108 THE BLOOD. 



cule has not yet been determined, although the formulae 

 of some varieties are known approximately; but it is very 

 complex, containing a large number of atoms. Like the 

 other members of the protein compound class, it contains 

 an albuminous substance, united in this case with an iron 

 compound. It easily unites with oxygen from the air, tak- 

 ing up one molecule of oxygen for each molecule of haemo- 

 globin and forming the readily-decomposable compound, 

 oxyhaemoglobin. It also forms compounds with carbon 

 monoxid (CO), nitric oxid (NO), and sulphur, all of which 

 are similar to its oxygen compound. 



Haemoglobin can be obtained from oxyhaemoglobin 

 by the removal of the oxygen. This may be effected either 

 by a vacuum, by driving it out by means of a gas which 

 itself does not act on the blood, or by the use of some 

 chemical reducing agent. It is obtained in the crystalline 

 state with more difficulty than its oxygen compound. It 

 is soluble in water, giving a reddish-purple solution. 



The spectrum of haemoglobin is of great value in test- 

 ing for its presence, and the same might be said in the case 

 of the haemoglobin derivatives. When a dilute solution of 

 blood is held before the slit of a spectroscope, the tube being- 

 turned toward a window, the solar spectrum, consisting of 

 the seven primary colors crossed by fine dark lines, is seen, 

 and in addition one or more dark bands, which are due 

 to the coloring matters of the blood. That of haemoglobin 

 has one broad band with rather indistinct edges lying be- 

 tween the D and E lines of the solar spectrum. If the 

 liquid in the tube be shaken with air oxyhaemoglobin is 

 formed, which has two dark bands. For clinical purposes 

 the direct-vision or pocket spectroscope will probably be 

 found to be the most convenient form of instrument. 

 (Figs. 1 and 2 on Plate IV show the spectra.) 



