Mr. Morgan’s Experiments , &c. 
mercury through a perforation (D) in the brafs cap (E) which 
covered the mouth of the ciftern (H), the whole was cemented 
together, and the air was exhaufted from the infide of the 
ciftern through a valve (C) in the brafs cap (E) juft mentioned, 
which producing a perfect vacuum in the gage (B) afforded an 
anftrument peculiarly well adapted for experiments of this 
kind. Things being thus adjufted (a fmall wire (F) having 
been previoufly fixed on the infide of the ciflern to form a com- 
munication between the brafs cap (E) and the mercury (G) 
into which the gage was inverted) the coated end (A) was ap- 
plied to the condudlor of an eledtricai machine, and notwith- 
ftanding every effort, neither the fmaileft ray of light, nor the 
flighteft charge, could ever be procured in this exhaufted gage. 
I need not obferve, that if the vacuum on its infide had been a 
conductor of eledlricity, the latter at leaft rauft have taken 
place , for it is well known (and I have myfelf often made dip 
experiment) that if a glafs tube be exhaufted by an air-pump, 
and coated on the outfide, both light and a charge may very 
readily be procured. If the mercury in the gage be imperfedtly 
boiled, the experiment will not fucceed ; but the colour of the 
eledtric light, which, in air rarefied by an exhaufter, is always 
violet or purple, appears in this cafe of a beautiful green, and, 
what is very curious, the degree of the air’s rarefadtion may be 
nearly determined by this means ; for I have known inftances, 
during the courfe of thefe experiments, where a fmall particle 
of air having found its way into the tube (B), the eledtric 
light became vifible, and as ufual of a green colour ; but the 
charge being often repeated, the gage has at length cracked at 
its fealed end, and in confequence the external air, by being 
admitted into the infide, has gradually produced a change in 
the eledtric light from green to blue, from blue to indigo, and 
Vol. LXXV. N n fo 
