warm, dry flannel, we shall find that it acquires the power of attracting light bodies, such as small pieces of paper, straw, pith, etc. After remaining in contact with the rubbed (or electrified) substance for a short time, the paper, etc., will fly off as if repelled; and this apparent repulsion will be more evident and more quickly produced if the experiment be performed over a metal tray. If a small pith-ball, the size of a pea, be suspended from the ceiling by a piece of fine cotton, previously damped and then approached by an ebonite comb which has been briskly rubbed, it will be vigorously attracted, and never repelled; but if for the cotton there be substituted a thread or fibre of very fine dry silk, the pith-ball will be first attracted and then repelled. This is owing to the fact that the damp cotton allows the electricity to escape along it: id est, damp cotton is a CONDUCTOR of electricity, while silk does not permit its dissipation; or, in other words, silk is a NON-CONDUCTOR. All bodies with which we are acquainted are found, on trial, to fall under one or other of the two heads—viz., conductors and non-conductors. Nature knows no hard lines, so that we find that even the worst conductors will permit the escape of some electricity, while the very best conductors oppose a measurable resistance to its passage. Between the limits of good conductors, on the one hand, and non-conductors (or insulators) on the other, we have bodies possessing varying degrees of conductivity.

§ 4. As a knowledge of which bodies are, and which are not, conductors of electricity is absolutely essential

to every one aspiring to apply electricity to any practical purpose, the following table is subjoined, giving the names of the commoner bodies, beginning with those which most readily transmit electricity, or are good conductors, and ending with those which oppose the highest resistance to its passage, or are insulators, or non-conductors:—

§ 5. TABLE OF CONDUCTORS AND INSULATORS.

Quality.Name of Substance.Relative Resistance.
Good ConductorsSilver, annealed1.
Copper, annealed1.063
Silver, hard drawn1.086
Copper, hard drawn1.086
Gold, annealed1.369
Gold, hard drawn1.393
Aluminium, annealed1.935
Zinc, pressed3.741
Brass (variable)5.000
Platinum, annealed6.022
Iron6.450
Steel, soft6.500
Gold and silver alloy, 2 to 17.228
Nickel, annealed8.285
Tin, pressed8.784
Lead, pressed13.050
German silver (variable)13.920
Platinum-silver alloy, 1 to 216.210
Steel, hard25.000
Antimony, pressed23.600
Mercury62.730
Bismuth87.230
Graphite145.000
Nitric Acid976000.000
Imperfect ConductorsHydrochloric acid[1]
Sulphuriacid1032020.000
Solutions of metallic saltsvaries with strength
Metallic sulphides[1]
Distilled water[1] 6754208.000
Inferior Conductors.Metallic salts, solid[1]
Linen}and other forms of cellulose[1]
Cotton
Hemp
Paper
Alcohol[1]
Ether[1]
Dry Wood[1]
Dry Ice[1]
Metallic Oxides[1]
Non-conductors,
or Insulators.		Ice, at 25 c.[1]
Fats and oils[1]
Caoutchouc1000000000000.
Guttapercha1000000000000.
Dry air, gases, and vapours[1]
Wool[1]
Ebonite1300000000000.
Diamond[1]
Silk[1]
Glass[1]
Wax[1]
Sulphur[1]
Resin[1]
Amber[1]
Shellac[1]
Paraffin1500000000000.

[1] These have not been accurately measured.

The figures given as indicating the relative resistance of the above bodies to the passage of electricity must be taken as approximate only, since the conductivity of all these bodies varies very largely with their purity, and with the temperature. Metals become worse conductors when heated; liquids and non-metals, on the contrary, become better conductors.

It must be borne in mind that dry air is one of the

best insulators, or worst conductors, with which we are acquainted; while damp air, on the contrary, owing to the facility with which it deposits water on the surface of bodies, is highly conducive to the escape of electricity.