Having thus noticed the principal facts which belong to terrestrial magnetism, we must return to the consideration of those phenomena which gradually led to a consistent magnetic theory. Gilbert observed that both smelted iron and hammered iron have the magnetic virtue, [220] though in a weaker degree than the magnet itself,[8] and he asserted distinctly that the magnet is merely an ore of iron, (lib. i. c. 16, Quod magnes et vena ferri idem sunt.) He also noted the increased energy which magnets acquire by being armed; that is, fitted with a cap of polished iron at each pole.[9] But we do not find till a later period any notice of the distinction which exists between the magnetical properties of soft iron and of hard steel;—the latter being susceptible of being formed into artificial magnets, with permanent poles; while soft iron is only passively magnetic, receiving a temporary polarity from the action of a magnet near it, but losing this property when the magnet is removed. About the middle of the last century, various methods were devised of making artificial magnets, which exceeded in power all magnetic bodies previously known.
[8] Lib. i. c. 9–13.
[9] Lib. ii. c. 17.
The remaining experimental researches had so close an historical connexion with the theory, that they will be best considered along with it, and to that, therefore, we now proceed.
CHAPTER II.
Progress of Magnetic Theory.
Theory of Magnetic Action.—The assumption of a fluid, as a mode of explaining the phenomena, was far less obvious in magnetic than in electric cases, yet it was soon arrived at. After the usual philosophy of the middle ages, the “forms” of Aquinas, the “efflux” of Cusanus, the “vapors” of Costæus, and the like, which are recorded by Gilbert,[10] we have his own theory, which he also expresses by ascribing the effects to a “formal efficiency;”—a “form of primary globes; the proper entity and existence of their homogeneous parts, which we may call a primary and radical and astral form;”—of which forms there is one in the sun, one in the moon, one in the earth, the latter being the magnetic virtue.
[10] Gilb. lib. ii. c. 3, 4
Without attempting to analyse the precise import of these expressions, we may proceed to Descartes’s explanation of magnetic phenomena. The mode in which he presents this subject[11] is, perhaps, the [221] most persuasive of his physical attempts. If a magnet be placed among iron filings, these arrange themselves in curved lines, which proceed from one pole of the magnet to the other. It was not difficult to conceive these to be the traces of currents of ethereal matter which circulate through the magnet, and which are thus rendered sensible even to the eye. When phenomena could not be explained by means of one vortex, several were introduced. Three Memoirs on Magnetism, written on such principles, had the prize adjudged[12] by the French Academy of Sciences in 1746.
[11] Prin. Phil. pars c. iv. 146.