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different application of the same substance, often causes this dif-ference. It appears from many experiments, that when two dif-ferent substances are rubbed together, that whose surface is rough-est, or whose electric power is strongest, generally acquires thepositive, and the other the negative. And if two electrics everyway equal are rubbed together, the substance, which suffers thegreatest friction, acquires the negative electricity, and the other
the positive. Wc here present the ingenious with Mr. Cavallo’sTable, which exhibits at one view what kind of electricity will beexcited in the best electric by rubbing with different substances." It might have been much extended, (he says) had he chosen tobring into it all (he minutia: as far as is known." But this lie thoughtboth unnecessary and impracticable.”
TARLE shewing the Kinds or Electricity excited in some Electrics by various Substances.
Substances used as rubbers.
The back of a cat.Smooth glass.
Rough glass.
Tourmalin.
Ilarc’s skin.
White silk.
Black silk.
Sealing-wax..
Baked wood.
Positive
Positive
j Positive( Negative( PositiveI Negative< Positive( Negative( Positive( Negative
S PositiveNegativef Positivef Negativei Positive(Negative
Every substance with which it has been hitherto tried.
Every substance hitherto tried, except the back of a cat.
Dry oiled silk, sulphur, metals.
Woollen cloth, quills, wood, paper, sealing-w ax, white wax, the human hand.Amber, air blown with a pair of bellows.
Diamond, the human hand.
Metals, silk, loadstone, leather, hand, paper, baked wood.
Other fine furs.
Black silk, metals, black cloth.
Paper, hand, hairs, weasel’s skin.
Sealing-wax.
Hare’s, weasel’s, and ferret’s skin, loadstone, brass,, silver, iron,, hand.Metals.
Hare’s, weasel’s, and ferret’s skin, hand, leather, woollen, cloth, paper.
Silk.
Flannel.
Of the Excitation of Electrics.
It is a well known law in electricity, that all electric substancesmay be excited by friction or rubbing. 'This, however, is not theonly method of exciting clectrics to exhibit electrical appearances.Several electrics are excited by melting or pouring a melted elec-tric into another substance; by heating or cooling ; and by eva-poration or effervescence. These methods are attended with pe-culiar phenomena. Sulphur, melted in an earthen vessel, and \left to cool upon conductors, is found strongly electrical; but not !at all when left to cool upon electrics. If melted in a glass vesseland left to cool, both the glass and the sulphur acquire a high de-gree of electricity; the former positive and the latter negative,whether left to cool upon electrics or conductors. And it is re-markable that the sulphur acquires no electricity till it begins tocool. Melted sulphur poured iuto a vessel of baked wood, ac-quires a negative electricity, while the wood acquires a positive:but when poured into rough glass, or sulphur, it shews none.Sulphur melted, and poured into a metal -cup, and there left tocool, exhibits no electrical appearances whilst in the cup; but ifthey be separated both will appear strongly electrified; the sulphurplus and the cup minus. Upon replacing the sulphur in the cup,every electrical sign vanishes; but if, while separate, the electri-city of either be withdrawn, they will both appear possessed of theother electricity, on being replaced. Melted wax, poured intowood or glass, acquires a negative electricity, and leaves the glass«r wood positive. Sealing-wax, poured into sulphur, acquires apositive electricity, and leaves the sulphur negative. If a stick ofsealing-wax be broken in two pieces, the extremities that werejoined will be electrified, the one positively, the other negatively.The late Mr. W. ilcnlv, F. R. S. discovered, that chocolate freshfrom the mill, becomes strongly electrical, as- it cools in the tinpans. It soon loses this property, but recovers it once or twice, byWeing melted in an iron ladle and poured into the tin pans. Whenthe mass becomes dry, the electricity cannot be restored by melt-ing, unless olive oil be mixed with it in the ladle; in which caseit completely recovers its electric power. The Tourmalin, orlapis electricus of Linnams, a hard semi-pellucid fossil, was firstobserved to exhibit electrical phenomena, by heating and cooling.This stone is common in the East Indies, and possesses many sin-gular electrical properties. It exhibits, however, no electricalappearances, while kept in an equal temperature ; but by increas-ing its heat, it becomes electrical, and still more so by diminishingit. Its electricity appears, not over its whole surface, but only ontwo opposite sides, which have-been stiled its poles, as they are in
a line with its centre, and in the same direction with its strata; inwhich direction it is opaque, though semi-transparent in the oilier.The tourmalin, while healing, has one of its sides electrified posi-tively, and the other negatively ; but while cooling, the former be-comes negative and the latter positive. If heated and allowed tocool without either side being touched, the former will be positive -and the latter negative, all the time it is heating or cooling. If ex-cited by rubbing, each of its sides, or both at once may be renderedpositive. If heated or cooled upon an insulated substance, thatsubstance will become possessed of the electricity contrary to thatof the side of the tourmalin, which was laid on it. The electricityof both sides, or of either, may be reversed by beating or cooling;the stone, in contact with other bodies. If a tourmalin be cut inpieces, each piece will have its positive and negative poles, as wellas the whole stone. All the above properties are observable in va-cuo. If this stone be covered over with wax, oil, or any similar,electric, it will exhibit the same electric signs, as without the co-vering. Mr. William Canton remarked a vivid light upon thisstone, while heating in the dark, by which lie could determine -which end of the tourmalin was positive or negative. When ex-cited, it emits very strong (lashes in the dark, trom the positive to -the negative end. Mr. Canton has also observed the Brasilian-emerald emit light while heating in the dark. Mr. Cavallo “ ima-gines that every other precious stone will shew it, if its electricpower be sufficiently strong; since the light is a consequence of.the passage of a sufficient quantity of electricity through the air,,or other partly resisting medium.” The electrical power of thetourmalin is sometimes improved, sometimes injured, sometimesnot in the least affected, by a strong fire. Most of the above pro-perties, which were supposed to be peculiar to the tourmalin, arepossessed by several hard precious stones, whicii are capable of be-coming electrical by heating and cooling, and have their positiveand negative sides lying in the direction of their strata, &c. The -met hod of exciting electricity by evaporation was lately discoveredby Mr. Volta; who observed that the evaporation of water, and'some othei fluids, as well as certain effervescences, generated elec-tricity. His experiments seem to prove, that fluids, or other bo-dies reduced to vapour, become electrified positively, and leave -the bodies from which they evaporated, electrified negatively ;.and that on the other hand, when the vapours are condensed into afluid, they become electrified negatively, ami leave the bodieswith which they were last in contact electrified positively.
Of Communicated Electricity.
“ Under the title ©f communicated electricity, (says M, Cavallo)
falls