the Light of Bodies m a State of Combufion . 201 
did in a barometer at the fame inftant, owing to the vapour 
which was formed by -the water. But through this rarefied wa- 
ter the eledlrical fpark pafled as luminoufly a& it does through 
air equally rarefied. 
exp. v. If, inflead of water, a few drops of fpirits of wine are 
placed 011 the fur face of the mercury, phenomena fimilar to 
thofe of the preceding experiment will be difcovered, with 
this difference only, that as the vapour in this cafe is more 
denfe, the eledlrical fpark in its palfage through it is not quite 
fo luminous as it is in the vapour of water. 
exp. vi. Good aether fubftituted in the room of the fpirits 
of wine will prefs the mercury down fo' low as the height of 
16 or 17 inches. The eledlrical fluid in paffing through this 
vapour (unlefs the force be very great indeed) is fcarcely lumi- 
nous. But if the preffure on the furface of the mercury m the 
bafon be gradually leflened by the aid of an air-pump, the va- 
pour will become more and more rare, and the eledtric fpark in 
paffing through it more and more luminous. 
exp. vii. I could not difcover that any vapour efcaped from 
the mineral acids when expofed in vacuo. To give them* 
therefore, greater rarity or tenuity, I found different methods 
neceflary. With a fine camel-hair pencil, dipped in the vi- 
triolic, the nitrous, or the marine acid, I drew upon a piece of 
glafs a line about one-eighth of an inch broad. In fome al- 
liances I extended this line to the length of 27 inches, and 
found that the contents of an eledtric battery, confiding of 10 
pint phials coated, would pafs over the whole length of this 
line with the greatefl brilliancy. If by widening the line, or 
by laying on a drop of the acid, its quantity was increafed 
in any particular part, the charge, in paffing through that part* 
never appeared luminous, Water, fpirits of wine, circum- 
Vol, LXXV. D d ftanced 
