This is more evidently the case in a theory like that of light, where the motions of a subsensible medium, the ether, are presented to the mind.  But no theory escapes the condition.  Newton took care not to encumber the idea of gravitation with unnecessary physical conceptions; but we know that he indulged in them, though he did not connect them with his theory.  But even the theory, as it stands, did not enter the mind as a revelation dissevered from the world of experience.  The germ of the conception that the sun and planets are held together by a force of attraction is to be found in the fact that a magnet had been previously seen to attract iron.  The notion of matter attracting matter came thus from without, not from within.  In our present lecture the magnetic force must serve as the portal into a new domain; but in the first place we must master its elementary phenomena.

The general facts of magnetism are most simply illustrated by a magnetized bar of steel, commonly called a bar magnet.  Placing such a magnet upright upon a table, and bringing a magnetic needle near its bottom, one end of the needle is observed to retreat from the magnet, while the other as promptly approaches.  The needle is held quivering there by some invisible influence exerted upon it.  Raising the needle along the magnet, but still avoiding contact, the rapidity of its oscillations decreases, because the force acting upon it becomes weaker.  At the centre the oscillations cease.  Above the centre, the end of the needle which had been previously drawn towards the magnet retreats, and the opposite end approaches.  As we ascend higher, the oscillations become more violent, because the force becomes stronger.  At the upper end of the magnet, as at the lower, the force reaches a maximum; but all the lower half of the magnet, from E to S (fig. 22), attracts one end of the needle, while all the upper half, from E to N, attracts the opposite end.  This doubleness of the magnetic force is called polarity, and the points near the ends of the magnet in which the forces seem concentrated are called its poles.

[Illustration:  Fig. 22.]

What, then, will occur if we break this magnet in two at the centre E?  Shall we obtain two magnets, each with a single pole?  No; each half is in itself a perfect magnet, possessing two poles.  This may be proved by breaking something of less value than the magnet—­the steel of a lady’s stays, for example, hardened and magnetized.  It acts like the magnet.  When broken, each half acts like the whole; and when these parts are again broken, we have still the perfect magnet, possessing, as in the first instance, two poles.  Push your breaking to its utmost sensible limit—­you cannot stop there.  The bias derived from observation will infallibly carry you beyond the bourne of the senses, and compel you to regard this thing that we call magnetic polarity as resident in the ultimate particles of the steel.  You come to the conclusion that each molecule of the magnet is endowed with this polar force.