with some minuteness, in order to discover the true 
relation which they bear to the present question. 
582. Dr Priestley, \vho, as we shall hereafter see, 
supposed that venal blood became red by imparting 
its phlogiston to the air, knew well that the blood in 
the lungs was separated from the air by a membra- 
nous substance, which, however, according to Dr 
Hales, does not, in thickness, exceed the -r^Vs- part of 
an inch. To ascertain the effect of this circumstance, 
he put some black blood into a bladder moistened 
with a little serum, and then tying the bladder very 
close, he hung it in a free exposure to the air. The 
next day, all the lower surface of the blood, which 
had been separated from the air by the intervention 
of the bladder, had acquired a coating of a florid red 
colour, as thick, it appeared, as it would have acqui- 
red, if it had been immediately exposed to the open 
air ; so that this membrane had been no impediment 
to the action of the air on the blood. This experi- 
ment was repeated, without previously moistening 
the bladder, and with the very same result *. 
583. But although, in these experiments, the blood 
was rendered red by the agency of the air, yet we are 
not entitled to conclude, that this redness was produced 
by the combination of its oxygen, unless we can shew 
not only that this substance comes into contact with the 
blood, but is likewise capable of changing it to a red 
colour. Dr Priestley himself, who believed the blood 
to become red by the loss of phlogiston, could draw 
* Obs. on Air, abridged, vol. iii. p. 
