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above); cyanide of potassium, sulpho-cyanide of potassium; carbonate, phosphate, chlorate, nitrate, silicate, manganate, chromate and bichromate of potash; carbonate, borate, phosphate, and nitrate of soda; nitrate of baryta and nitrate of strontia; acid phosphate of lime (Phosphor-glas), borate of zirc; borate, phosphate, sulphate, and nitrate of lead; phosphate and nitrate of copper; sulphate of mercury and nitrate of silver. (Faraday.)

The decomposibility of liquids by the electric current as connected with their conducting power will be considered further on.

c. Air rarefied by heat or exhaustion is a conductor. The more completely the space above the mercurial column is freed from air and vapour of mercury, the fainter is the light produced by electricity passing through it. (H. Davy.) The most perfect vacuum obtainable by the air-pump does not conduct nearly so well as metals: when the metallic conductors are placed at a considerable distance from one another, the electricity does not pass through. (Harris, Pogg. 41, 99.)

The Torricellian vacuum does not conduct the current of a voltaic battery. (Masson, Institut. Nr. 249.)

The current of a single pair of zinc and copper plates in water acidulated with sulphuric acid passes slowly—as may be seen by the decomposition of iodide of potassium introduced into the circuit—through the flame of charcoal urged by the bellows, that of hydrogen gas, coal gas, alcohol, and ether, even when the metallic conductors are placed i} inch apart: it passes through the charcoal flame more easily, and through that of coal-gas less easily, than through the flame of alcohol. But even with a battery of 20 pairs, the current is very feeble, the tension of the pole suffering no diminution, because the electricity is generated faster than it is carried off. Flame conducts positive electricity better than negative electricity. Hence the conduction is more complete when the negative wire is wound into a coil, and its surface thereby made larger than that of the positive wire. When the negative wire is placed in contact with the charcoal and the positive wire in the flame, the current passes more freely than when the arrangement is made the contrary way. Moreover, if into the flame in which the two polar wires are placed, there be introduced a third wire in contact with the earth, the tension of the positive pole diminishes, because the positive electricity is carried off, while that of the negative pole increases. In the heated air above the flame of an Argand spirit-lamp, no conduction takes place from one wire to another; but if the negative wire be connected with the brass cylinder of the lamp and the positive wire wound into a spiral form and introduced into the hot air above the flame, conduction will take place,—but not when the wires are arranged in the contrary way, on account of the difficult conduction of the negative electricity. (Andrews.)

Non-conductors, Insulators (Faraday's Dielectric Bodies). a. Solids. Diamond, boron, phosphorus, sulphur, selenium, iodine,-. sulphuret of manganese; zinc-blende, bismuth-glance, cupreous bismuth, cinnabar. (Fox.)—Red lead, sưlphuret of lead, black oxide of iron, green vitriol, oxide of copper, cyanide of mercury, cyanide of silver (gently ignited?), charcoal from oak-wood (Walcker),– the substances enumerated on page 311 as becoming conductors when fused (Faraday),—oil, dry vegetable fibres, silk, wool and hair.

The following substances are non-conductors both in the solid and in

the fused state: phosphorus, sulphur, boracic acid, green glass, iodide of sulphur, biniodide of tin, realgar, orpiment, glacial acetic acid, a mixture of tallow and oleic acid, camphor, naphthaline, artificial camphor, tallow, cocoa-fat, spermaceti, common resin, sandarac, shellac, sugar, and caffein. (Faraday.)

b. Liquids. Besides those above enumerated: bromine, chlorine, and sulphurous acid. (De la Rive.)

Terchloride of arsenic and its hydrate, and bichloride of tin. (Faraday.) Olive-oil conducts the electricity of a Zamboni's pile less quickly than other fat oils: solid tallow does not conduct so well as oily fat; and solid animal fat more slowly than the same when liquid. (Roussean, Ann. Chim. Phys. 25, 373.)

c. Elastic fluids. Air and all other gases under the ordinary atmospheric pressure and at ordinary temperatures, are perfect insulators. They conduct so much the more quickly as their density is diminished.

A conducting body surrounded by non-conductors is said to be Insulated.

I Gutta Percha has lately been shown by Faraday to be one of the best among non-conducting bodies, its insulating power being fully equal to that of shellac. It may be used in the form of either sheet, rod, or filament. When formed into cylinders about half an inch in diameter, it forms excellent insulating pillars; and in the form of fine threads, it is exceedingly convenient for insulating light bodies, such as feathers, pithballs, &c. A sheet of it is easily converted into an electrophorus, or it may be coated and used as a Leyden jar. When rubbed, it shows negative electricity:-All specimens of gutta percha are not equally good as insulators. The cut surface of a piece which insulates well has a resinous lustre and a compact ch er, which is very distinctive; whilst that which conducts has not the same degree of lustro, appears less translucent, and has more the aspect of a turbid solution solidified. By heating it in a cnrrent of hot air, as over the chimney of a low gas-flame,—and then stretching, doubling, and kneading it for a time between the fingers, as if with the intention of dissipating the moisture within, its insulating power may be made equal to that of the best specimens. (Phil. Mag. J. 32, 165.) T.

