73 
electrical phenomena exhibited in vacuo. 
constantly by the mercury. The results with tin must be 
regarded as more equivocal ; because as this substance cannot 
be boiled in vacuo, it may be always suspected to have 
emitted a small quantity of the rare air or gas to which it 
has been exposed; yet, supposing this circumstance, such gas 
must be at least as highly expanded as the vapour from 
cooled mercury, and can hardly be supposed capable of afford- 
ing the dense light, which the passage of the electricity of 
the charged Leyden phial through the vacuum produces. 
When the intense heat produced by electricity is consi- 
dered, and the strong attractive powers of differently elec- 
trified surfaces, and the rapidity of the changes of state, it 
does not seem at all improbable, that the superficial particles 
of bodies, which, when detached by the repulsive power of 
heat, form vapour, may be likewise detached by electrical 
powers, and that they may produce luminous appearances in 
a vacuum, free from all other matter by the annihilation of 
their opposite electrical states. 
In common cases of electrical action, the quantity of the 
heat generated by the annihilation of the different electrical 
states depends, as I stated in my last communication to the 
Society, upon the nature of the matter on which it acts ; and 
in cases when electrical sparks are taken in fluids, vapour or 
gas is always generated; and in elastic fluids, the intensity of 
the light is always greater, the denser the medium. The 
luminous appearances therefore, it is evident from all the 
statements, must be considered as secondary ; whilst the uni- 
form exertions of electrical attractions and repulsions, under 
all circumstances, in rare and dense media and in vacuo, and 
with respect to solids, fluids, and gases, point them out (whe- 
MDCCCXXl I. L 
