2r4 Mr. Morgan's Experiments toafcertain the 



' fo on to violet and purple, till the medium has at lall: become fo 

 Azw^t as no longer to be a conductor of ele6lricity. I think 

 there can be little doubt from the above experiments of the 

 non-condu6ling power of a perfe(5l vacuum ; and this fa6l is 

 flill more flrongly confirmed by the phaenomena which appear 

 upon the admiflion of a very minute particle of air into the 

 iniide of the gage. In this cafe the whole becomes imme- 

 diately luminous upon the flightefl application of electricity, 

 and a charge takes place, which continues to grow more and 

 more powerful in proportion as frefh air is admitted, till the 

 denfity of the conducting medium arrives at its maximum, 

 which it always does when the colour of the ele(ftric light is 

 indigo or violet. Under thefe circumftances the charge may 

 be fo far increafed as frequently to break the glafs. In fome 

 tubes, which have not been completely boiled, 1 have obferved, 

 that they will not condud the eledlric fluid when the mercury is 

 fallen very low in them, yet upon letting in air into the ciftern 

 (H), fo that the mercury fhall rife in the gage (B), the elec- 

 tric fluid, which was before latent in the iniide, (hall now be- 

 come vifible, and as the mercury continues to rife, and of con- 

 fequence the medium is rendered lefs rare, the light fhall grow 

 more and more vifible, and the gage fhall at lafl be charged, 

 notwithflanding it has not been near an eledlrical machine for 

 two or three days. This feems to prove, that there Is a limit 

 even in the rarefa£llon of air, which fets bounds to its con- 

 dudling power ; or, in other words, that the particles of air may 

 be fo far feparated from each other as no longer to be able to 

 tranfmit the ele£lric fluid ; that if they are brought within a 

 certain diflance of each other,their conducing power begins, and 

 continually increafes till their approach alfo arrives at its limit, 

 when the particles again become fo near as to refiil the paflage 

 4 of 



