at this point, whether he believed a "properly balanced"
terrella would be a _perpetuum mobile_.
The next induction that Gilbert made was that as the magnet possesses
verticity and turns towards the poles, so the loadstone-earth
possesses a verticity and turns on an axis fixed in direction.[84] He
could now discuss the motions of a loadstone in general, in terms of
its nature, just as an Aristotelian discussed the motion of the
elements in terms of their nature.
[84] M: pp. 68, 70-71, 97, 129, 179-180, 311, 315, 317-335
Gilbert implied (M: p. 166), that a terrella does not rotate
as Peregrinus said, due to resistance (M: p. 326), or due to
the mutual nature of coition (M: p. 166); or even to the
rotation of the earth (M: p. 332). However (M: p. 129), he
also mentioned that a terrella would revolve by itself!
But before reaching this point in his argument, Gilbert digressed to
classify the different kinds of attractions and motions which the
elements produce. In particular, he distinguished electric attraction
from magnetic coition, and pointed out the main features of electrical
attraction. Since the resultant motions were different, the essential
natures of electric and magnetic substances had to differ.
Gilbert introduced his treatment of motion by discussing the
attraction of amber. All sufficiently light solids[85] and even
liquids,[86] but not flame or air[87] are attracted by rubbed amber.
Heat from friction,[88] but not from alien sources like the sun[89] or
the flame,[90] produce this "affection." By the use of a detector
modeled after the magnetic needle, which we would call an electroscope
but which he called a "versorium,"[91] Gilbert was able to extend the
list of substances that attract like amber.[92] These Gilbert called
"electricae."[93]
[85] M: pp. 78, 82, 84, 86.
[86] M: pp. 78, 89, 91.
[87] M: pp. 89, 95.
[88] M: pp. 83, 86.
[89] M: pp. 81, 86, 87.
[90] M: pp. 80, 81, 86, 87.
[91] M: p. 79.
[92] M: pp. 77-78, 79.
[93] M: p. 78. The definition Gilbert gave of an electric
in the glossary at the beginning of his treatise was not an
experimental one: "Electricae, quae attrahunt eadem ratione
ut electrum."
Possibly as a result of testing experimentally statements like that of
St. Thomas, on the effect of garlic on a loadstone, Gilbert discovered
that the interposition of even the slightest material (except a fluid
like olive oil) would screen the attraction of electrics.[94] Hence
the attraction is due to a material cause, and, since it is invisible,
it is due to an effluvium.[95] It must be much rarer than air,[96] for
if its density were that of air or greater, it would repel rather than
attract.[97]
[94] M: pp. 86, 91, 135.
[95] M: pp. 96, 135.
[96] M: p. 89.
[97] M: pp. 90, 92, 95.
The source of the effluvia could be inferred from the properties of
the electrics. Many but not all of the electrics are transparent, but
all are firm and can be polished.[98] Since they retain the appearance
and properties of a fluid in a firm solid mass,[99] Gilbert concluded
that they derived their growth mostly from humors or were concretions
of humors.[100] By friction, these humors are released and produce
electrical attraction.[101]
[98] M: pp. 83, 84, 85.
[99] M: p. 84.
[100] M: pp. 84, 89. See also Aristotle, _op. cit._ (footnote
45), _Meteorologica_, bk. 4.
[101] M: p. 90.
This humoric source of the effluvia was substantiated by Gilbert in a
number of ways. Electrics lose their power of electrical attraction
upon being heated, and this is because the humor has been driven
off.[102] Bodies that are about equally constituted of earth and
humor, or that are mostly earth, have been degraded and do not show
electrical attraction.[103] Bodies like pearls and metals, since they
are shiny and so must be made of humors, must also emit an effluvium
upon being rubbed, but it is a thick and vaporous one without any
attractive powers.[104] Damp weather and moist air can weaken or even
prevent electrical attraction, for it impedes the efflux of the humor
at the source and accordingly diminishes the attraction.[105] Charged
bodies retain their powers longer in the sun than in the shade, for in
the shade the effluvia are condensed more, and so obscure
emission.[106]
[102] M: pp. 84, 85.
