non-conductor. I take the chain off from the cushion, and put it on the conductor, so as to hang on the ground, while I turn the machine. Will the balls be affected now, if you hold them to the conductor?

James. No, they are not?

Tutor. Take them to the cushion.

Charles. They are attracted and repelled now by being brought near the cushion, as they were before, by being carried to the conductor.

Tutor. Yes, and you may now take sparks from the cushion as you did just now from the conductor: in both cases, it must be evident that the electric fluid is brought from the earth.

Some machines are furnished with two conductors, one of which is connected with the cushion, the other such as we have described. Turn the cylinder, and both conductors will be electrified; but any body which is brought within the influence of these, will be attracted by one of the conductors, and repelled by the other: and if a chain or wire be made to connect the two together, neither will exhibit any electric appearances: they seem, therefore, to be in opposite states; accordingly electricians say, that the conductor connected with the cushion is negatively electrified, and the other is positively electrified.

CONVERSATION XXXI.

Of Electrical Attraction and Repulsion.

James. What is this large roll of sealing-wax for?

Tutor. As I mean to explain, this morning, the principles of electrical attraction and repulsion, I have, besides the electrical machine, brought out for use a roll of sealing-wax, which is about fifteen inches long, and an inch and a quarter in diameter; and the long glass tube. Charles. Are they not both electrics, and capable of being excited?

Tutor. They are; but the electricity produced by exciting them has different or contrary properties.

James. Are there two kinds of electrics then? Tutor. We will show you an experiment before we attempt to give any theory.-I will excite the glass tube, and Charles shall excite the wax. Now do you bring the pith-balls, which are suspended on silk (Fig. 3.) to the tube. They are suddenly drawn to it, and now they are repelled from one another, and likewise from the tube, for you cannot easily make them touch it again :-but take them to the excited

wax.

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James. The wax attracts them very powerfully now they fall together again, and appear in the same state as they were in before they were brought to the excited tube.

Tutor. Repeat the experiment again and again, because on this two different theories have been formed. One of which is, that there are two electricities, called by some philosophers the vitreous or positive electricity, and resinous or negative electricity.

Charles. Why are they called vitreous and resinous ?

Tutor. The word vitreous is Latin, and signifies any glassy substance; and the word resinous is used to denote that the electricity produced by resins, wax, &c. possesses different qualities from that produced by glass.

James. Is it not natural to suppose that there are two electricities, since the excited wax attracts the very same bodies that the excited glass repels ?

Tutor. It may be as easily explained, by supposing that every body, in its natural state, possesses a certain quantity of the electric fluid, and if a part of it be taken away, it endeavours to get it from other bodies; or if more be thrown upon it than its natural quantity, it yields it readily to other bodies that come within its influence.

Charles. I do not understand this.

Tutor. If I excite this glass tube, the electri

city which it exhibits is supposed to come from my hand; but if I excite the roll of wax in the same way, the effect is, according to this theory, that a part of the electric fluid naturally belonging to the wax, passes from it through my hand to the earth: and the wax being surrounded by the air, which, in its dry state, is a non-conductor, remains exhausted, and is ready to take sparks from any body that may be presented to it.

James. Can you distinguish that the sparks come from the glass to the hand; and, on the contrary, from the hand to the wax?

Tutor. No: the velocity with which the electric spark moves, renders it impossible to say what course it takes; but I shall show you other experiments which seem to justify this theory: and as Nature always works by the simplest means, it seems more consistent with her usual operations, that there should be one fluid rather than two, provided that known facts can be equally well accounted for, by one as by two. Charles. Can you account for all the leading facts by either theory?

Tutor. Yes, we can.

You saw when the pith-balls were electrified, they repelled one another. It is a general principle in electricity that two bodies having more than their natural share of the electric fluid, will repel one another. But if one have more,

and the other less, than its share, they will attract one another.

James. How is this shown?

Tutor. I will hold this ball, which is insulated, by a silk thread, to the conductor, and do you, Charles, do the same with the other. Let us now bring them together.

Charles. No, we cannot: they fly from one another.

Tutor. I will hold mine to the insulated cushion, and you shall hold yours to the conductor while the machine is turned; now I suspect they will attract one another.

James. They do indeed.,

Charles. The reason is this; that the cushion, and whatever is in contact with it, parts with a portion of its electricity; but the conductor, and the adjoining bodies, have more than their share; therefore, the ball applied to the cushion, being negatively electrified, will attract the one connected with the conductor, which is positively electrified.

Tutor. Here is a tuft of feathers, which I stick in a small hole in the conductor: now see what happens when I turn the cylinder.

James. They all endeavour to avoid each other, and stand erect, in a beautiful manner. Let me take a spark from the conductor: now they fall down in a moment.

Tutor. When I turned the wheel they all had more than their share of the electric fluid, and