( 39 ) 
in the due Fermentation of the Blood, and other Juices 
of the Body, he (hews how neceffary the Air is for the 
continuing of Life, both in Plants and Animals, which 
the former that want Lungs, do yet receive in VeHels 
Analogous thereto; having fliewn that a Vivifick 
Spirit or Aura, generated out of the Blood by the Brain^ 
is the Principle of Motion in Animals ; and that the di- 
lating of the Thorax in breathing depends on this Aura 
only, which by the direftion of the Soul is Cent into the 
Nerves of all the Members, fo caufing Refpiration 
(which he makes a voluntary Adion ) and accompanies 
the Air, received by the Veflels, to the Blood. In the 
Third tenure he Examines the feveral EfTefts of the 
Caves exhaling thcle Damps, attempting to explain their 
caufes, viz. Lighted Torches are extinguiiht by the 
thick and compafl: Mineral Exhalations which oppofe 
the Particles giving or compofing Fire, and comprefs 
them fo, that they are forced to give way, and are dif- 
fipated. That in Animals thefe Vapours rifing by little 
and little out of the Veins of the Earth, and united to- 
gether in thefe Subterraneous Caverns, forcibly enter the 
Organs of Refpiration, and either wholly conquer or 
drive out the Vivifick ^//r^, or at lead fo contrad: it, 
and lellen its Power, as to flop the Motion of the Blood, 
and indeed all other Motions, and Life it felf. That 
deadly Lakes kill only the Birds that fly over them, by 
reafon of the Mineral fteams that from the bottom pe- 
netrating the Water, arife in a thin weakned Exhalation, 
and Birds requiring k more refined and purer Air than 
other AhimajsV are fooneft afFefted by ir. Then he Que- 
ries upon what Account Avernus , formerly from its 
deadly Quality dedicated to ?luto^ is now fo altered, as 
to have been hurtlefs both to Fifli and Birds for a long 
time, and fubfcribcs to their Opinion who refer thecaule 
to the burning of the adjacent Gaurus, now called Barh- 
rus, and fuppofe that the Mortal Vapours formerly pe- 
F z netrating 