[103] M: p. 84.
[104] M: p. 90. See also p. 95.
[105] M: pp. 78, 85-86, 91. (see particularly the heated
amber experiment described on p. 86).
[106] M: p. 87.
All these examples seemed to justify the hypothesis that the nature of
electrics is such that material effluvia are emitted when electrics
are rubbed, and that the effluvia are rarer than air. Gilbert realized
that as yet he had not explained electrical attraction, only that the
pull can be screened. The pull must be explained by contact
forces,[107] as Aristotle[108] and Aquinas[109] had argued.
Accordingly, he declared, the effluvia, or "spiritus,"[110] emitted
take "hold of the bodies with which they unite, enfold them, as it
were, in their arms, and bring them into union with the
electrics."[111]
[107] M: p. 92.
[108] Aristotle, _Physics_, translated by P. H. Wicksteed and
F. M. Cornford, Loeb Classical Library, London, 1934, bk. 7,
ch. 1, 242b25.
[109] St. Thomas Aquinas, _op. cit._ (footnote 19), vol. 2,
_Physicorum Aristotelis expositio_, lib. 7, lect. 2 (In
moventibus et motis non potest procedi in infinitum, sed
oportet devenire ad aliquid primum movens immobile), cap. d,
p. 96.
[110] M: p. 94.
[111] M: p. 95.
It can be seen how this uniting action is effected if objects floating
on water are considered, for solids can be drawn to solids through the
medium of a fluid.[112] A wet body touching another wet body not only
attracts it, but moves it if the other body is small,[113] while wet
bodies on the surface of the water attract other wet bodies. A wet
object on the surface of the water seeks union with another wet object
when the surface of the water rises between both: at once, "like drops
of water, or bubbles on water, they come together."[114] On the other
hand, "a dry body does not move toward a wet, nor a wet to a dry, but
rather they seem to go away from one another."[115] Moreover, a dry
body does not move to the dry rim of the vessel while a wet one runs
to a wet rim.[116]
[112] M: p. 93.
[113] M: pp. 92, 93.
[114] M: p. 93.
[115] M: p. 94.
[116] M: p. 94.
By means of the properties of such a fluid, Gilbert could explain the
unordered coming-together that he called coacervation.[117] Different
bodies have different effluvia, and so one has coacervation of
different materials. Thus, in Gilbert's philosophy air was the earth's
effluvium and was responsible for the unordered motion of objects
towards the earth.[118]
[117] M: p. 97.
[118] M: p. 92 (see also p. 339). Although Gilbert does not
make it explicit, this would solve the medieval problem of
gravitation without resorting to a Ptolemaic universe. In
addition, since coacervation is electric, and electric forces
can be screened, it should have been possible to reduce the
downward motion of a body by screening!
The analogy between electric attraction and fluids is a most concrete
one, yet lying beneath this image is a hypothesis that is difficult to
fix into a mechanical system based upon contact forces. This is the
assumption that under the proper conditions bodies tend to move
together in order to participate in a more complete unity.[119] The
steps in electrical attraction were described as occurring on two
different levels of abstraction: first one has physical contact
through an effluvium or "spiritus" that connects the two objects
physically. Then, as a result of this contact, the objects somehow
sense[120] that a more intimate harmony is possible, and move
accordingly. Gilbert called the motion that followed contact,
attraction. However, this motion did not connote what we would call a
force:[121] it did not correspond directly to a push or pull, but it
followed from what one might term the apprehension of the possibility
of a more complete participation in a formal unity. The physical unity
due to the "spiritus" was the prelude to a formal organic unity, so
that _humor_ is "rerum omnium unitore." Gilbert's position can be best
seen in the following:[122]
Spiritus igitur egrediens ex corpora, quod ab humore aut
succo aqueo concreverat, corpus attrahendum attingit,
attactum attrahenti unitur; corpus peculiari effluviorum
radio continguum, unum effecit ex duobus: unita confluunt in
conjunctissimam convenientiam, quae attractio vulgo dicitur.
