THE 'BLOOD 115 



Stokes's reagent. 1 One gramme of haemoglobin will combine with 

 1'34 c.c. of oxygen. 



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

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

 (venous) tint of haemoglobin. On once more allowing oxygen to 

 come into contact with the haemoglobin, as by shaking the solution 

 with the air the bright arterial colour returns. 



These colour-changes may be more accurately studied with the 

 spectroscope, and the constant position of the absorption bands seen 

 constitutes an important test for blood pigment. 



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 with the red, passing through orange, 

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

 is seen in nature in the rainbow. 



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 F, in the green ; G and H, in the violet. These lines are 

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

 light 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 lines and one violet line ; and the other elements, w r hen 

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

 If now the flame of a lainp 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 

 absent, and in its place a dark line, or rather two dark lines very 

 close together, occupying the same position as the two bright lines 

 of the sodium spectrum. The sodium vapour thus absorbs the same 

 rays as those which it itself produces at a higher temperature. Thus 

 the D line, as we term it, in the solar spectrum is due to the presence 

 of sodium vapour in the solar atmosphere. The other dark lines are 

 similarly accounted for by other elements. 



1 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 

 reaction is alkaline. 



i 2 



