Chapter 4

The strength of a loadstone depends upon its shape and mass. A bar magnet has greater powers than a spherical one because it tends to concentrate the magnetic powers more in the ends.[162] For a given purity and shape, the heavier the loadstone, the greater its strength.[163] A loadstone has a maximum degree of magnetic force that cannot be increased.[164] However, weaker ones can be strengthened by stronger ones.[165] Similarly, the shape and weight of the iron determine the magnetic force in coition.[166]

[162] M: pp. 131, 132, 153-158.

[163] M: pp. 141, 152, 153, 158, 161, 191, 222.

[164] M: p. 146.

[165] M: p. 165.

[166] M: p. 153.

The formal forces of a loadstone emanate in all directions from it,[167] but there is a bound to it that Gilbert called the "orbis virtutis."[168] The shape of this "orbis virtutis" is determined by the shape of the stone.[169] This insensible effusion is analogous to the spreading of light that reveals its presence only by opaque bodies.[170] Similarly, the magnetic forms are effused from the stone,[171] and can only reveal their presence by coition with another loadstone or by "awakening" magnetic bodies within the "orbis virtutis."[172] Unmagnetized iron that comes within the "orbis virtutis" is altered, and the magnetic virtue renews a form that is already potentially in the iron.[173] The formal energy is drawn not only from the stone but from the iron.[174] This is not generation, or alteration in the sense of a new impressed quality, but alteration in the sense of the entelechy or the activation of a form potentially present.[175] Those bodies magnetized by coming within the "orbis virtutis" have in turn an efflux of their own.[176] Iron can also receive verticity directly from the earth without the intervention of an ordinary loadstone.[177] Such verticity can be expelled and annulled by the presence of another loadstone.[178]

[167] M: pp. 121, 123, 124, 304, 305, 306, 307, 309.

[168] Gilbert defined the _orbis virtutis_ in the glossary at the beginning of his treatise as, "... totum illud spatium, per quod quaevis magnetis virtus extenditur." This is the core of the difference between electric and magnetic forces. The substantial form of an electric could not be "effused," but was "imprisoned" in matter (as the Neoplatonic soul in the human body); while the primary form of a magnet did not require a material carrier and its effusion was similar to the propagation of a species in light.

[169] M: pp. 124, 150, 151.

[170] M: pp. 123, 307.

[171] M: pp. 304-307. See also p. 310, where it is stated that the sun and earth could awaken souls.

[172] M: pp. 101, 110, 112, 123, 148, 149, 304, 305. This awakening of the iron within the "orbis virtutis" is comparable (pp. 216, 350) to the birth of a child under the influence of the stars.

[173] M: pp. 110, 111, 112, 189, 216, 217. See also footnote 36.

[174] M: p. 106.

[175] M: pp. 106, 109, 110.

[176] M: pp. 113, 114.

[177] M: pp. 190, 192, 210-216.

[178] M: p. 209.

Although one does not normally find iron to be magnetized, a loadstone always has some magnetism. That two bodies such as iron and loadstone should have different properties is the result of the loss of a form by the iron, but this form is still potentially present in the iron. The iron that has been obtained from an ore has been deformed,[179] for it has been placed "outside its nature" by the fire.[180] The nature has not been removed, since, once the iron has cooled, the confused form can be reformed by a loadstone. [181] The latter "awakens" the proper form of iron.[182] After smelting, the magnetized iron may manifest stronger powers than a loadstone of equal weight, but this is because the primary matter of the earth is purer in the iron than in the loadstone.[183] If fire does not deform a loadstone too much, it can be remagnetized,[184] but a burnt loadstone cannot be reformed.[185] Corruption from external causes may also deform a loadstone or iron so that it can not be magnetized.[186] Bodies mixed with the degenerate substance of the earth or with aqueous humor spoilt by contamination with earth, do not show either electric attraction or magnetic coition.[187]

[179] M: pp. 107, 110, 111.

[180] M: p. 108.

[181] M: pp. 111, 112, 113.

[182] M: pp. 109, 111, 112, 148, 149.

[183] M: pp. 112, 149.

[184] M: pp. 142, 189.

[185] M: p. 190.

[186] M: pp. 85, 105, 113, 143, 226.

[187] M: p. 84.

In a manner suggestive of Peregrinus, Gilbert wrote that, "magnetic bodies seek formal unity."[188] Thus a dissected loadstone not only tends to come back together, as in the unordered coacervation of electric attraction, but to restore the organization it had before dissection.[189] Accordingly, opposite poles appear on the interfaces of the sections, not "from an opposition" but from "a concordance and a conformance."[190] This ensures that when the parts are joined together again, they have the same orientation as before. Gilbert compared this power of restoring the original loadstone with that of a plant's vital power under the process of cutting and grafting; the plant can be revived only when the parts are in a certain order.[191]

[188] M: p. 186.

