Prof. Stokes on the Colouring Matter of the Blood. 395 



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 requi- 

 site 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 of forming an emulsion. When enough acetic 

 acid has been added, the acid ether rises charged with nearly the 

 whole of the colouring matter, while the substance which caused the 

 precipitate remains in the acid watery layer below*. The acid ether 

 solution shows in perfection the characteristic spectrum fig. 3. When 

 most of the acid is washed out the substance falls, remaining in the 

 ether near the common surface. If after removing the wash-water 

 a solution, even a weak one, of ammonia or carbonate of soda be 

 added, the colouring matter readily dissolves in the alkali. The 

 spectrum of the transmitted light is quite different from that of scarlet 

 cruorine, and by no means so remarkable. It presents a single band 

 of absorption, very obscurely divided into two, the centre of which 

 nearly coincides with the fixed line D, so that the band is decidedly 

 less refrangible than the pair of bands of scarlet cruorine. The rela- 

 tive proportion of the two parts of the band is liable to vary. The 

 presence of alcohol, perhaps even of dissolved ether, seems to favour 

 the first part, and an excess of caustic alkali the second, the fluid 

 at the same time becoming more decidedly dichroitic. The blue 

 end of the spectrum is at tbe same time absorbed. The band of 

 absorption is by no means so definite at its edges as those of scarlet 

 cruorine, and a far larger quantity of the substance is required to 

 develope it. 



* If I may judge from the results obtained with the precipitate given by acetic 

 acid and a neutral salt, a promising mode of separation of the proximate consti- 

 tuents of blood-crystals would be to dissolve the crystals.in glacial acetic acid and 

 add ether, which precipitates a white albuminous substance, leaving the haematine 

 in solution. 



2D 2 



