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



bottle, or even iu a tall narrow vessel open at the top, it presently changes 

 in colour from a bright to a dark red, decidedly purple in small thicknesses. 

 This change is perceived even before the solution has begun to stink in the 

 least perceptible degree. The tint agrees with that of the purple cruorine 

 obtained immediately by reducing agents ; and if a little of the solution be 

 sucked up from the bottom into a quill-tube drawn to a capillary point, and 

 the tube be then placed behind a slit, so as to admit of analyzing the 

 transmitted light without exposing the fluid to the air, the spectrum will 

 be found to agree with that of purple cruorine. On shaking the solution 

 with air it immediately becomes bright red, and now presents the optical 

 characters of scarlet cruorine. It thus appears that scarlet cruorine is 

 capable of being reduced by certain substances, derived from the blood, 

 present in the solution, which must themselves be oxidized at its expense. 



10. When the alkaline tartaric solution of protoxide of tin is added in 

 moderate quantity to a solution of scarlet cruorine, the latter is presently 

 reduced. If the solution is now shaken with air, the cruorine is almost 

 instantly oxidized, as is shown by the colour of the solution and its spec- 

 trum by transmitted light. On standing for a little time, a couple of minutes 

 or so, the cruorine is again reduced, and the solution may be made to go 

 through these changes a great number of times, though not of course in- 

 definitely, as the tin must at last become completely oxidized. It thus 

 appears that purple cruorine absorbs free oxygen with much greater avidity 

 than the tin solution, notwithstanding that the oxidized cruorine is itself 

 reduced by the tin salt. I shall return to this experiment presently. 



1 1 . When a little acid, suppose acetic or tartaric acid, which does not 

 produce a precipitate, is added to a solution of blood, the colour is quickly 

 changed from red to brownish red, and in place of the original bands (fig. 1) 

 we have a different system, nearly that of fig. 3. This system is highly 

 characteristic ; but in order to bring it out a larger quantity of substance 

 is requisite than in the case of scarlet cruorine. The figure does not exactly 

 correspond to any one thickness, for the bands in the blue are best seen 

 while the band in the red is still rather narrow and ill-defined at its edges, 

 while the narrow inconspicuous band in the yellow hardly comes out till 

 the whole of the blue and violet, and a good part of the green, are absorbed. 

 The difference in the spectra figs. 1 and 3 does not alone prove that the 

 colouring matter is decomposed by the acid (though the fact that the 

 change is not instantaneous favours that supposition), for the one solution 

 is alkaline, though it may be only slightly so, while the other is acid, and 

 the difference of spectra might be due merely to this circumstance. As 

 the direct addition of either ammonia or carbonate of soda to the acid 

 liquid causes a precipitate, it is requisite in the first instance to separate the 

 colouring matter from the substance so precipitated. 



This may be easily effected on a small scale by adding to the watery 

 extract from blood-clots about an equal volume of ether, and then some 

 glacial acetic acid, and gently mixing, but not violently shaking for fear 