[189] M: pp. 185-188. See also footnote 31.

[190] M: pp. 186, 193.

[191] M: pp. 199-200.

A hypothesis similar to that used to explain electric attraction lay beneath the explanation of magnetic coition: that bodies brought into contact will move together. In electric attraction, the contact is material and due to the "spiritus" from the electric body; in magnetic coition, it is formal and depends on the action of a primary form that spreads from a magnetized body to its limit of effusion, the "orbis virtutis." If iron is inside the "orbis virtutis," the two bodies "enter into alliance and are one and the same"[192] for within it "they have absolute continuity, and are joined by reason of their accordance, albeit the bodies themselves be separated."[193]

Gilbert's treatment of coition can be analyzed into the same two steps as can electric attraction. First occurs a contact, which in this case is not physical but formal, and from this initial formal contact follows movement to a more complete unity. Both the contact and the movement to unity are described on the same level of abstraction, instead of on two different levels as in electric attraction. Again one does not find any clear-cut concept of force as a push or pull,[194] but instead, a motion to a formal unity, this time a cooperative motion. The parts of a magnetic body are in greater harmony when they are assembled in a certain pattern and so they move accordingly.

[192] M. p. 111.

[193] M: p. 112.

[194] See, however, M: pp. 112, 113.

As to the nature of the primary form itself, Gilbert agreed with Thales that it is like a soul,[195] "for the power of self-movement seems to betoken a soul."[196] With Galen and St. Thomas he placed the form of the loadstone superior to that of inanimate matter.[197] In a sense, Gilbert even made it superior to organic matter, for it is incapable of error.[198] Like the soul, the primary form cannot be fragmented; when a loadstone is divided, one does not separate the poles but each part acquires its own poles and an equator.

[195] M: pp. 109, 312.

[196] M: p. 109.

[197] M: p. 309.

[198] M: pp. 311-312.

Like the soul, fire does not destroy it.[199] Like the soul of astral bodies, and of the earth itself, it produces complex but regular motions; the motion of two loadstones on water offers such an example.[200] Like the soul of a newborn child, whose nature depends on the configuration of the heavens, the properties in the newly awakened iron depend upon its position in the "orbis virtutis."[201]

Whence Gilbert declared:

... the earth's magnetic force and the animate form of the globes, that are without senses, but without error ... exert an unending action, quick, definite, constant, directive, motive, imperant, harmonious through the whole mass of matter; thereby are the generation and the ultimate decay of all things on the superficies propagated.[202] The bodies of the globes ... to the end that they might be in themselves, and in their nature endure, had need of souls to be conjoined to them, for else there were neither life, nor prime act, nor movement, nor unition, nor order, nor coherence, nor _conactus_, nor _sympathia_, nor any generation nor alteration of seasons, and no propagation; but all were in confusion....[203] Wherefore, not with reason, Thales ... declares the loadstone to be animate, a part of the animate mother earth and her beloved offspring.[204]

Gilbert ended book 5 of his treatise on the magnet with a persuasive plea for his magnetic philosophy of the cosmos, yet his conceptual scheme was not too successful an induction in the eyes of his contemporaries. In particular the man from whom the Royal Society took the inspiration for their motto, "Nullius in verba," did not value his magnetic philosophy very highly. Whether Francis Bacon was alluding to Gilbert when he expounded his parable of the spider and the ant[205] is not explicit, but he certainly had him in mind when he wrote of the Idols of the Cave and the Idols of the Theater.[206]

[199] M: p. 108.

[200] M: p. 110.

[201] M: p. 216.

[202] M: p. 311.

[203] M: pp. 310, 311.

[204] M: p. 312.

[205] Francis Bacon, _op. cit._ (footnote 42), vol. 1, _Novum organum_, bk. 1, ch. 95, p. 306.

[206] _Ibid._, ch. 54 and ch. 64 (pp. 259 and 267).

Few of the subsequent experimenters and writers on magnetism turned to Gilbert's work to explain the effects they discussed. Although both his countrymen Sir Thomas Browne[207] and Robert Boyle[208] described a number of the experiments already described by Gilbert and even used phrases similar to his in describing them, they tended to ignore Gilbert and his explanation of them. Instead, both turned to an explanation based upon magnetic effluvia or corpuscles. The only direct continuation of Gilbert's _De magnete_ was the _Philosophia magnetica_ of Nicolaus Cabeus.[209] The latter sought to bring Gilbert's explanation of magnetism more directly into the fold of medieval substantial forms.

[207] Sir Thomas Browne, _Pseudodoxia epidemica_, ed. 3, London, 1658, bk. 2, ch. 2, 3, 4.

[208] Robert Boyle, _Experiments and notes about the mechanical production of magnetism_, London, 1676.

