it may be conveyed away again by means of other conductors.

Tutor. Whatever body is supported, or prevented from touching the earth, or communicating with it, by means of glass or other nonconducting substances, is said to be insulated. Thus a body suspended on a silk line is insulated, and so is any substance that stands on glass, or resin, or wax, provided that these are in a dry state, for moisture will conduct away the electric fluid from any charged body.

CONVERSATION XXX.

Of the Electrical Machine.

Charles. What is that shining stuff which I saw you put to the rubber yesterday?

Tutor. It is called amalgam: the rubber, by itself, would produce but a slight excitation : its power, however, is greatly increased by laying upon it a little of this amalgam, which is made of quicksilver, zinc, and tinfoil, with a little tallow or mutton suet.

James. Is there any art required in using this amalgam ?

Tutor. When the rubber and silk flap are very clean and dry, and in their place, then spread a little of the amalgam upon a piece of leather, and apply it to the upper part of the glass cylinder, while it is revolving from you; by this means, particles of the amalgam will be carried by the glass itself to the lower part of the rubber, and will increase the excitation.

Charles. I think I once saw a globe, instead of a cylinder, for an electrical machine.

Tutor. You might: globes were used before cylinders, but the latter are the most convenient of the two. The most powerful electrical machines are fitted with flat plates of glass. our experiments, we shall be content with the cylinder, which will answer every purpose of explaining the principles of the science.

James. As I was able to conduct the electricity from the tin conductor to the ground, could I likewise act the part of the chain, by conducting the fluid from the earth to the cushion?

Tutor. Undoubtedly: I will take off the chain, and now do you keep your hand on the cushion, while I turn the handle.

James. I see the machine works as well as when the chain was on the ground.

Tutor. Keep your present position, but stand on the stool with glass legs; by which means there is now all communication cut off between

the cushion and the earth; in other words, the cushion is completely insulated, and can only take from you what electricity it can get from your body. Go, Charles, and shake hands with your brother.

Charles. It does not appear that the machine had taken all the electricity from him, for he gave me a smart spark.

Tutor. You are mistaken; he gave you nothing, but he took a spark from you.

Charles. I stood on the ground; I was not electrified how then could I give him a spark?

Tutor. The machine had taken from James the electricity that was in his body, and by standing on the stool, that is, by being insulated, he had no means of receiving any more from the earth, or any surrounding objects; the moment, therefore, you brought your hand near him, the electricity passed from you to him.

Charles. I certainly felt the spark, but whether it went out of, or entered into, my hand, I cannot tell have I then less than my share now?

Tutor. No: what you gave to your brother was supplied immediately from the earth. Here is another glass-legged stool; do you stand on this, but at the distance of a foot or two from your brother, who still keeps his place. I take the electricity from him by turning the machine, and as he stands on the stool, he has now less than his share. But you have your natural

share because though you also are insulated, yet you are out of the influence of the machine; extend, therefore, your hand, and give him a part of the electric fluid that is in you.

Charles. I have given him a spark.

Tutor. And being yourself insulated, you have now less than your natural quantity, to supply which you shall have some from me: give me your hand. You draw it back without my touching it!

Charles. I did; but it was near enough to get a strong spark from you.

Tutor. When a person has less electricity than his natural share, he is said to be electrified minus, or negatively: but if he has more than his natural share, he is said to be electrified plus, or positively.

James. Then before Charles gave me the spark, I was electrified minus; and when he had given it to me, he was minus till he received it from you.

Tutor. That is right. Suppose you stand on a stool and hold the rubber, and Charles stand on another stool, and touch the prime conductor L, while I turn the machine, which of you will be plus, and which minus electrified?

James. I shall be minus, because I give to the rubber and Charles will be plus, because he receives from the conductor what I gave to the rubber, and which is carried by the cylinder to the conductor.

Tutor. You then have less than your share, and your brother has more than he ought to have. Now if I get another glass-legged stool, I can take from Charles what he has too much, and give it to you who have too little.

Charles. Is it necessary that you should be insulated for this purpose?

Tutor. By being insulated I may perhaps carry back to James the very electricity which passed from him to you. But if I stand on the

ground, the quantity which I take from you will pass into the earth, because I cannot, unless I am insulated, retain more than my natural share.

James. And what is given by you to me is likewise instantaneously supplied by the earth

Tutor. It is. Let us make another experiment to show that the electric fluid is taken from the earth. Here are some little balls (Plate vII. Fig. 3.) made of the pith of elder: they are put on thread, and being very light, are well adapted to our purpose.

While the chain is on the cushion, and I work the machine, do you bring the balls near the conductor by holding the thread at D.

James. They are attracted by it, and now the two balls repel each other, as in the figure

X.

Tutor. I ought to have told you, that the upper part D of the line is silk, by which means you know the balls are insulated, as silk is a