286 
the relative proportions of the expired air should 
thus vary ; but the fact proves only the retention of 
oxygen in the lungs, but not its absorption by the 
blood. Should it even be maintained that oxygen 
was absorbed, because, in these two experiments, a 
portion of it disappeared, then, by the same mode of 
reasoning, we must also contend, that, in the thirteen 
preceding experiments, no absorption of oxygen 
took place, because no part of it was retained ; and 
as these last experiments alone come near to the 
natural exercise of this function, they authorise us to 
conclude, that such supposed absorption of oxygen 
constitutes no necessary part of healthy respiration. 
In truth, in some instances where a mixture of oxy- 
gen and hydrogen gases was respired, the oxygen 
and carbonic acid in the expired air uniformly ex- 
ceeded, by one per cent, the total oxygen inspired *; 
from which it may be inferred, that these variations 
in the proportions of the expired air proceed entirely 
from accidental causes, and are totally independent 
of any absorbent function in the lungs. 
578. But, beside the argument derived from the 
supposed loss of oxygen in respiration, analogies and 
experiments of a different nature have been brought 
forward in support of the doctrine that oxygen com- 
bines with the blood. It is known that the blood 
becomes red, when it is placed in contact with oxy- 
* Phil. Trans. 1809, p. 425. 
