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ELECTRICITY.

son is said to be insulated, when he is set with his feet upon a cakeof resin, or on a stool with p,lass feet, or any other good electric,so that the communication between his body and the earth maybe thereby cut off.

Electrification*. Any body, to which the power of attrac-tion and repulsion, &c. is communicated, is said to be electrified.

Or Conductors and Non-Conductors.

One of the first principles in electricity is, that all the substancesin nature are either-electrics or conductors. Numberless experi-ments prove that a substance which is a conductor cannot be ex-cited ; and on the other hand, that a substance which can be ex-cited is not a conductor. This law, however, must not he takenin an absolute sense; for, strictly speaking, there is no substancein nature that can be justly stiled a perfect electric, or a perfec tconductor. In the best conductors the electric fluid finds someresistance ; and it is partially transmitted through or along the sur-face of most, if not all electrics. These two classes so far ap-proach each other’s limits, that the less perfect conductors maybe excited, and even some that are pretty good conductors. Wcinsert the following lists of electrics and conductors, in the orderof their perfection, as they are classed by Mr. Cavallo, who be-gins with the most perfect in each:

Electrics. 1 . Glass and all vitrifications whatever, 2. Pre-cious stones; of which the most transparent are the best. 3. Allresins and resinous compounds; or such consistent oily vegetableproductions, as are inflammable, and not soluble in water. 4.Amber. 5. Sulphur. 6. Baked wood. 7. Bituminous sub-stances. S Wax. 9. Silk. 10. Cotton. 11. Feathers, wool,hair, and all other dry animal substances. 12. Paper. 13. Whitesugar and sugar-candy. 14. Air. 15. Oils. 16. Metallic calces.17. Ashes of animal and vegetable substances. 18. All dry ve-getable substances. 19. All hard stones. Many of these, when■very hot, lose their electric property and become conductors; asred hot glass, melted resin, hot air, &c. The best vitrified glassalso sometimes becomes a conductor.

Conductors. 1 Gold. 2. Silver. 3. Copper. 4. Brass. 5. Iron,6. Tin. 7. Quick-silver. 8. Lead. 9. Semi-metals. 10. Metallicores. 11. Charcoal, of animal or vegetable substances. 12. Fluidsof animal bodies. 13. All other fluids, except air and oils. 14. Theeffluvia of flaming bodies. 15. Ice. 16. Snow. 17. Metallic salts,and most other saline substances. 18. Stony substances. 19. Smoke.20< Steam. Electricity also pervades the vacuum made by anair-pump. Green vegetables, raw meat, Sic. are rendered conduc-tors by the fluids they contain. Hence all electrics before exci-tation should be well dried and some even heated, to free themfrom every particle of dampness, otherwise they may act as con-ductors. W e cannot close this part of our subject, without tak-ing notice of the changes made in the same substance by differentpreparations. A piece of wood just cut from a tree is a good con-ductor: bake it and it will become an electric: burn it to char-coal, and it is a good conductor again ; reduce it to ashes, andthese will be found electrics. Similar changes are observable inmany other bodies.

Of Positive and Negative Electricity.

If a fine downv feather, or any light body, be tied to a silkthread, and electrified strongly, by touching it with the excitedglass tube, it immediately flies from, or is repelled l>v, the glasstube. If we now present an excited stick of sealing-wax, the fea-ther immediately flies towards it. Thus what was attracted byexcited wax, is repelled by excited glass. This experiment hasgiven rise to a very important distinction in electricity, implyinga contrariety of agency. Let two light balls formed of cork, orthe pith of elder, be suspended by fine linen threads from smallcylinders of wood, and insulated upon a common wine glass, thatis wiped dry, and free from dust. Upon electrifying the two balls,thus suspended, by excited glass, they will repel each other; hutwe may destroy this electricity only by touching them with thefinger. If again we electrify them, but with excited wax, theywill again repel each other. But bring the balls electrified bywax towards those electrified by glass, and they immediately flytowards each other. From these and similar experiments, thetwo electricities were at first specifically distinguished by thenames of Vitreous and Resiuous, That which was thought to be

