On the Electrical PkcBiiomena exhibited in Vacuo. 181 



■was very rarely perceptible, and always disappeared when the 

 tube was inverted, and the mercury made to strike the top 

 with some force, led me to conclude that the minute space was 

 really filled with the vapour of mercury ; the attraction of the 

 particles of the fluid mercuiy for each other preventing their 

 actual contact with the glass, except when this contact was 

 forcibly made by mechanical means ; and I soon proved that 

 this was the case : for by causing the mercury, when its co- 

 lumn was short, to descend into the more perfect from the less 

 perfect vacuum, with more or less velocity, I could make the 

 space more or less, or cause its disappearance altogether, in 

 which last case the cohesion between the mercury and the glass 

 was always extremely strong. 



I found that in all cases when the mercurial vacuum was 

 perfect, it was permeable to electricity, and was rendered lu- 

 minous by either the common spark, or the shock from a Ley- 

 den jar, and the coated glass surrounding it became charged ; 

 but the degree of intensity of these phaenomena depended upon 

 the temperature : when the tube was very hot, the electric light 

 appeared in the vapour of a bright green colour, and of great 

 density ; as the temperature diminished, it lost its vividness ; 

 and when it was artificially cooled to 20° below zero of Fah- 

 renheit, it was so faint as to require considerable darkness to 

 be perceptible. 



The charge likewise communicated to the tm or platinum 

 foil was higher the higher the temperature ; which, like the 

 other phaenomenon, must depend upon the different density of 

 the vajiour of mercury ; and at 0° Fahrenheit it was very 

 feeble indeed. 



A very beautiful phaenomenon occurred in boiling the mer- 

 cury in the exhausted tube, which showed the great brilliancy 

 of the electrical light in pure dense vapour of mercury. In 

 the formation and condensation of the globules of mercurial 

 vapour, the electricity produced by the friction of the mercm-y 

 against the glass, was discharged through the va2;)our with 

 sparks so bright as to be visible in day light. 



In all cases when the iniiuitest quantity of rare air was in- 

 troduced into the mercurial vacuum, the colour of the light 

 produced by the passage of the electricity changed from green 

 to sea green ; and, by increasing the quantity, to blue and 

 purple ; and when the temperature was low, the vacuum be- 

 came a nuich better conductor. 



I tried to get rid of a portion of the mercurial vapour, by 

 using a diHicullty fusible iunalgam of mercury and tin, which 

 was made to crystulli/.c by cooling in the tube; but the results 

 were precisely the same as when pure mcrcuj-y was used. 



J (ried 



