289 
colour or shape in nature, which, in their turn, these 
philosophers have not made oxygen to exhibit and 
assume* And if it was absurd in the Phlogistians to 
ascribe effects to an agent which they could not 
prove any where to exist, it is, surely, not less absurd 
in their successors to attribute similar effects to oxy- 
gen, even where it exists not. 
581. It is, farther, an objection to this supposed 
operation of oxygen, that, in the lungs, the blood 
and air do not come into contact, and, therefore, al- 
though the combination of oxygen with that fluid 
might be conceived to happen when they are placed 
together out of the body, yet the intervention of or- 
ganised membranes may be supposed to prevent 
such an union in the living system. In the ordinary 
operations of chemistry, such an interposition of ani- 
mal substance would be considered sufficient to vi- 
tiate the result of any similar experiment in which it 
was employed ; but in the application of this science 
to the living body, neither membranes nor blood- 
vessels are conceived to oppose any obstacle to the 
exertion of chemical action, or, in the smallest degree, 
to affect its result. In support of this supposed ope- 
ration of oxygen on the blood, some experiments of 
Dr Priestley have been appealed to, as affording de- 
cisive evidence that this substance has the power of 
penetrating a compact membranous body, and may, 
consequently, penetrate the cells and blood-vessels of 
the lungs. The importance which has been attached 
to these experiments, in all the late hypotheses which 
have been proposed to explain the function of respi- 
ration, renders it necessary for us to examine them 
