At one time it was possible to say that magnetism could not be produced except by antecedent magnetism; that there was no known way of generating it spontaneously; yet that, since it undoubtedly occurs in certain rocks of the earth, it must have come into existence somehow, at date unknown. It could also be said, and it can be said still, that, given an initial magnet, any number of others can be made, without loss to the generating magnet. By influence or induction exerted by proximity on other pieces of steel, the properties of one magnet can be excited in any number of such pieces,—the amount of magnetism thus producible being infinite; that is, being strictly without limit, and not dependent at all on the very finite strength of the original magnet, which indeed continues unabated. It is just as if magnetism were not really manufactured at all, but were a thing called out of some infinite reservoir: as if something were brought into active and prominent existence from a previously dormant state. And that indeed is the fact. The process of magnetisation, as conducted with a steel magnet on other pieces of previously inert steel, in no case really generates new lines of magnetic force, though it appears to generate them. We now know that the lines which thus spring into corporeal existence, as it were, are essentially closed curves or loops, which cannot be generated; they can be expanded or enlarged to cover a wide field, and they can be contracted or shrunk up into insignificance, but they cannot be created, they must be pre-existent; they were in the non-magnetised steel all the time, though they were so small and ill-arranged that they had no perceptible effect whatever; they constituted a potentiality for magnetism; they existed as molecular closed curves or loops, which, by the operation called magnetisation, could, some of them, be opened out into loops of finite area and spread out into space, where they are called "lines of force." They then constitute the region called a magnetic field, which remains a seat of so-called "permanent" magnetic activity, until by lapse of time, excessive heat, or other circumstance, they close up again; and so the magnet, as a magnet, dies. The magnetism itself, however, has not really died, it has a perpetual existence; and a fresh act of magnetisation can recall it, or something indistinguishable from it, into manifest activity again; so that it, or its equivalent, can once more interact with the rest of material energies, and be dealt with by physicists, or subserve the uses of humanity. Until that time of re-appearance its existence can only be inferred by the thought of the mathematician: it is indeed a matter of theory, not necessarily recognised as true by the practical man.