274 Mr. Morgan’s Experiments to ajceriain the 
fo on to violet and purple, till the medium has at laft become fo 
denfe as no longer to be a conductor of electricity. I think 
there can be little doubt from the above experiments of the 
non - con diahing power of a perfeCt vacuum; and this faCt is 
flill more ftrongly confirmed by the phenomena which appear 
upon the admiffion of a very minute particle of air into the 
infide of the gage. In this, cafe the whole becomes imme- 
diately luminous upon the flighted 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 eleCtric 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, I have obferved, 
that they will not conduCt the electric 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 infide, fhall 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, 
not withftan ding it has not been near an eleCtrical machine for 
two or three days. This feems to prove, that there is a limit 
even in the rarefaction of air, which fets bounds to its con- 
ducting 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 elearic fluid ; that if they are brought within a 
certain diftance of each other, their conducting power begins, and 
continually increafes till their approach alfo arrives at its limit, 
when the particles again become fo near as to refill the paffage 
