[ 269 J 
<c limits of the atmofphere; and contracting them - 
ct felves into a narrower compafs, by that principle 
“ of nature that congregates the homogene a, may lie 
“ like a train of gunpowder in the aether, till catch- 
“ ing fire by fome internal ferment, the flame may 
tc be communicated to its continued parts, and fo 
<c run on like a train fired,” &c *. But this hypo- 
thecs of that excellent naturalift feems liable to the 
following difficulties, i. What are the experiments 
which fhew, that inflammable vapours have any fort 
of a vis centrifuga - y or will rife either in an abfolute 
vacuum, or in a medium where the meteors are often 
found, and where, according to the doCtor, the air 
muff be five hundred times rarer than in a common 
exhaufted receiver fl- ? 2. Altho’ fulphureous vapours 
may afcend from the earth, upon being agitated by 
heat, will not that volatility foon ceafe, by the cold- 
nefs of the air, long before they can arrive at the up- 
per regions ? 3. Does not all matter, capable of 
taking flame, fuppofe a proportion of an acid fait, 
adherent to the phlogiftic principle, and confequently 
fome gravitation ? 4. And will not the weight be 
increafed by the large quantity of condenfed air, or 
fome other elaftic fluid combined with the inflam- 
mable vapours, which is the caufe of the explofion 
and report? 5. Are we not led to this notion about 
the innate levity and vis centrijuga of igneous matter, 
from finding, that heat has a greater tendency upwards 
* Phil. Tranf. N° 360. p. 978. Abr. Vol. IV. Par. II. Ch. 1. 
§28. 
f His expreflion is, above three hundred thoufand times rarer than 
what we breathe, 
than 
