Obfervations on inflammable Air . 
expirated, which are neceffary to give the lungs all the 
neceffary expanfion ; whereas, in the other cafe, the in- 
flammable air ferves to fill up fpace, and, together with 
the common air, contributes to fill the lungs ; fo that the 
inflammable air, confidered under thefe circumftances, 
and under this point of view, may be faid to be ufeful 
for animal refpiration. This explanation feems moft 
evidently demonftrated by the following experiment. If 
thirty-five cubic inches of common air are introduced 
into the bladder, and this air be breathed when the lungs 
are in a natural ftate, it will be found, that one may 
breathe it for twenty times or longer; whereas, when the 
bladder was empty, it could not be breathed more than 
nine times at moft. 
Before I finilh this paper it will be proper to mention 
another caufe, which, perhaps, alfo contributes to render 
the inflammable air of the bladder lefs noxious : this is 
the levity of the inflammable air itfelf with refpedt to 
common air, which hinders the inflammable mixing 
With the common air. The inflammable air fwims con- 
tinually upon the common air, juft as aether fwims 
upon water; and the inflammable air fwims ftill better 
than aether, becaufe it is much lighter in comparifon than 
aether. Various experiments made upon volatile fub- 
ftances have convinced me of this truth. If equal quan- 
Vol. LXIX. B b b tities 
