ELECTRICITY.
387
by a change of temperature which determines the existence of electric ‘ matter ’ in aform of palpable activity: and when these charged masses approach each other, eitherby electric attraction, or by the motion of air currents or other causes—then therestitution of the integral and original state of the electricity question takes place;the action of which, when of a destructive character, appears and terminates violently,along and at the end of the chain of intermediate polarized atoms of air, &c., in theforms of sheet lightning, forked lightning, or as the fire-ball or ‘ thunderbolt.’ Whenof a harmless description, it will be as the ‘glow discharge’* (to a point).—as the‘ brush discharge ’ (from a point), or as the ‘ summer lightning,’ which confines itsactivity within the precincts of the cloud;—these two groups comprising all theknown varieties of lightning.
Although the greater conducting power of metals is thus considered as only a moreintense and rapid form of induction,—and relative as the expressions ‘ conductor’ and‘ non-conductor ’ are,—yet the difference of those powers in certain bodies isenormous; that of iron, for instance, being estimated at 400,000,000 times greaterthan that of water.
The following Table gives in an approximate way the order of precedence in con-ductive power.
TABLE I.f
Conductors.
Non-Conductors or Insulators.
'All known metals.
flee at 0° of Fahrenheit.
£
Well-burned charcoal.
o
Dried vegetable substances.
O)
Plumbago.
Dried animal substances, generally.
Burning gaseous matter, as flame.
<L)
P. 1
Parchment, leather, feathers.
o
Smoke.
Baked wood.
_Concentrated acids.
Oils and fattv substances.
["Dilute acids.
L Silk.
Saline fluids.
f
Living animals.
Dry gases, including air.
OJ J
P.1
Living vegetables.
Pure steam of high elasticity.
to
Wood, in its ordinary state.
,<U
Glass and all vitrefactions.
<u
Snow , and ice from 32° to 0°.
Diamond and transparent gems.
Water.
•+J
Talc.
'Aqueous vapour.
Common earth and stone.
o
Amber.
All resins and resinous bodies.
*£
Dry chalk and lime.
Brimstone.
O) 1
p.
[_Shell-lac.
s
Paper.
Alkaline matter.
The ratios of heat evolved, and of those of conducting power, are shewn as follows.
TABLE II.J
Heat evolved. Conducting power.
Silver . . . .
. . 6
120
Copper . . . .
. . 6
120
Gold.
80
Zinc.
40
Platinum . . .
. . 30
24
Iron.
. . 30
24
Tin.
20
Lead . . . .
. . 72
12