Mr. Morgan's Experiments, ScCs 273 



-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 iniide of tlie 

 ciftern through a valve (C) in the brafs cap (E) jufl: mentioned, 

 ■which producing a perfect vacuum in the gage (B) afforded an 

 inftrument peculiarly well adapted for experiments of this 

 kind. Things being thus adjufled (a fmall wire (F) having 

 been prevloully 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 conductor of an electrical machine, and notwith- 

 ftanding every effort, neither the fmalleft ray of light, nor the 

 ilighteft 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 eleCtricity, the latter at leaft muft have taken 

 place , for it is well known (and I have myfelf often made the 

 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 imperfectly 

 boiled, the experiment will not fucceed ; but the colour of thc- 

 eleCtric 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 rarefaction 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 eleCtrlc 

 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 eleCtrIc light from green to blue, from blue to indigo, and 

 Vol. LXXV. N n io 



