22 On Respiration and Animal Heat. 
the whole change effected consistsin removing 
the cornice, and substituting another in its 
place. We must now enquire in which state 
the edifice presents the most symmetrical ap- 
pearance. 
According to La Grange and Hassenfratz, 
oxygen enters the blood in the lungs. How 
does it enter? By mechanical or chemical 
means? Not by mechanical; for then azote 
would enter four times more copiously, owing 
to its greater density. It must enter by che- 
mical means.—How does the blood attract 
oxygen through the membrane of sensible 
thickness which separates them? Granting 
the fact, how does the elastic fluid enter into 
combination with a liquid, without depositing 
its heat in the lungs, a circumstance so much 
to be guarded against on this hypothesis ?—If 
the heat be given out to the blood in the 
lungs, there will be none left to be extricated 
during the circulation in order to form carbo- 
nic acid. Passing by all those difficulties, how 
is the carbonic acid to escape through the 
membranes of the lungs ito the air cells? 
Not by chemical means, for there is no agent 
to attract it; mechanical means nrust be used ; 
simple pores will not effect the business, be- 
cause air might enter as well as escape; there 
must then be air pores with valves opening 