For methods of determining the conducting power of bodies for eleo. tricity of small tension, vid. Wollaston (Phil. Trans. 1823, 20); Rousseau (Ann. Chim. Phys. 25, 373).

The difference between conductors and insulators is one of degree only. Electricity of bigh tension like that developed by the electrical machine—and in a less degree, that of a voltaic battery of 150 pairs of plates-is conducted even by ice, and still better by iodide of potassium. The communication of one kind of electricity, either to conductors or nonconductors, first produces a polarization of their particles, a peculiar electric distribution, so that the particle situated next to the one at which the electricity enters, takes up the opposite kind of electricity, the next particle the electricity opposite to that, and so on throughout the whole mass. This produces a discharge of electricity between the neighbouring particles: the sooner this takes place, and the smaller the electrical tension required to produce it, the better does the body conduct; the more slowly the discharge takes place and the higher the tension required to produce it, the better does the body insulate. Conductors cannot

remain permanently polarized; insulators, on the other hand, retain their polarized condition firmly. (Faraday.)

3. The two electricities exist in bodies in the state either of rest or of motion.

a. At rest. Peltier's Statical Electricity. a. Either positive or negative electricity may be accumulated in insulated conductors in various quantities. The greater this quantity in proportion to the surface of the conductor (it is immaterial whether the conductor be hollow or solid) the greater is the electrical Intensity or Tension of the conductor, and the stronger the effort which the electricity makes to leave the conductor and unite with the opposite electricity in its neighbourhood; and this greater tension probably gives rise to more rapid motion when the electricity is conducted away:

B. In an Insulator the two kinds of electricity are always present at the same time, because the accumulation of one electricity at a particular point of such a body always causes by polarisation the appearance of the other electricity at the opposite point; and the tension of the two electricities is higher, the greater their quantity in proportion to the surface;they also adhere to the insulator very strongly, so that they can only be gradually removed by conductors, unless the latter touch the whole sur. face of the insulator.

b. In Motion. Peltier's Dynamical Electricity. The two electricities issuing from two sources or two conductors charged with them meet in a conductor and combine together, and thus produce an Electrical Current.



The two electricities bave a very great affinity for each other, and exert a powerful tendency to combine together. From their combination results Latent or Quiescent Electricity, which, according to the electrochemical theory, is nothing else than light and heat.

By various causes, the latent electricity in bodies is resolved into positive and negative electricity, which are set free at various points.

1. Combination of the two Electricities with each other. When a body in which positive electricity is accumulated is brought near another charged with negative electricity, the two electricities combine together; and if they are present in equal quantities, both bodies appear perfectly neutral after the combination has taken place. The combination is attended with the development of light, heat, and magnetic phenomena. Insulators which oppose the combination are often violently torn asunder.

The tendency of the two electricities to unite shows itself, when the two bodies in which they exist are moveable, in an approximation of the bodies themselves:-Bodies oppositely electrified attract each other.

If the two bodies are separated by a thin film of a non-conductor, the

two electricities break through it-provided they have sufficient tension -and a spark is produced accompanied by a noise. (Electric Shock.) The duration of an electric spark is, according to Wheatstone, only 113200 second. If the non-conductor be solid, like glass, it is pierced; the same effect takes place even with semi-conductors and good conductors—as with a card or tinfoil-when they are surrounded with air and placed in a thin film between the two oppositely electrified conductors. When a card is pierced in this way, the edges of the hole are often raised in the direction of both conductors, positive as well as negative. Tinfoil, according to Moll (J. Phys. 90, 396), often exhibits two holes with their edges turned in opposite directions, as if the positive electricity had gone through the one and the negative through the other.

The light of the spark is variously coloured, partly according to the density of the elastic medium, partly according to its peculiar nature. (H. Davy, Ann. Chim. Phys. 20, 168: Grotthuss, Schw. 14, 163; Faraday.)

