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linen thread, about twelve inches long; then, doubling the thread in the middle, these two balls, suspended at the ends of the thread, will be in contact.

Then fasten the middle of the thread to one end of one of these pairs of rods. The threads should be very smooth, and free from any little fibres; and must be dipped once in salt water. * Proceed in the same manner with regard to the other two balls, and the other two rods.

Make little hemispherical holes in each pair of rods, to place the balls in, when you wish to shut up the apparatus.

MODE OF USING IT.

To make use of this apparatus, take three large drinking-glasses, and warm and dry them well before the fire.

On one of the glasses, place one pair of the rods open, the hinge being on the upper side, and the little balls suspended at the end. Place the other pair in like manner on the second glass.

Place these two glasses near a corner of the table, in such a position that the ends of the rods from which the balls are suspended may extend beyond the table, and thus the balls may hang entirely free of the table, on either side of the same corner, while the two ends without balls are an inch apart, and the rods in a line with each other.

Rub the third glass with a silk handkerchief, of any color; a piece of black silk, however, is better than any thing else.

* The reason why the threads should be dipped once in salt water is, that they may always continue to be good conductors; for, unless this is done, in dry weather they would sometimes become too dry to conduct freely. For this ingenious method, we are indebted to Mr. Cavendish

1. Bring the rubbed glass near one pair of balls; they will be attracted by it, and receive electricity from it; and, when you withdraw the glass, the balls will hang apart, because they will repel each other.

2. To show that this electricity is a sublile fluid, which penetrates wood, and passes readily through it from end to end, being at the same time susceptible of division and of communication;

Without touching the wood, bring the glasses nearer each other, so that the ends of both pairs of rods will meet; and you will immediately perceive that the two separated balls will approach each other by half the distance that divides them, and that the two balls which were hanging in contact will recede from each other, to an equal distance.

3. To show that electricity does not pass into wax, although it passes into wood;

Touch the wood with a stick of sealing-wax; and you will perceive no change in the respective positions of the balls.

4. A proof that it does not enter glass is, that it is retained as long as the rods are supported on the glass.

5. But, to prove that it enlers metals and animal bodies, touch the rods either with a key, or with the finger, and the electricity will instantly pass off into the ground, whence it was attracted by rubbing the glass, and each pair of balls will come together again.

6. To show that the particles of the fluid mutually repel each oiher, and that the natural quantity of it contained in any substance whatever can be put in motion by repulsion;

Rub the glass well, and, having separated the two pairs of rods, hold the glass over that end of one pair from which the balls are not suspended; on the approach of the glass, you will see the balls separate and recede from each other. Remove the glass, and they will come together again; which shows that the divergence of the balls was not occasioned by any electricity communicated, for none remains ; but solely by the motion of the quantity naturally contained in the wood, which the repellent power of that of the rubbed glass has driven from one end of the wood to the other, so that it is accumulated at the end where the balls are suspended, the end next the glass being deprived of it in proportion. By withdrawing the glass it is made to resume its place, because the equilibrium is restcred, and the balls reunite.

7. Again hold the glass over the end of one pair of rods; and, when the natural quantity of electricity is driven to the end from which the balls are suspended, and has separated them, touch this end with the finger, which will carry off the accumulated electricity, leaving in this end and the balls only their natural quantity, and the balls will consequently come together again ; then withdraw at the same moment the glass and the finger, and you will see the balls again recede from each other; but they are now, as well as the wood, in a negative state; for, on removing the glass, the natural quantity of electricity, which the finger had left at the other end, returns and is diffused equally throughout the wood; and, as this wood has lost a portion of its natural quantity, which was carried off by the finger, what remains is, in reference to the whole substance, less than the natural quantity.

8. To prove that these balls are now in a negative state, present the rubbed glass to them, and it will attract them, whereas it would repel them if they were in a positive state. On the other hand, they would be repelled by a stick of sealing-wax that had been rubbed, whose electricity is negative; instead of which

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this same rubbed wax would attract them, if they were in a positive state.

9. But you may obtain the most convincing proof, that they are in a negative state, in the following manner. Electrify the other pair of rods positively, as was directed above; and, when the balls of each pair have receded to the same distance from each other, which shows that there is as much positive electricity in one pair as there is negative in the other, bring the ends of the rods in contact, and you will see the balls on each pair of rods come together instantly, one pair of rods restoring to the other the quantity which it had parted with, so that both are restored to their natural state. When both pairs are electrified plus, or both minus, separately and equally, in vain would you bring them in contact; no effect whatever would be produced.

To perform these experiments well, care must be taken to keep the glasses always dry; and the best way to do this is to warm them from time to time, if the air is damp.

TO M. DUBOURG.*

On the Analogy between Magnetism and Electricity.

London, 10 March, 1773. SIR, As to the magnetism, which seems produced by electricity, my real opinion is, that these two powers of nature have no affinity with each other, and that the

• This letter and the three following it are translated from M. Dubourg's French edition, (Tom. I. pp. 277, 312, 332.) – EDITOR.

apparent production of magnetism is purely accidental. The matter may be explained thus.

1st. The earth is a great magnet.

2dly. There is a subtile fluid, called the magnetic fluid, which exists in all ferruginous bodies, equally attracted by all their parts, and equally diffused through their whole substance; at least where the equilibrium is not disturbed by a power superior to the attraction of the iron.

3dly. This natural quantity of the magnetic fluid, which is contained in a given piece of iron, may be put in motion so as to be more rarefied in one part and more condensed in another; but it cannot be withdrawn by any force that we are yet made acquainted with, so as to leave the whole in a negative state, at least relatively to its natural quantity; neither can it be introduced so as to put the iron into a positive state, or render it plus. In this respect, therefore, magnetism differs from electricity.

4thly. A piece of soft iron allows the magnetic fluid which it contains to be put in motion by a moderate force; so that, being placed in a line with the magnetic pole of the earth, it immediately acquires the properties of a magnet, its magnetic fluid being drawn or forced from one extremity to the other; and this effect continues as long as it remains in the same position, one of its extremities becoming positively magnetized, and the other negatively. This temporary magnetism ceases as soon as the iron is turned east and west, the fluid immediately diffusing itself equally through the whole iron, as in its natural state.

5thly. The magnetic fluid in hard iron, or steel, is put in motion with more difficulty, requiring a force greater than the earth to excite it; and, when once it has been forced from one extremity of the steel to the

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