[209] Nicolaus Cabeaus, _Philosophia magnetica_, Ferarra, 1629.

However, Gilbert's efforts towards a magnetic philosophy did find approval in two of the men that made the seventeenth century scientific revolution. While Galileo Galilei[210] was critical of Gilbert's arguments as being unnecessarily loose, he nevertheless saw in them some support for the Copernican world-system. Johannes Kepler[211] found in Gilbert's explanation of the loadstone-earth a possible physical framework for his own investigations on planetary motions.

[210] Galileo Galilei, _Dialogue on the great world systems_, in the translation of T. Salusbury, edited and corrected by G. de Santillana, University of Chicago Press, 1953, pp. 409-423.

[211] Cassirer, _op. cit._ (footnote 3), vol. 1, p. 359-367.

Yet Galileo and Kepler had moved beyond Gilbert's world of intellectual experience. They were no longer concerned with determining the nature of material things in order to explain their qualities. Instead, they had passed into the realm of the mathematical relations of kinematics: quantitative law had replaced qualitative experience of cause and effect. Gilbert had some intimations of the former, but he was primarily concerned with explaining magnetism in terms of substance and attribute. He had to ascertain the nature of the loadstone and of the earth in order to explain their properties and their motions. He even went further and explained the nature of the form of the loadstone.

His method of determining the nature of a substance was a rather primitive one--it was not by a process of induction and deduction, nor by synthesis and analysis, nor by "resolutio" and "compositio," but by the use of analogies. He compared the natural history of metals and rocks with that of plants, and gave the two former the same kind of principle as the last. He determined the nature of the entity behind electric attraction by finding that such attractions could be screened, and hence it had to be corporeal. After comparing this "corporeal" attraction with that of the surface forces of a fluid, he concluded that the entity was a subtle fluid. He determined the nature of the entity behind magnetic coition by (incorrectly) finding that it cannot be screened, and hence the cause had to be a formal one. Since both stars and the loadstone can carry out regular motions, and stars had souls, the form of the loadstone had to be a soul. The method of analogy was used again in his comparison of the properties of a magnetized needle placed over a terrella with the properties of a compass placed over the earth, whence he concluded the earth to be a giant loadstone. Since the earth resembled the other celestial globes, it had to have, the circular inertia of these globes.[212] As for his magnetic experiments to show physically that the earth moved, and his unbridled speculations on the "animae" of the celestial globes, one is inclined to agree with Bacon's estimate of his magnetic philosophy.

One might consider Gilbert's book as a Renaissance recasting of Aristotle's _De caelo_ with the earth in the role of a heavenly body. So it might well be, for Gilbert was still concerned with distinguishing the nature of the heavenly body, earth, that caused the coitional and revolving motions, from those natures for which up and down, and coacervation were the natural motions. Because the natural motions were different, the natures had to be different, and these different natures led to a universe and a concept of space neither of which were Aristotelian. One no longer had a central reference point for absolute space; there was no "motor essentialis" focused upon the earth but one had only the mutual motion of the heavenly bodies. The natural distinction between heaven and earth was gone, for the earth was no longer an inert recipient but a source of wonder, and so the stage was set for the universe of Giordano Bruno.[213] The Aristotelian philosophy of nature was used to justify a new cosmology, but there was no break with the past such as one finds in Galileo and Kepler. Instead he followed the chimera of the world organism, as Paracelsus had, and of the world soul, as Bruno had. Consequently Gilbert's physiology did not enter into the main stream of science.

[212] Because the earth has the same nature as a celestial globe, its revolution and circular inertia require no more explanation than those of any other heavenly body.

[213] One wonders if Bruno might not have been another of the stimuli for Gilbert. The latter's interest in magnetism began shortly before Bruno visited England and lectured on his interpretation of the Copernican theory.

Yet this is not to deny Gilbert's services to natural philosophy. Although not all of his experimental distinction between electric and magnetic forces has been retained, still, some of it has. His "orbis virtutis" was to become a field of force, and his class of electrics, insulators of electricity. His practice of arming a loadstone was to be of considerable importance in the period before the invention of the electromagnet. His limited recognition of the mutual nature of forces and their quantitative basis in mass was ultimately to appear in Newton's second and third laws of motion. In spite of the weaknesses of the method of analogy, Gilbert's experimental model of the terrella to interpret the earth's magnetism was as much a contribution to scientific method as to the theory of magnetism.

Consequently, in spite of an explanation of electricity and magnetism that one would be amused to find in a textbook today, we can still read his _De magnete_ with interest and profit. But more important than his scientific speculations, is the insight he can give us into a Renaissance philosophy of nature and its relation to medieval thought. One does not find in _De magnete_ a prototype of modern physical science in the same sense one can in the writings of Galileo and Kepler. Instead one finds here a full-fledged example of an earlier kind of science, and this is Gilbert's main value to the historian today.