Professor Leslie on Electrical Theories. H 
When glass-powder is strongly electrified, it continues to glow 
for some seconds ; sealing-wax and rosin, spars, and other fos- 
sils, if treated in the same manner, manifest a similar property, 
only the appearance is of shorter duration H If dry wood 
is made the conductor of a charge , it assumes a temporary lus- 
tre ; but, if water forms the communication, it gives a. sudden 
flash f. Thus, the duration of the brightness in these sub- 
stances depends, when other circumstances are equal, on the 
slowness or the rapidity of transmission. 1 he quantity of light 
that is emitted will be determined also by the same cause, and 
will therefore become extremely small when the effect is momen- 
tary ; and the impression made on the organ of sight may be 
so prodigiously diminished, as entirely to escape observation. 
Consequently, in the sudden communication of the electrical 
virtue through continuous metallic bodies, the momentary bril- 
liancy cannot be perceived. But air is one of the slowest con- 
ducting substances, and, if strongly electrified, it must make a 
profuse emission of light. Hence the brightness of the star , 
the pencil, the spark, and the discharge . 
In these cases, it is unnecessary to recur to the supposition of 
an electric fluid. No argument can be adduced to prove its ex- 
istence. It is founded only on prejudice, and on a fallacious 
inference drawn from the single and unassisted indication of the 
sense of sight. Some writers, indeed, are unwilling to admit the 
possibility of action at a distance , and, like the poor Indian who 
placed the world on the back of a tortoise, they have recourse 
* It is easy to discover whether the light proceeds from the body itself, or flows 
either wholly or partly from the surrounding medium ; for, in the former case, the 
surface will be distinctly defined, but, in the latter, the glare will extend a short 
distance all round, and gradually melt away. Thus, a ball of quartz, at a white 
heat, appears environed with a luminous bur, which vanishes vrhen the contigu- 
ous air is exhausted. The particles of glass, rosin, &c., when strongly electrified, 
are brilliant, and nicely defined, but the metallic points communicating with a large 
mass are crowned with light. The reason of this will afterwards be perceived, 
-f- A. long narrow stripe of gold leaf is illuminated by the discharge of a jar, 
so that even metals, if sufficiently thin and narrow, such as to retard electrical 
communication, will appear luminous. This also shows, that bodies conduct 
through their mass, and not over their surfaces only. 
VOL, XI. NO. 21 . JULY - 1824 - 
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