will become magnetic, that is, if the needle be accurately suspended on a small piece of cork, in a basin of water, one end will, of itself, point to the north, and the other to the south.

Example 10. I will lay this chain on a sheet of writing paper, and send the charge of the battery through the chain; and you will see black marks will be left on the paper, in those places where the rings of the chain touch each other.

Example 11. Place a small piece of very dry wood between the balls of the universal dischargers, so that the fibres of the wood may be in the direction of the wires, and pass the charge of the battery through them, the wood will be torn in pieces. The points of the wires being run into the wood, and the shock passed through them, will effect the same thing.

Example 12. Here is a glass tube, open at both ends, six inches long, and a quarter of an inch in diameter. These pieces of cork, with wires in them, exactly fit the ends of the tube. I put in one cork, and fill the tube with water, then put the other cork in, and push the wires so that they nearly touch, and pass the charge of the battery through them, you see the tube is broken, and the water dispersed in every direction*.

* To prevent accidents, a wire cage, such as is used in some experiments on the air-pump, should be put over the tube before the discharge is made. Young persons should not attempt this experiment by themselves.

Charles. If water is a good conductor, how is it that the charge did not run through it without breaking the tube?

Tutor. The electric fluid, like common fire, converts the water into a highly elastic vapour, which occupying very suddenly a much larger space than the water, bursts the tube before it can effect any means of escape.

CONVERSATION XXXVI.

Of the Electric Spark, and Miscellaneous Experiments.

Tutor. I wish you to observe some facts connected with the electric spark. By means of the wire inserted in this ball, I fix it to the end of the conductor, and bring either another brass ball, or my knuckle to it, and if the machine act pretty powerfully, a long, crooked, brilliant spark will pass between the two balls, or between the knuckle and ball. If the conductor is negative, it receives the spark from the body; but if it is positive, the ball or the knuckle receives the spark from the conductor.

Charles. Does the size of the spark depend at all on the size of the conductor?

Tutor. The longest and largest sparks are obtained from a large conductor, provided the machine act very powerfully. When the quantity of electricity is small, the spark is straight; but when it is strong, and capable of striking at a greater distance, it assumes what is called a zig-zag direction.

James. If the electric fluid is fire, why does not the spark, which excites a painful sensation, burn me, when I receive it on my hand?

Tutor. Ex. 1. I have shown you that the charge from a battery will make iron wire red hot, and inflame gunpowder. Now stand on the stool with glass legs, and hold the chain from the conductor with one hand. Do you, Charles, hold this spoon, which contains some spirit of wine, to your brother, while I turn the machine, and a spark taken from his knuckle, if large, will set fire to the spirit.

Charles. It has indeed. Did you do nothing with the spirit?

Tutor. I only made the silver spoon pretty warm before I put the spirit into it.

Ex. 2. If a ball of box-wood be placed on the conductor instead of the brass ball, a spark taken from it will be of a fine red colour.

Ex. 3. An ivory ball placed on the conductor will be rendered very beautiful and luminous, if a strong spark be taken through its centre.

Ex. 4. Sparks taken over a piece of silver

leather appear of a green colour, and over gilt leather of a red colour.

Ex. 5. Here is a glass tube (Plate VII. Fig. 13.) round which, at small distances from each other, pieces of tin foil are pasted in a spiral form, from end to end: this tube is inclosed in a larger one, fitted with brass cups at each end, which are connected with the tin foil of the inner tube. I hold one end A in my hand, and while one of you turns the machine, I will present the other end B to the conductor, to take sparks from it. But first shut the windowshutters.

Charles. This is a very beautiful experiment.

Tutor. The beauty of it consists in the distance which is left between the pieces of tin foil, and by increasing the number of these distances, the brilliancy is very much heightened.

Ex. 6. The following is another experiment of the same kind. Here is a word, with which you are acquainted (Plate VIII. Fig. 14.) made on glass, by means of tin foil pasted on glass, fixed in a frame of baked wood. I hold the frame in my hand at н, and present the ball G to the conductor, and at every considerable spark, the word is beautifully illuminated.

Ex. 7. A piece of sponge, filled with water, and hung to a conductor, when electrified in a dark room, exhibits a beautiful appearance.

Ex. 8. This bottle is charged: if I bring the brass knob that stands out of it, to a basin of

water which is insulated, it will attract a drop; and on the removal of the bottle, it will assume a conical shape, and if brought near any conducting substance, it will fly to it in luminous streams.

Ex. 9. Place a drop of water on the conductor, and work the machine, the drop will afford a long spark, assume a conical figure, and carry some of the water with it.

Ex. 10. On this wire I have fixed a piece of sealing-wax, and having fixed the wire into the end of the conductor, I will light the wax, and the moment the machine is worked, the wax will fly off in the finest filaments imaginable.

Ex. 11. I will wrap some cotton-wool round one of the knobs of my discharging-rod, and fill the wool with finely bruised resin: I now discharge a Leyden jar, or a battery, in the common way, and the wool is instantly in a blaze. The covered knob must touch the knob of the jar, and the discharge should be effected as quickly as possible.

You will remember that the electric fluid always chooses the nearest road, and the best conductors to travel by; in proof of which take the following experiment:

Ex. 12. With this chain I make a sort of w, (Plate VII. Fig. 15.) the wire w touches the outside of a charged jar, and the wire x is brought to the knob of the jar, and in the dark a brilliant w is visible. But if the wire w is