Prof. Stokes on the Colouring Matter of the Blood. 393 



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 



Fig. 1. 



Fig. 2. 



Fig. 3. 



Fig. 4. 



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. 



6. Of course it is necessary to assure oneself that the single band 

 in the green is not due to absorption produced merely by the reagent, 

 as is readily done by direct observation of its spectrum, not to men- 

 tion that in the region of the previous dark bands, or at least the 

 outer portions of it, the solution is actually more transparent than 

 before, which could not be occasioned by an additional absorption. 

 Indeed the absorption due to the reagent itself in its different stages 

 of oxidation, unless it be employed in most unnecessary excess, may 

 almost be regarded as evanescent in comparison with the absorp- 

 tion due to the colouring matter ; though if the solution be repeatedly 

 put through its changes, the accumulation of the persalt of iron will 

 presently tell on the colour, making it sensibly yellower than at first 

 for small thicknesses of the solution. 



7. That the change which the iron salt produces in the spectrum 

 is due to a simple reduction of the colouring matter, and not to the 

 formation of some compound of the colouring matter with the re- 

 agent, is shown by the fact that a variety of reducing agents of very 

 different nature produce just the same effect. If protochloride of tin 

 be substituted for protosulphate of iron in the experiment above de- 

 scribed, the same changes take place as with the iron salt. The tin 

 solution has the advantage of being colourless, and leaving the visi- 

 ble spectrum quite unaffected, both before and after oxidation, and 

 accordingly of not interfering in the slightest degree with the optical 

 examination of the solutions, but permitting them to be seen with 



Phil. Mag. S. 4. Vol. 28. No. 190. Nov. 1864. 2 D 



