298 
stratum of water to prevent entirely this action of air 
on the blood *. M. Cigna found the same thing to 
take place, when a pellicle of oil was interposed fj 
and Dr Wells ascribes a similar effect to a solution 
of gum arabic. These substances, however, act lit- 
tle, if at all, in changing the air ; and no change of 
colour, therefore, takes place in the blood. That 
black blood should have the power of attracting the 
oxygen of the air, through several inches of serum, 
and yet lose this power when a thin stratum of water 
is interposed, seems somewhat surprising, if the in- 
tervening fluid be, in each case, considered to be 
equally passive ; but proceeding on the fact, that the 
serum exerts an action on the air, which the water 
is incapable of effecting, a new circumstance comes 
into view, and upon it the colouration of the blood 
may probably depend. 
594. If, then, it appear, that the interposition of 
substances between the blood and the air necessarily 
prevents that contact which is essential to the che- 
mical union of oxygen with that fluid ; if it also ap- 
pear, that the colour of the blood is never, in such 
cases, changed, unless such substances be interposed, 
as are themselves capable of acting on the air ; and 
if, lastly, it be proved, that when the blood exhibits 
this change of colour, the air also suffers change, 
and that its oxygen, instead of combining with the 
blood, is really contained in the carbonic acid that is 
formed, we must conclude, that, whatever be the 
* Obs. on Air, abridged, vol, iii. p. 370. 
fr Ibid. 
