continued to m, the electric fluid takes shorter road to x, and, of course, only half of the w is seen, viz. that part marked m zy: but if, instead of the wire w m, a dry stick be laid in its place, the electric matter will prefer a longer circuit, rather than go through a bad conductor, and the whole w will be illuminated.

Ex. 13. Here is a two ounce phial, half full of sallad oil; through the cork is passed a piece of slender wire, the end of which, within the phial, is so bent as to touch the glass just below the surface of the oil. I place my thumb opposite the point of the wire in the bottle, and in that_position_take a spark from the charged conductor. You observe that the spark, to get to my thumb, has actually perforated the glass. In the same way, I can make holes all round the phial.

Charles. Would the experiment succeed with water, instead of oil?

Tutor. No, it would not.

James. At any rate, we see the course of the electric fluid in this experiment, for the spark comes from the conductor down the wire, and through the glass to the thumb.

Tutor. Its direction is, however, better shown in this way.

Ex. 14. At that end of the conductor which is farthest from the machine, I fix a brass wire, about six inches long, having a small brass ball

on its extremity. To this ball, when the machine is at work, I hold the flame of a wax taper.

Charles. The flame is evidently blown from the ball, in the direction of the electric fluid: it has a similar effect to the blast of a pair of bellows.

Ex. 15. I will fix a pointed wire upon the prime conductor, with the point outward, and another like wire upon the insulated rubber. Shut the window-shutter, and I will work the machine: now observe the points of the two wires.

James. They both are illuminated, but differently. The point on the conductor sends out a sort of brush of fire, but that on the rubber is illuminated with a star.

Tutor. You see then the difference between the positive and negative electricity.

CONVERSATION XXXVII.

Miscellaneous Experiments-Of the Electrophorus-Of the Electrometer, and the Thunder House.

Tutor. I shall proceed this morning with some other experiments on the electrical machine.

Ex. 1. Here are two wires, one of which is connected with the outside of this charged Leyden jar, the other is so bent as easily to touch the knob of the jar. The two straight ends I bring within the distance of the tenth of an inch of one another, and press them down with my thumb, and in this position, having darkened the room, I discharge the jar. Do you look upon my thumb.

Charles. It was so transparent that I think I even saw the bone of the thumb. But did it not hurt you very much?

Tutor. With attention, you might observe the principal blood vessels, I believe; and the only inconvenience that I felt was a sort of tremour in my thumb, which is by no means painful. Had the wires been at double the distance, the shock would have probably made my thumb the circuit, which must have caused a more powerful and unpleasant sensation, but being so close, the electric fluid leaped from one wire to the other, and during this passage it illuminated my thumb, but did not go through it.

Ex. 2. If, instead of my thumb, a decanter full of water, having a flat bottom, were placed on the wires, and the discharge made, the whole of the water will be beautifully illuminated.

Ex. 3. This small pewter bucket is full of water, and I suspend it from the prime conductor, and put in a glass siphon, with a bore so narrow, that the water will hardly drop out. VOL. III.-S

See what will happen when I work the machine; but first make the room dark.

James. It runs now in a full stream, or rather in several streams, all of which are illuminated. Tutor. Ex. 4. If the knob a (Plate VIII. Fig. 16.) communicate with the outside of a charged Leyden jar, and the knob b with the inside coating, and each be held about two inches from the lighted candle x, and opposite to one another, the flame will spread towards each, and a discharge will be made through it: this shows the conducting power of flame.

This instrument (Plate VIII. Fig. 17.) which consists of two circular plates, of which the largest B is about fifteen inches in diameter, and the other A fourteen inches, is called an electrophorus. The under plate в is made of glass, or sealing-wax, or of any other non-conducting substance: I have made one with a mixture of pitch and chalk boiled together. The upper plate A is sometimes made of brass, and sometimes of tin plate, but this is of wood, covered very neatly with tin foil: x is a glass handle fixed to a socket, by which the upper plate is removed from the under one.

Charles. What do you mean by an electrophorus ?

Tutor. It is, in fact, a sort of simple electrical machine, and is thus used. Rub the lower plate B with a fine piece of new flannel, or with rabbit's, or hare's, or cat's skin, and when it is

well excited, place upon it the upper plate A, and put your finger on the upper plate: then remove this plate by the glass handle x, and if you apply your knuckle, or the knob of a coated jar, you will obtain a spark. This operation may be repeated many times, without exciting again the under plate.

James. Can you charge a Leyden jar in this way?

Tutor. Yes, it has been done, and by a single excitation, so as to pierce a hole through a card.

Here is another kind of electrometer (Plate VIII. Fig. 18.) which is by far the most sensible that has been yet invented; that is, it is capable of discovering the smallest quantities of electricity. A is a glass jar, в the cover of metal, to which are attached two pieces of gold leaf x, or two pith-balls suspended on threads: on the sides of the glass jar are two narrow strips of tin foil.

Charles. How is this instrument used?

Tutor. Any thing that is to be electrified is brought to the cover, which will cause the pieces of gold leaf, or pith-balls to diverge; and the sensibility of this instrument is so great, that the brush of a feather, the throwing of chalk, hair-powder, or dust, against the cap B, evinces strong signs of electricity.

Ex. 5. Place on the cap в a little pewter, or any other metallic cup, having some water in