SS8 Sir Humphry Davy on Electrical Phenomena in Vacno. 
found to depend on the temperature. When the tube wa» 
very hot, the electric light appeared in the vapour of a bright, 
green colour^ and of great density ; but it became less vivid as 
the temperature diminished, and when it was cooled down to 20'’ 
below the zero of Fahrenheit’s scale, it was so faint that it could 
only be seen when the darkness was considerable. The charge 
communicated to the tin or platinum foil F, increased also with 
the temperature, and became extremely feeble at O'’ of Fahren- 
heit. 
In boiling the mercury in the exhausted tube, Sir Humphry 
observed a very beautiful phenomenon. In the formation and 
condensation of the globules of the pure and dense mercurial 
vapour, whicli was thus generated, the electricity produced by 
the friction of the mercury against the glass, was discharged 
through the vapour, in sparks so extremely brilliant, that they 
were visible in day-light. 
By introducing the minutest quantity of rare air into the mer- 
curial vacuum, the colour of the electric light changed from 
green to sea-green^ and by increasing the quantity, to blue and 
2 mrplc ; and when the temperature was low, the vacuum be- 
came a much better conductor. 
Sir Humphry Davy next proceeded to form a vacuum by 
means of fused tin. By using freshly cut pieces of grain tin, 
and melting them in a tube made void after being filled with 
hydrogen, and by applying long continued heat and agitation, 
he obtained a column of fused tin entirely free of gas. In the 
vacuum thus formed, the same electrical phenomena were pro- 
duced as in the mercurial vacuum, at a temperature below 0°, 
The electric light was yellow, and of the palest phosphorescent 
kind, requiring almost absolute darkness to be perceived ; and 
it was not perceptibly increased by heat *. 
In some of the first of these experiments. Sir Humphry Da- 
vy connected the metal with the stop-cock by means of a wire ; 
but afterwards the rarified air or gas was the only chain of 
communication, and he was enabled by that circumstance to as- 
certain, that the feebleness of the light in the more perfect va^ 
• Sir Humphry Davy also found, that electrical and raagnetical attractions ami 
repulsions took place as they would have done in air. 
