724 ON THE INFLUENCE OF PHYSICAL AND CHEMICAL AGENTS UPON BLOOD. 
In concluding this paper, it was my intention to make some remarks on the reciprocal 
action of hsematin and atmospheric air ; for, as stated in a communication on the con- 
dition of ox gen absorbed into the blood during respiration*, which I had the honour 
of making to the Royal Society some years ago, the colouring- matter of the blood 
appears to possess a more powerful effect in altering the composition of atmospheric air 
than any other individual constituent of that liquid. The recent researches of Professor 
Stokes, however, cause me to pause before again publishing my views on animal colour- 
ing-matters. For the interesting results obtained by that gentleman with the prism, 
although in accordance with my facts, may nevertheless induce me to modify my theory ; 
not regarding the action, but regarding the nature of these substances. I have hitherto 
held the view that all the animal pigments spring from one colourless radical, and that 
the difference in tint between hsematin, urohsematin, and biliverdin was simply due to 
the different stages of oxidation of the radical. It would appear, however, from the 
researches of Professor Stokes, that all these substances, although closely allied, are 
nevertheless chemically distinct. I consequently prefer reinvestigating the subject 
before communicating to the Society the data which are at present before me. 
* Proceedings of the Boyal Society, vol. viii. p. 82. 
