CI
CHAPTER XIV.
Electricity.
418–447.
§251. 1) The nature of electricity. 418. If we rub a
glass-rod with a piece of flannel, we see the following phe
nomena:
a) Light substances are first attracted and then repelled
again.
b) A peculiar smell is experienced.
c) If the finger is brought near, a crackling sound will be
heard and a spark leaps over. The force by which these phe
nomena are caused, is called electricity (known to the Greeks
by rubbing amber).
§ 252. Conductors (metals) and non-conductors (glass,
sealing-wax). Good conductors, if touched by an electric body
have the electricity spread over their entire surface; if touched
again, they lose in the same manner all their electricity. Non-
conductors receive electricity only at the spot they are touched
and lose it only there, if touched again. The human body is a
good conductor, dry wood and other bodies are semi-conductors.
To separate a good conductor from other good conductors,
means to insulate it. The difference between good and non-
conductors was discovered by Gray in 1729.
§ 253. Sources of electricity are: a) mechanical ones,
especially friction. 418. b) chemical sources by composition
or decomposition of bodies. Thus different metals by being
connected with each other or placed in acids produce the electric
current. 419. Distinction of two kinds of electricity: vitreous
electricity (positive), produced by friction of glass; and resinous