Quae unitas iuxta Pythagorae opinionem rerum omnium
principium est, per cuius participationem unaquaeque res una
dicitur. Quoniam enim nullo actio a materia potest nisi per
contactum, electrica haec non videntur tangere, sed ut
necesse erat demittitur aliquid ab uno ad aliud, quod proxime
tangat, et eius incitationis principium sit. Corpora omnia
uniuntur & quasi ferruminantur quodammodo humore ...
Electrica vero effi via peculiaria, quae humoris fusi
subtilissima sunt materia, corpuscula allectant. Aer (commune
effluvium telluris) & partes disjunctis unit, & tellus
mediante aere ad se revocat corpora; aliter quae in
superioribus locis essent corpora, terram non ita avide
appelerent.
Electrica effluvia ab aere multum differunt, & u aer telluris
effluvium est, ita electrica suahabent effluvia & propria;
peculiaribus effluviis suus cuique; est singularis ad
unitatem ductus, motus ad principium, fontem, & corpus
effluvia emittens.
A similar hypothesis will reappear in his explanation of magnetic
attraction.
[119] M: pp. 91, 92: "This unity is, according to Pythagoras,
the principle, through participation, in which a thing is
said to be one" (see footnotes 30 and 122).
[120] "Sense" is probably too strong a term, and yet the
change following contact is difficult to describe in
Gilbert's phraseology without some such subjective term. See
Gilbert's argument on the soul and organs of a loadstone, M:
pp. 309-313.
[121] M: pp. 112, 113.
[122] Gilbert, _De magnete_, London, 1600, bk. 2, ch. 2, pp.
56-57.
Following the tradition of the medieval schoolmen Gilbert started his
examination of the nature of the loadstone by pointing out the
different kinds of motion due to a magnet. The five kinds (other than
up and down) are:[123]
(1) coitio (vulgo attractio, dicta) ad unitatem magneticam
incitatio.
(2) directio in polos telluris, et telluris in mundi
destinatos terminos verticitas et consistentia.
(3) variatio, a meridiano deflexio, quem motum nos depravatum
dicimus.
(4) declinatio, infra horizontem poli magnetici descensus.
(5) motus circularis, seu revolutio.
Of the five he initially listed, three are not basic ones. Variation
and declination he later explained as due to irregularities of the
surface of the earth, while direction or verticity is the ordering
motion that precedes coition.[124] This leaves only coition and
revolution as the basic motions. How these followed from "the
congregant nature of the loadstone can be seen when the effusion of
forms has been considered."
Coition (he did not take up revolution at this point) differed from
that due to other attractions. There are two and only two kinds of
bodies that can attract: electric and magnetic.[125] Gilbert refined
his position further by arguing that one does not even have magnetic
attraction[126] but instead the mutual motion to union that he called
coition.[127] In electric attraction, one has an action-passion
relation of cause and effect with an external agent and a passive
recipient; while in magnetic coition, both bodies act and are acted
upon, and both move together.[128] Instead of an agent and a patient
in coition,[129] one has "conactus." Coition, as the Latin origin of
the term denoted, is always a concerted action. [130] This can be seen
from the motions of two loadstones floating on water.[131] The mutual
motion in coition was one of the reasons for Gilbert's rejection of
the perpetual motion machine of Peregrinus.[132]
[123] _Ibid._, ch. 1, pp. 45-46.
[124] M: pp. 110, 314.
[125] M: pp. 82, 105, 170, 172, 217.
[126] M: p. 98.
[127] M: pp. 100, 112, 113, 143, 148. It need hardly be
pointed out that coitus is not an impersonal term.
