^d m a barometer at th^ f^ma inftant, ©wing t-o the vapour 

 which was formed by the water* But thraugh this rai^eiSed wa« 

 ter the electrical fpark pafled m lurninouilj aa k daes through 

 air equally rareiied.. 



EXP. V. If, inflead of water, a few drops of fpirits of wrae aru 

 placed on the furface of the mercury, phaenomena 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 electrical fpark in its paffage through it is not c^uita 

 fo luminous as it is- in the vapour of water, 



EXP. VI, Good JEther fubftituted in the room of the fpirits 

 of wine will prefs the mercury down fo low as the height of 

 1 6 or ly inches. The eledlricai fluid in paffing through this 

 vapour (unlefs the force be very great indeed) is fcarcely lumi- 

 nous. But if the prefTure on the furface of the mercury in the 

 bafon be gradually leflened by the. aid of an air-pump, the va- 

 pour will become more and more rare, and the eleOric fpark in 

 paffing through it more and more luminous, 



EXP. vji. I could not dlfcover that any vapour eicapedfrom 

 the mineral acids when expofed in vacuo. To give them,, 

 therefore, greater rarity or tenuity, I found different methods 

 neceflary. With a fme 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 in- 

 ftances I extended this line to the length of 27 inches, and 

 found that the contents of an eledric battery, confiding of la 

 pint phials coated, would pafs over the whole length of this 

 line with the greateft 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 palling through that part, 

 never appeared luminous. Water, fpirits of wine, circum- 

 Vol. LXXY. D d ft^nced 



