We have also read of positive and negative electricities, and we must not fancy there is any particular reason for this distinction. It was Du Fay, whom we have mentioned, who gave the names “vitreous” and “resinous” to the two kinds, as one was developed by rubbing glass, and the other by rubbing resin. But, as shown by our experiments, either kind of electricity can be excited in glass or sealing-wax, and both kinds are produced at once. You cannot get “positive” without negative electricity. “Positive” is the term applied to the kind produced by rubbing glass with silk or wool; “negative” is the term applied to the kind developed by rubbing sealing-wax, but the kind developed by friction depends on the rubbing substance and certain conditions. Bodies charged with the same electricity repel; if charged with different kinds they attract each other. The more readily displaced particles when bodies are rubbed become negatively electrified as a rule. Similar electricities repel each other with a force inversely proportional to the squares of the distance between their centres, as established by Coulomb. So if the space between any two similarly electrified bodies be reduced by say _one-half_, the force of the repulsion will be increased _four_ times. The rule for attraction is similar; so when two bodies are charged with opposite electricities, and the distance between them is increased, the attractive force is diminished in proportion as the square of the distance between them. Many confirmations of this theory were made by the late friend of our boyhood, Sir W. Snow Harris, and published in the _Philosophical Transactions_. The following full list of conductors and non-conductors (copied from Professor Noad’s Text-book of Electricity, and compared with De La Rive’s Treatise) may be useful:— CONDUCTING BODIES IN ORDER OF CONDUCTING POWER. All the metals. Well-burnt charcoal. Plumbago. Concentrated acids. Powdered charcoal. Dilute acids. Saline solutions. INSULATORS IN THE INVERSE ORDER OF INSULATING POWER. Dry metallic oxides. Oils (heavier the better). Vegetable ashes. Transparent dry crystals. Ice below 13° Fahr. Phosphorus. Lime. CONDUCTING BODIES IN ORDER OF CONDUCTING POWER. Metallic ores. Animal fluids. Sea-water. Spring-water. Rain-water. Ice above 13° Fahr. Snow. Living vegetables. Living animals. Flame smoke. Steam. Salts soluble in water. Rarefied air. Vapour of alcohol. Vapour of ether. Moist earth and stones. Powdered glass. Flowers of sulphur. INSULATORS IN THE INVERSE ORDER OF INSULATING POWER. Dry chalk. Native carbonate of baryta. Lycopodium. Caoutchouc. Camphor. Silicious and argillaceous stones. Dry marble. Porcelain. Dry vegetables. Baked wood. Leather. Parchment. Dry paper. Hair. Wool. Dyed silk. Bleached silk. Raw silk. The following may be added to the Insulators, viz.:—