoglobin and thus disasso- 

 ciation 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 sul- 

 phate kept from precipitation by the presence of tartaric acid, be added to 

 a solution of haemoglobin, or even to an unpurified solution of blood cor- 

 puscles 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 recognized at once by the characteristic 

 change of color ; from a bright scarlet the solution becomes of a purplish- 

 claret color, when seen in any thickness, but greenish when sufficiently 

 thin ; the color 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 re- 

 duced hcemoglobin, as we may now call it, offers a spectrum (Fig. 97, 5) very 

 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 between the two absorption bands of the unreduced solution, 

 though the redward edge of the band shades away rather further toward the 

 red than does the other edge toward 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. Hence the difference in color between haemoglobin which retains 

 the loosely combined oxygen, 1 and haemoglobin which has lost its oxygen 

 and become reduced. In tolerably concentrated solutions, or tolerably thick 

 layers, the former lets through the red and orange-yellow rays, the latter the 

 red and the bluish-green rays. Accordingly, the one appears scarlet, the 

 other purple. In dilute solutions, or in a thin layer, the reduced haemo- 

 globin lets through so much of the green rays that they preponderate over 

 the red, and the resulting impression is one of green. In the unreduced 

 haemoglobin or oxy-haemoglobin, the potent yellow which is blocked out in 

 the reduced haemoglobin makes itself felt, so that a very thin layer of oxy- 

 haemoglobin, as in a single corpuscle seen under the microscope, appears 

 yellow rather than red. 



It must be remembered that when we speak of reduced haemoglobin (or 

 more briefly haemoglobin) with a purple color and a characteristic one- 

 banded spectrum, we mean haemoglobin which has lost all its loosely asso- 

 ciated oxygen. If a quantity of oxy-haemoglobin be exposed to an insuffi- 



1 For brevity's sake we may call the haemoglobin containing oxygen in loose combina- 

 tion, oxy-hxmogloMn. and the haemoglobin from which this loosely combined oxygen has 

 been removed, reduced haemoglobin or simply haemoglobin. 



