new inflammable Air or Gafls , See. 405 
with the alkaline fait, an hepar flulpburis , which riling in 
tough bubbles confines this explofive air generated. 
At length, however, the increafing heat, whieh fets fire 
to the fulphur, fets this explofive air on fire alfo; which 
then following its own nature explodes with fo much 
the more force from its having been entangled and con- 
fined within the bubbles of the hepar flulpburis. 
After what has already been faid, it will not be diffi- 
cult to explain, why a fingle fpark of fire propagates the 
combuftion with great rapidity through the whole mafs 
of gunpowder, however great. If we put a fingle 
grain of gunpowder upon a red-hot iron, we fee the par- 
ticles of red-hot charcoal projected with great rapidity in 
every direction by the forcible explofion of the two airs 
extricated in the manner before explained. Thus, if 
one or more grains, among a heap of others, are fet fire 
to, the particles of red-hot charcoal being driven with great 
violence againft the furrounding grains communicate 
their heat to all the particles of charcoal they hit, which 
particles, by heating the particles of nitre in clofe contact 
with them, extricate their dephlogifticated air at the fame 
time that the charcoal yields its inflammable air; in con- 
fequence of which a more powerful explofion happens. 
This fecondary explofion projects with a much greater 
force the particles of charcoal furrounded by the explofive 
Vol. LXIX. Hhh flame 
