6 INFLUENCE OF THE AIR. 
contact with our blood ;\ and this, from the 
form and structure of the lungs, through- 
out an extent of surface which might seem to 
you incredible, and which, at the least, far 
exceeds the whole surface of the exterior 
of the body. Well, through the circulation 
of the blood, which is effected by means of the 
palpitation of the heart, the whole quantity 
of the blood is estimated to pass through the 
lungs seventeen times in an hour ; and, while 
passing through the lungs, the blood is che- 
mically purified, or else vitiated, by the atmos- 
pheric air which those for the time contain. 
Now, if it is purified, one of the great conse- 
quences is, that it becomes more limpid, and 
therefore circulates, or flows, more freely; and 
immediately, therefore, (that is, at its first re- 
turn to the heart,) it lightens the heart ; and 
the heart, thus lightened, becomes itself more 
free to act; drives the blood more freely tc- 
ward the lungs, the head, and the feet, and 
from this cause arises all that vivacity, and 
all that sense of lightness, and that cheerful- 
ness of temper, which we call good health and 
spirits. But this purifying of the blood, is, 
among other things, the cooling it; the re- 