[128] M: p. 110.
[129] M: p. 110.
[130] M: pp. 109, 115, 148, 149, 155, 166, 174.
[131] M: pp. 110, 155.
[132] M: pp. 166, 332. See also footnote 84.
Magnetic coition, unlike electric attraction, cannot be screened.[133]
Hence it cannot be corporeal for it travels freely through bodies[134]
and especially magnetic bodies;[135] one can understand the action of
the armature on this basis.[136] Since coition cannot be prevented by
shielding, it must have an immaterial cause.[137]
[133] M: pp. 90, 106, 107, 108, 113, 132, 135, 136, 158. This
is, of course, contrary to modern experience.
[134] M: pp. 106, 107, 108, 114, 134, 136, 140, 162.
[135] M: pp. 106, 109, 114, 159, 162.
[136] M: pp. 137-140.
[137] M: p. 109.
Yet, unless one has the occult action-at-a-distance, change must be
caused by contact forces. Gilbert resolved the paradox of combining
contact forces with forces that cannot be shielded, by passing to a
higher level of abstraction for the explanation of magnetic phenomena:
he saw the contact as that of a form with matter.
Although Gilbert remarked that the cause of magnetic phenomena did
not fall within any of the categories of the formal causes of the
Aristotelians, he did not renounce for this reason the medieval
tradition. Actually there are many similarities between Gilbert's
explanation of the loadstone's powers and that of St. Thomas. Magnetic
coition is not due to any of the generic or specific forms of the
Aristotelian elements, nor is it due to the primary qualities of any
of their elements, nor is it due to the celestial "generans" of
terrestrial change.[138]
Relictis aliorum opinionibus de magnetis attractione; nunc
coitionis illius rationem, et motus illius commoventem
naturam docebimus. Cum vero duo sint corporum genera, quae
manifestis sensibus nostris motionibus corpora allicere
videntur, Electrica et Magnetica; Electrica naturalibus ab
humore effluviis; Magnetica formalibus efficientiis, seu
potius primariis vigoribus, incitationes faciunt. Forma ilia
singularis est, et peculiaris, non Peripateticorum causa
formalis, et specifica in mixtis, est secunda forma, non
generantium corporum propagatrix; sed primorum et praeciporum
globorum forma; et partium eorum homogenearum, non
corruptarum, propria entitas et existentia, quam nos
primariam, et radicalem, et astream appellare possumus
formam; non formam primam Aristotelis; sed singularem illam,
quae globum suum proprium tuetur et disponit. Talis in
singulis globis, Sole, lunas et astris, est una; in terra
etiam una, quae vera est ilia potentia magnetica, quam nos
primarium vigorem appellamus. Quare magnetica natura est
telluris propria, eiusque omnibus verioribus partibus,
primaria et stupenda ratione, insita; haec nec a caelo toto
derivatur procreaturve, per sympathiam, per influentiam, aut
occultiores qualitates; nec peculiari aliquo astro: est enim
suus in tellure magneticus vigor, sicut in sole et luna suae
formae; frustulumque; lunae, lunatice ad eius terminos, et
formam componit se; solarque; ad solem, sicut magnes ad
tellurem, et ad alterum magnetem, secundum naturam sese
inclinando et alliciendo. Differendum igitur de tellure quae
magnetica, et magnes; tum etiam de partibus eius verioribus,
quae magneticae sunt; et quomodo ex coitione difficiuntur.
Instead, he declared it to be due to a form that is natural and proper
to that element that he made the primary component of the earth.[139]
To understand his argument, let us briefly recall the peripatetic
theory of the elements. In this philosophy of nature each element or
simple body is a combination of a pair of the four primary qualities
that informs inchoate matter. These qualities are the instruments of
the elemental forms and determine the properties of the element. Thus
the element fire is a compound of the qualities hot and dry, and the
substantial form of fire acts through these qualities. Similarly for
the other elements, earth, water, and air: their forms determine a
proper place for each element, and a motion to that place natural to
each element.[140]
[138] M: p. 105, and Gilbert, _De magnete_, London, 1600, bk.
