_Apparatus._ A thin bar magnet, B M (No. 21); iron filings; a sheet of paper. Fig. 9 shows simply the edge of B M and the paper. B M should be magnetized as directed in Exp. 9. [Illustration: Fig. 9.] =41. Directions.= (A) Sprinkle some iron filings upon a sheet of paper. (B) Bring one pole of B M in contact with the filings (Fig. 9), and lightly sweep it through them several times, always in the same direction. Are the filings _simply_ pushed about? (C) Do the same with a stick, and compare the result with that produced with B M. _=42. Theory of Magnetism; Magnetic Saturation.=_ This bringing into line the particles of iron indicates that each particle became a magnet. This experiment should aid in understanding what is thought to take place when steel is magnetized. The pile of filings represents the body to be magnetized, and each little filing stands for a particle of that body. A bar of steel is composed of extremely small particles, called _molecules_. They are very close together and do not move from place to place as easily as the pieces of filings. A magnet, however, when properly rubbed upon the steel, seems to have power to make the molecules point in the same direction. This produces an effect upon the whole bar. Each molecule of the steel is supposed to be a magnet. When these little magnets pull together, the whole bar becomes a strong magnet. When a magnet is jarred, and the little magnetized molecules are mixed again, they pull in all sorts of directions upon each other. This lessens the attraction for outside bodies. Steel is said to be _saturated_, when it contains as much magnetism as possible. A piece of steel becomes slightly longer when magnetized. It is thought, by many, that there is a current of electricity around each molecule, making a little magnet of it. (See electro-magnets.) =EXPERIMENT 19. To find whether soft iron will permanently retain magnetism.= _Apparatus._ A piece of soft iron wire, 3 or 4 in. (7.5 to 10 cm.) long (No. 4); the horseshoe magnet, H M; iron filings; flat cork, F C (No. 2), and the dish of water used in Exp. 10 (Fig. 4). =43. Directions.= (A) Magnetize the wire (Exp. 9). Notice that the wire clings strongly to H M. (B) Test the lifting power of the little wire magnet by seeing about how many iron filings its poles will raise. (C) Test the pointing power (§ 25) of the wire by floating it on F C (Fig. 4). (D) Holding one end of the wire in the hand, thoroughly jar it by striking the other end several times against a hard surface. (E) Test the lifting and pointing powers, as in B and C.