subtile matter surrounding the molecules of the magnet, each fluid repelling its own kind, and attracting the other

kind.

According to this hypothesis, a body is magnetized when these fluids are separated and driven to its opposite extremities. The difference of the two poles arises from the nature

B

Fig. 239.

of the fluids which predominate in them; the poles which contain the same kind of fluid, repel, those which contain opposite kinds, attract each other. The attraction and re pulsion are mutual.

Another theory supposes but one kind of magnetic fluid, and explains the phenomena by supposing this to exist in excess at one pole, and in defect at the opposite pole. Either theory explains the phenomena, but that of two fluids is the most easily applied, and for that reason, solely, it is adopted.

The earth, as we shall see hereafter, resembles a huge magnet, acting upon magnetic needles in the same way that magnetized bars

When is a body magnetized according to this theory? What other theory is there? Describe the magnetic action of the earth.

do. Its magnetic poles are near the geographic poles of the earth, and the neutral line coincides very nearly with the equator. Consequently the fluid which is supposed to predominate near the north pole of the earth is called the boreal fluid, and that which is supposed to predominate near the south pole of the earth is called the austral fluid.

Because dissimilar poles attract and similar ones repel, it follows that the pole of a balanced magnetic needle which turns towards the north must contain the austral fluid, whilst the one which turns towards the south must contain the boreal fluid.

Laws of Attraction and Repulsion.

348. The following laws have been established by theory and confirmed by experiment:

1. Magnetic poles of contrary names attract, and those of the same name repel each other.

2. The forces of attraction and repulsion both vary inversely as the square of the distance between the attracting and repelling poles.

Magnetic and Magnetized Bodies.

349. A MAGNETIC BODY is one which contains the two magnetic fluids, but in a state of equilibrium, that is, balancing each other; thus, iron, steel, nickel, and cobalt, are such bodies.

MAGNETIZED BODIES also contain the two fluids, but the difference between them and magnetic bodies is, that in the former the two fluids are separated, each producing an opposite effect, whilst in the latter the fluids are combined and produce no effect. In a word, magnetic bodies are

What is the boreal fluid? The austral fluid? Which turns towards the north? Why? (348) What is the first law of magnetic attraction and repulsion? The second law? (349.) What is a Magnetic Body? Examples. What are Magnetized Bodies?

capable of being magnetized, but are not yet magnets; they present neither poles nor neutral line.

When a magnetic substance is brought into contact with one of the poles of the magnet, as the boreal pole, for example, the latter, acting by its attraction upon the austral fluid, and by its repulsion upon the boreal fluid, separates them, giving rise to poles, producing a real magnet.

If a magnetized bar be presented to a magnetic body, as an iron ring, it converts it into a magnet in the manner just described. If a second ring be presented to the first, it is in like manner converted

Fig. 240.

into a magnet, and so on for a third, fourth, &c. The magnets thus formed adhere to each other, as shown in Fig. 240. If the bar be removed, the rings cease to be magnets, the chain falls to pieces, and the rings separate. This mode of exciting magnetic phenomena is called magnetizing by induction. According to the theory of two fluids, it is in consequence of this action that a magnet is capable of attracting magnetic bodies. It first acts by induction to convert them into magnets, and then it attracts them according to the laws laid down in the last article.

Fig. 241 represents a common child's toy. A small swan made of glass has a piece of iron in its head, and on presenting to it a magnet, the swan approaches it, swimming along the surface of the water upon which it is placed. The magnet may be concealed in a

How are magnets produced? Illustrate. What is magnetic induction? Explain it on the two fluid theory. Explain the magnetic swan.

The Coercive Force.

350. The force required to separate the two fluids in a magnetic body is called the COERCIVE FORCE.

The fluids are not separable with equal ease in all bodies. In some, as, for example, in soft iron, they yield easily and separate at once; in others, as in hardened steel, for example, the fluids yield with difficulty, and a powerful magnet is required to effect the separation, and it is effected only after a greater or shorter length of time. The harder and better tempered the steel, the more difficult it becomes to separate the two fluids.

(350.) What is the Coercive Force? How is it in different bodies?

In soft iron the coercive force required to separate the fluids is very small, in hardened steel it is very great. Soft iron brought in contact with a bar magnet becomes a magnet instantly, and on being removed returns to its neutral condition, ceasing to be a magnet. With hardened steel the reverse is the case; it takes considerable force and some time to render it a magnet, and on being removed from the bar it continues to be a magnet. The force which resisted the separation of the fluids in the first instance, now acts to prevent their reunion, so that the steel magnet retains its magnetism for a long time.

II. TERRESTRIAL

MAGNETISM.-COMPASSES.

Directive Force of Magnets.

351. When a permanent magnet is balanced so that it can turn freely in a horizontal direction, it assumes, after a few oscillations, a determinate direction, which is very nearly north and south.

Fig. 242 shows the manner of balancing a needle, and indicates the north and south direction which it assumes. In this figure, as in all others illustrating the subject of magnetism, the pole which contains the austral fluid is designated by the letter A, whilst that which contains the boreal fluid is designated by the letter B.

It will be noticed that it

is the austral pole which turns towards the north, and the

Illustrate. (351.) What direction does a free magnet take? How is a needle balanced? In what other way may it be balanced?