When the combination of the two electricities takes place through a conductor brought in contact with both the electrified bodies, and the quantity of the combining electricities is considerable, the conductor becomes heated to the most vivid incandescence, and fuses. Platinum and iron wire fuse in the circuit of a powerful voltaic battery; if the wire be immersed in water, the water boils. A given quantity of common electricity heats a wire to the same degree, whatever may be its intensity. (Harris.)-Currents from different voltaic batteries which produce equal deflections of the magnetic needle, raise the temperature of the conducting wire in the same degree: if a wire 0·1 metre in length produces a deviation of 20°, and becomes heated 10°, a wire several metres long will also have its temperature raised 10', when, by a proportionate increase in the power of the battery, its deviating power has been made equal to 20°. With the same wire, an augmentation in the power of the battery raises the temperature in a higher proportion than it increases the deflecting power: when the deflecting power is doubled, the degree of heating is trebled. With wires of different thicknesses, the heating power increases more rapidly than the transverse section diminishes; when the latter is reduced one-half, the former is increased three-fold. (Peltier.) If the connecting wire of the voltaic battery consists of three pieces of platinum and three pieces of silver soldered together alternately, the platinum alone becomes red-hot. If a platinum and a silver wire are connected together, the platinum becomes red-hot before the silver: so likewise platinum with zinc or gold. When platinum is joined with tin or lead, the latter metals fuse at the junction before the platinum becomes heated to redness; but even in this case the greatest rise of temperature takes place in the platinum. When platinum and iron are united, the platinum becomes first incandescent, then the iron more strongly than the platinum. When iron is united with gold or zinc, the iron becomes incandescent. Zinc is more strongly heated than silver, copper more strongly than gold, The more slowly metals conduct, the more intensely do they become heated; and according to these experiments, they succeed one another in the following order, beginning with the best conductors: Silver, zinc, gold, copper, iron, platinum. At high temperatures, iron seems to conduct less easily than platinum; for after the action has continued a certain time, the iron becomes the more incandescent of the two. (Children, Phil. Tr. 1815, 363; also Schw. 16, 359.) If a concentrated solution of chloride of calcium be connected with the positive pole of a strong voltaic battery by a thick wire, and with the negative pole by a thin wire of platinum, the latter becomes incandescent, fuses, and falls into the solution in drops which follow one another very quickly; but if the thin wire conveys the positive electricity, it ignites but does not fuse. (Hare.) -A metal becomes the more strongly heated, the more slowly it conducts and the less capacity it has for heat. (Riess, Pogg. 44,78.)

If a zinc wire be soldered to an iron or copper wire, the point of junction becomes more strongly heated when the zinc is connected with the negative than when it is connected with the positive pole. If the compound wire consists of copper and bismuth, the junction becomes heated wben the copper is connected with the positive, but cooled when that metal is in connection with the negative pole. The opposite relation is exhibited when copper is united with antimony, the cooling taking place when the copper touches the positive pole. Antimony and bismuth give the strongest variations of temperature, exhibiting a rise of temperature at the junction when the positive electricity proceeds from the antimony, and a fall in the contrary case.

[Many metals, especially antimony, appear to conduct positive better than negative electricity; others, particnlarly bismuth, seem to be more capable of conducting negative electricity than positive. When antimony is connected with the positive, and bismuth with the negative pole, the former conducts positive and the latter negative electricity to the junction, which becomes heated from the combination of the two electricities; but when the antimony is connected with the negative pole, the heat of the junction is resolved into positive electricity, which passes on to the negative pole through the antimony which conducts it most freely,-and negative electricity which passes on, through the bismuth which gives it the readiest passage, to the positive pole. The combination of the two electricities and the consequent development of heat take place where the antimony touches the negative, and the bismuth the positive pole; but the junction is cooled by the decomposition of a portion of its heat*.]

The cooling of the junction amounts to 3° R; water placed in a hole made at that point becomes frozen in five minutes, if the rod has been previously cooled down to 0. When the experiment is continued for a longer time, the temperature of the junction again rises, because the bismuth becomes gradually heated (much sooner than the antimony,) and communicates its temperature to the junction. (Lenz.).

If the copper wires connecting the poles of a Daniell's battery of 160 pairs be gradually separated to the distance of or inch, the electrical current will pass from one to the other in the form of a luminous arch (probably carrying fine particles of the conductor along with it). The positive wire alone becomes incandescent. Similar phenomena are exhibited by brass, iron, and platinum wires. (Gassiot, Phil. Mag. J. 13, 436; also Pogg. 46, 330.). (This seems to show that the negative electricity of the luminous arch is conducted more readily than the positive electricity, and that consequently the two electricities combine together in the wire.]

Charcoal placed between the poles of the deflagrator emits as much light as 1600 candles. (Hare.)

If the direction of the conducting wire is from North to South and a

* All remarks enclosed within three-cornered brackets contain explanations according to the author's theory given in Pogg. 44, 1, which, whether established or not, may serve as a guide to conduct us through the chaotic region of the theory of electricity, however much it may leave uuexplained.

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