356 Prof. Stokes on the Reduction and Oxidation [June 16, 



characteristic dark bands seen before are now replaced by a single band, 

 somewhat broader and less sharply defined at its edges than either of the 

 former, and occupying nearly the position of the bright band separating 

 the dark bands of the original solution. The fluid is more transparent for 

 the blue, and less so for the green than it was before. If the thickness be 

 increased till the whole of the spectrum more refrangible than the red be 

 on the point of disappearing, the last part to remain is green, a little be- 

 yond the fixed line 6, in the case of the original solution, and blue, some 

 way beyond F, in the case of the modified fluid. Figs. 1 and 2 in the accom- 

 panying woodcut represent the bands seen in these two solutions respec- 

 tively. 



4. If the purple solution be exposed to the air in a shallow vessel, it 

 quickly returns to its original condition, showing the two characteristic 

 bands the same as before ; and this change takes place immediately, pro- 

 vided a small quantity only of the reducing agent were employed, when the 

 solution is shaken up with air. If an additional quantity of the reagent be 

 now added, the same effect is produced as at first, and the solution may 

 thus be made to go through its changes any number of times. 



5. The change produced by the action of the air (that is, of course, by 

 the absorption of oxygen) may be seen in an instructive form on partly 

 filhng a test-tube with a solution of blood suitably diluted, mixing with a 

 little of the reducing agent, and leaving the tube at rest for some time in 

 a vertical position. The upper or oxidized portion of the solution is readily 

 distinguished by its colour ; and if the tube be now placed behind a slit 

 and viewed through a prism, a dark band is seen, having the general form 

 of a tuning-fork, like figs. 1 and 2, regarded now as a single figure, the 

 line of separation being supposed removed. 



