you see they all move to it. That is called electrical attraction.

Charles. They jump back again now, and now they return to the glass.

Tutor. They are, in fact, alternately attracted and repelled, and this will last several minutes if the glass be strongly excited. I will rub it again; present your knuckle to it in several parts, one after another.

James. What is that snapping? I feel likewise something like the pricking of a pin.

Tutor. The snapping is occasioned by little sparks which come from the tube to your knuckle, and these give the sensation of pain.

Let us go into a dark room, and repeat the experiment.

Charles. The sparks are evident enough now, but I do not know where they can come from.

Tutor. The air, and every thing is full of the fluid which appears in the shape of sparks; and whatever be the cause, which I do not attempt to explain, the rubbing of the glass with the hand collects it from the air, and having now more than its natural share, it parts with it to you, or to me, or to any body else that may be near enough to receive it.

James. Will any other substance besides the hand, excite the tube?

Tutor. Yes, many others, and these, in this science, are called the rubbers; and the glass

tube, or whatever is capable of being thus excited, is called an electric.

Charles. Are not all sorts of solid substances capable of being excited?

Tutor. You may rub this poker, or the round ruler for ever, without obtaining an electric spark from them.

James. But you said one might get a spark from the mahogany table, if it had more than its share.

Tutor. So I say you may have sparks from the poker, or ruler, if they possess more than their common share of the electric fluid.

Charles. How do you distinguish between bodies that can be, and those that cannot be, excited?

Tutor. The former, as I have told you, are called electrics, as the glass tube; the latter, such as the poker, the ruler, your body, and a thousand other substances, are denominated conductors.

Charles. I should be glad to know the reason of the distinction, because I shall be more likely to remember it.

Tutor. That is right: when you held your knuckle to the glass tube, you had several sparks from the different parts of it: but if I, by any means, overcharged a conductor, as this poker, all the electricity will come away at a single spark, because the superabundant quan

tity flows instantaneously from every part to that point where it has an opportunity of getting away. I will illustrate this by an experiment. But first of all, let me tell you, that all electrics are called also non-conductors.

James. Do you call the glass tube a non-conductor because it does not suffer the electric fluid to pass from one part of it to another?

Tutor. I do:-silk, if dry, is a non-conductor. With this skein of sewing-silk, I hang the poker or other metal substance A (Plate VII. Fig. 1.) to a hook in the ceiling, so as to be about twelve inches from it; underneath, and near the extremity, are some small substances, as bits of paper, &c. I will excite the glass tube, and present it to the upper part of the poker.

Charles. They are all attracted, but now you take away the glass they are quiet.

Tutor. It is evident that the electric fluid passed from one part of the tube through the poker, which is a conductor, to the paper, and attracted it:-if the glass be properly excited, you may take sparks from the poker.

James. Would not the same happen if another glass tube were placed in the stead of the poker?

Tutor. You shall try.-Now I have put the glass in the place of the poker, but let me excite the other tube as much as I will, no effect can be

F

produced on the paper:-there are no signs of electrical attraction, which shows that the electric fluid will not pass through glass.

Charles. What would have happened if any conducting substances had been used, instead of silk, to suspend the iron poker?

Tutor. If I had suspended the poker with a moistened hempen string, the electric fluid would have all passed away through that, and there would have been no (or very trifling) appearances of electricity at the end of the poker.

You may vary these experiments till you make yourselves perfect with regard to the distinction between electrics and conductors. Sealing-wax is an electric, and may be excited as well as a glass tube, and will produce similar effects. I will give you a list of electrics, and another of conductors, disposed according to the order of their perfection, beginning in each list with the most perfect of their class: thus glass is a better electric than amber, and gold a better conductor than silver.

ELECTRICS.

Glass of all kinds.

TABLE.

CONDUCTORS.

All the metals, in the follow-
ing order:

Gold; silver;
Copper; platina;
Brass; iron;

Tin; quicksilver;
Lead.

The semi-metals.*
Metallic ores.*
Charcoal.

The fluids of an animal body,
Water, especially salt water
and other fluids, except
oil.

* This, and other chemical terms, are explained and familiarly illustrated in a work just published, by the author of the Scientific Dialogues, entitled "Dialogues in Chemistry," &c.

VOL. III.-0