CH. XXVI.] 



ABSORPTION SPECTRA 



435 



a dilute one. The one band (y band) of haemoglobin (spectrum 3) is 

 not so well denned as the a or ft bands. On dilution it fades rapidly ; 

 so that in a solution of such strength that both bands of oxyhaemoglobin 

 would be quite distinct, the single band of haemoglobin has disappeared 

 from view. The oxyhaemoglobin bands can be distinguished in a 

 solution which contains only one part of the pigment to 10,000 of 

 water, and even in more dilute solutions which seem to be colourless 

 the a band is still visible. , 



Haemoglobin and its compounds also show absorption bands in 

 the ultra-violet portion of the spectrum. This portion of the spectrum 

 is not visible to the eye, but can be rendered visible by allowing the 

 spectrum to fall on a fluorescent screen, or on a sensitive photographic 



ABC D E6 F Q ABC D Eb F 



I II 



PlO. 338. Graphic lepreseutalioas of the amount of absorption of light by solution of (I) oxyhaemo- 

 globin, (II) of haemoglobin, of different strengths. The shading indicates the amount of absorption 

 of the spectrum ; the figures on the right border express percentages. (Eollett.) 



plate. In order to show absorption bands in this part of the spectrum 

 very dilute solutions of the pigment must be used. 



Oxyhaemoglobin shows a band (Soret's band) between the lines G 

 and H. In haemoglobin, carbonic oxide haemoglobin, and nitric oxide 

 haemoglobin, this band is rather nearer G. Methaemoglobin and 

 haematoporphyrin show similar bands. 



We owe most of our knowledge of the " photographic spectrum " 

 to Prof. Gamgee, through whose kindness I am enabled to present 

 reproductions of two of his numerous photographs (figs. 369 and 370). 



Methaemoglobin. This may be produced artificially in various 

 ways, as by adding potassium ferricyanide or amyl nitrite to blood, 

 and as it also may occur in certain diseased conditions in the urine, 

 it is of considerable practical importance. It can be crystallised, and 

 is found to contain the same amount of oxygen as oxyhaemoglobin, 

 only combined in a different way. The oxygen is not removable by 

 the air-pump, nor by a stream of neutral gas like hydrogen. It can 