2 ch. 4, p. 65.
[139] M: p. 105.
[140] M: pp. 289, 322.
Gilbert had previously declared that the primary substance of the
earth is an element. Since it is an element, it has a motion natural
to it, and this motion is magnetic coition. As an Aristotelian
considered the substantial form of the element, fire, to act through
the qualities of hot and dry, and to cause an upward motion; so
Gilbert argued that the substantial form of his element, pure
loadstone, acts through the magnetic qualities and causes magnetic
coition. This motion is due to its primary form, and is natural to the
element earth.[141] It is instilled in all proper and undegenerate
parts of the earth,[142] but in no other element.[143]
[141] M: pp. 26, 68, 105, 179, 198, 307, 335, 343. For
rotation, see footnote 147.
[142] M: pp. 67, 71. That each part is informed with the
properties of the whole is an argument favoring an animistic
explanation of the nature of this form.
[143] M: p. 109.
To the medieval philosopher, the "generantia" of the occult powers of
the loadstone are the heavenly bodies. Gilbert, however, endowed the
earth with these heavenly powers which were placed in the earth in the
beginning[144] and caused all magnetic materials to conform with it
both physically and formally.[145] Such magnetic powers are the
property of all parts of the earth;[146] they give the earth its
rotating motion[147] and hold the earth together in spite of this
motion.[148]
[144] M: pp. 111, 188.
[145] M: pp. 67, 105, 179, 183.
[146] M: pp. 101, 105, 217.
[147] M: pp. 179, 304, 305, 311, 322, 326, 328, 330-334,
338-343.
[148] M: pp. 142, 179; see also electric attraction, p. 97.
Indeed, each of the main stellar bodies, sun, moon, stars, and earth,
has such a form or principle unique to itself that causes its parts
not only to conform with itself but to revolve.[149] Thus, if one
removes a piece of the moon from this body, it will tend to align
itself with the moon and then to return to its proper place; and a
fragment of the sun would similarly tend to return after proper
orientation.[150] Moreover, there is a farther-ranging, though weaker,
mutual action of the heavenly bodies so that one has a causal
hierarchy of these specific conforming powers. The form of the sun is
superior to that of the inferior globes and is responsible for the
order and regularity of planetary orbits.[151] In like manner, the
moon is responsible for the tides of the ocean.[152]
[149] M: pp. 308, 317-343.
[150] M: pp. 106, 340.
[151] M: pp. 308, 309, 311, 330, 333, 344, 347.
[152] M: pp. 136, 334, 345.
By virtue of the causal hierarchy of forms, the loadstone acquires its
magnetic powers from the earth.[153] As the earth has its natural
parts, so has the stone.[154] Although the geometrical center of a
terrella is the center of the magnetic forces,[155] objects do not
tend to move to the center but to its poles,[156] where the magnetic
energy is most conspicuous.[157] However, in a sense, the energy is
everywhere equal: the virtue is spread throughout the entire mass of
the loadstone,[158] and all the parts direct the forces to the
poles.[159] The poles become the "thrones" of the magnetic
powers.[160] On the other hand, the directive force is stronger where
coition is weaker and accordingly, verticity is most prominent at the
equator.[161]
[153] M: pp. 184-186, 190, 232. This is not quite the same
argument as that the powers of the loadstone are identical
with those of the earth. See footnote 78.
[154] M: pp. 125, 180.
[155] M: p. 151.
[156] M: pp. 121, 150.
[157] M: pp. 115, 151, 165.
[158] M: pp. 106, 118, 151, 191, 205, 221, 243.
[159] M: pp. 116, 117, 119, 131, 183, 188, 221.
[160] M: p. 31.
[161] M: pp. 116, 151, 200.
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.