Mr. Morgan's Experiments, &c. 273 



'tnercury through a perforation (D) In the brafs cnp (E) which 

 ■covered the mouth of the cill:ern (H), the whole was cemented 

 together, and the air was exhaufted from the infide of the 

 cillern through 2. valve (C) in the brafs cap (E) jufl: mentioned, 

 w^hich producing a perfect vacuum in the gage (B) afforded aa 

 -Inlh-ument pecuharly well adapted for experiments of this 

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

 been previoully fixed on the intide of the ciftern to form a com- 

 munication between the brals cap (E) and the mercury (G) 

 into which the gage was inverted) tlie coated end (A) was ap- 

 plied to the conductor of an electrical machine, and notvvith- 

 ftan ding every effort, neither the Inialieft 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 electricity, the latter at leall: muff 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 the 

 electric light, wdiich, 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 eleCtric 

 light became vjfible, and as ufual of a green colour ; but the 

 cliarge being often repeated, the gage has at length crack^^^f ■•' 

 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 

 Vql.. LXXV. N n io 



