92 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



pure solutions of haemoglobin. The respiratory oxygen can be 

 removed, for example, in the Torricellian vacuum of a mercurial air- 

 pump, or by passing a neutral gas like hydrogen through the blood, 

 or by the use of reducing agents like ammonium sulphide or Stokes's 

 reagent.^ One gramme of haemoglobin v^^ill combine vpith 1'34 c.c. 

 of oxygen. 



If any of these methods for reducing oxyhaemoglobin is used, the 

 bright red (arterial) colour of oxyhaemoglobin changes to the purplish 

 (venous) tint of haemoglobin. On once more allovs^ing oxygen to 

 come ■ into contact w^ith the haemoglobin, as by shaking the solution 

 withjthe air, the bright arterial colour returns. 



These colour-changes may be more accurately studied v^ith the 

 spectroscope, and the constant position of the absorption bands seen 

 constitutes an important test for blood pigment. It will be first 

 necessary to describe briefly the instrument used. 



The Spectroscope. — When a ray of white light is passed through 

 a prism, it is refracted or bent at each surface of the prism ; the 

 whole ray is, however, not equally bent, but it is split into its con- 

 stituent colours, which may be allowed to fall on a screen. The 

 band of colours beginning vdth the red, passing through orange, 

 yellow, green, blue, and ending with violet, is called a spectrum : this 

 is seen in nature in the rainbow. It may be obtained artificially by 

 the glass prism or prisms of a spectroscope. 



The spectrum of sunlight is interrupted by numerous dark lines 

 crossing it vertically, called Fraunhofer's lines. These are perfectly 

 constant in position, and serve as landmarks in the spectrum. 

 The more prominent are A, B, and C, in the red ; D, in the yellow ; 

 E, b, and P, in the green ; G and H, in the violet. These lines are 

 due to certaini volatile substances in the solar atmosphere. If the 

 lights from burning sodium or its compounds is examined spectro- 

 scopically, it will be found to give a bright yellow line, or rather 

 two bright yellow lines very close together. Potassium gives two 

 bright red hues and one' violet line ; and the other elements, when 

 incandescent, give characteristic lines, but none so simple as sodium. 

 If now the flame of a lamp be examined, it will be found to give a 

 continuous spectrum like that of sunlight in the arrangement of its 

 colours, but unlike it in the absence of dark lines ; but if the light 

 from the lamp be made to pass through sodium vapour before it 

 reaches the spectroscope, the bright yellow light will be found 



' Stokes's reagent must always be freshly prepared : it is a solution of ferrous 

 sulphate to which a little tartaric acid has been added, and then ammonia till the j 

 reaction is alkaline. 