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the constant production of rubbed glass, was stiled the VitreousElectricity ; and that which was first observed to be producedby wax and oilier resinous substances was denominated the Kk-sinous Electricity. But the vitreous, for reasons which willafterwards appear, is now called by the best electricians, Positiveor Plus Electricity, and the resinous is denominated Nega-tive or Minus Electricity. I,el a person be insulated uponany good eleclric, holding a dry glass tube in his left hand, andrubbing it with his right: both the person and the tube will liequickly electrified. If feathers, thread, hits of paper, or any otherlight substances, be presented to his body, they will be attractedand repelled. If another person presents his finger to him, a sparkof iigiit will follow the linger with a slight snapping noise. In aword he will exhibit every electrical appearance that the tubedocs. Yet the electricity of the person and that of the tube, arequite the opposite of each oilier ; the one attracting what tlmother repels. Tims if a small piece of cork or any otlier lightbody, insulated or suspended by a silk thread, has been attractedand repelled by the glass tube, if no conducting substance comein contact with it in this state, it will not again be attracted by thetube. The same happens with the insulated person : if the lightsubstance has been attracted and afterwards repelled by his body,it will not be again attracted. If, however, in this state of repul-sion, the tube he presented to this light substance, it will be at-tracted violently by the tube; and when repelled by the tube, itwill be again attracted by the insulated person. If two or moreof these light insulated bodies be separately attracted by the tube,and when repelled brought near each other, they will repel eachother, and continue in this electrified and repulsive state for sometime, l'he same will happen if they be presented to the insulat-ed person ; after which they will likewise repel each other. Butif one or more of them he attracted and repelled by the class tube,and one or more others by the person, and afterwards all of thembe brought near eaeli other, they will then mutually attract insteadof repelling each other ; and instead of remaining in an electrifiedstate, they will extinguish every appearance of electricity. Thesetwo electricities, therefore, appear to be the opposites of each,other, and as if tlie one was an affirmative or positive power, andthe other a negative, they balance each other, and lose everyelectrical property and appearance. There is another character*istical difference between them too, in the appearance of theirlight. If a needle, a wire, or any other pointed body, be [’re-sented to the excited tube in the dark, a small lucid globe, resem-bling a star, will be seen upon the point ; but if the needle, orwire, be presented to the person, a lucid pencil of rays will appearissuing from the point, and diverging towards the person. Theserays appear best when the needle is presented to the person, oneinch from his body, while he is rubbing the tube. Anothercharacteristical difference has been observed in some experiments.The electricity of the tube, when in the act of passing from a bodvovercharged w ith it to another, cither not electrified, or possessedof the contrary electricity, shews an indisputable current from theformer to the latter ; and the electricity of tlic insulated person*when in the act of passing from a body overcharged with it to ano-ther, either not electrified, or possessed of the contrary electri-city, shew s clearly a current from the latter to the former. These,two electricities are not only observed in the above-mentioned ex-periment, hut in several other cases also ; and they always ac-company each other; for when different electriesare rubbed, somewill acquire one electricity, and others will acquire the contrary;the rubber, if insulated, shewing at the same time signs of theelectricity contrary to that acquired by the excited electric: be-sides this, almost alt electrics may be made to shew at pleasure theone or the other electricity, according to the substance used for arubber. Hence the following corollaries may he deduced ; viz,1. Whenever two different substances (being both insulated, oronly that which is a conductor) are rubbed together, except theyarc both equally good conductors, they will be both electrified,and one acquire the electricity contrary to the electricity of theother. 2. Almost all the electrics may be made to acquire, atpleasure, the one or the other electricity by using proper rubbers.Many causes operate to produce a difference in the electricity.Often the same electric, rubbed with the same rubber, exhibits atone time signs of positive and at another of negative electricity.A slight variation in the surface, «r in the degree of dryness, ora

different