Book II
The justly famous Second Book contains Gilbert’s electrical work and, as is generally known, the second chapter thereof is the earliest ever published on electricity. We are here introduced to Gilbert’s versorium—a rotating needle electroscope[33]—and are given the results of his many experimental observations[34] and the opinions of others relative to magnetic coition or attraction. We find, throughout the whole of the second chapter, the first systematic study of amber, with an interesting list of electrics and the recognition of a group of anelectrics—non-electrics. After pointing out the different kinds of attractions admitted by Galen and other ancient writers, we are told that:
“Only feeble power of attraction is possessed by some electrics (all which have their own distinct effluvia) in favouring dry atmosphere: observable in midwinter while the atmosphere is very cold, clear and thin, when the electric effluvia of the earth offer less impediment and electric bodies are harder—that these bodies then draw, as well, all metals, wood, leaves, stones, earths, even water and oil, in short, whatever things appeal to our senses or are solid.
“All bodies are attracted by electrics, save those which are afire or flaming or extremely rarefied.
“Very many electric bodies do not attract at all, unless they are first rubbed. An ordinary piece of amber does not attract by heat, even when brought to the flaming point, but it attracts by friction, without which latter few bodies give out their true natural electric emanation and effluvium. By friction, the amber is made moderately hot and also smooth; these conditions must in most cases concur; but a large polished piece of amber or of jet attracts even without friction, though not so strongly; yet, if it be carefully brought nigh to a flame or a red coal, it does not attract corpuscles; further, the sun’s heat heightened by means of a burning-glass imparts no power to amber, for it dissipates and spoils all the electric effluvia. Again, flaming sulphur and burning sealing-wax (of lac) do not attract.
“The loadstone, though susceptible of very high polish, has not the electric attraction. The force does not come through the lustre proceeding from the rubbed and polished electric; for the vincentina, diamond and pure glass attract when they are rough. Effluvia that attract but feebly when the weather is clear produce no motion at all when it is cloudy. For the effluvium from rock crystal, glass, diamond—substances very hard and very highly compressed—there is no need of any notable outflow of substance. Such an electric as sound cypress-wood, after a moment’s friction, emits powers subtle and fine, far beyond all odours; but sometimes an odour is also emitted by amber, jet, sulphur, these bodies being more readily resolved; hence it is that, usually, they attract after the gentlest friction because their effluvia are stronger and more lasting.
“Rock crystal, mica, glass, and other electric bodies do not attract if they be burned or highly heated, for their primordial humour is destroyed by the heat, is altered, is discharged as vapour. All bodies that derive their origin principally from humours and that are firmly concreted attract all substances whether humid or dry; but bodies consisting mostly of humour and not firmly compacted by nature, wherefore they do not stand friction but either fall to pieces or grow soft or are sticky, do not attract corpuscles.
“Electrical movements come from the matter (materia) but magnetic from the prime form (forma). Moist air blown from the mouth, moisture from steam, or a current of humid air from the atmosphere chokes the effluvium. But olive oil that is light and pure does not prevent it; and, if a sheet of paper or a linen cloth be interposed, there is no movement. But loadstone, neither rubbed nor heated, and even though it be thoroughly drenched with liquid, and whether in air or water, attracts magnetic bodies, and that though solidest bodies or boards or thick slabs of stone or plates of metal stand between.
“Electrics attract all things save flame and objects aflame, and thinnest air ... for it is plain that the effluvia are consumed by flame and igneous heat ... yet they draw to themselves the smoke from an extinguished candle; and, the lighter the smoke becomes as it ascends, the less strongly is it attracted, for substances that are too rare do not suffer attraction.”
This Chapter II ends with the following explanation of the difference between electric and magnetic bodies, viz. all magnetic bodies come together by their joint forces (mutual strength); electric bodies attract the electric only, and the body attracted undergoes no modification through its own native force, but is drawn freely under impulsion in the ratio of its matter (composition). Bodies are attracted to electrics in a right line toward the centre of electricity: a loadstone approaches another loadstone on a line perpendicular to the circumference only at the poles, elsewhere obliquely and transversely, and adheres at the same angles. The electric motion is the motion of conservation of matter; the magnetic is that of arrangement and order. The matter of the earth’s globe is brought together and held together by itself electrically. The earth’s globe is directed and revolves magnetically; it both coheres, and, to the end it may be solid, it is in its interior fast joined.
Of the other interesting chapters in this Book II, attention is called more particularly to:
Chap. IV. “Of the strength of a loadstone and its form: the cause of coition.” The magnetic nature is proper to the earth and is implanted in all its real parts ... there is in the earth a magnetic strength or energy (vigour) of its own ... thus we have to treat of the earth, which is a magnetic body, a loadstone. An iron rod held in the hand is magnetized in the end where it is grasped and the magnetic force travels to the other extremity, not along the surface only but through the inside, through the middle.... Iron instantly receives from the loadstone verticity and natural conformity to it, being absolutely metamorphosed into a perfect magnet. As soon as it comes within the loadstone’s sphere of influence it changes instantly and has its form renewed, which before was dormant and inert, but now is quick and active.
Chaps. VI and XXVII illustrate the Orbis Virtutis (Orb of Virtue, or the magnetic atmosphere surrounding both earth and loadstone alike), showing how the earth and loadstone conform magnetic movements, the centre of the magnetic forces of the earth being the earth’s centre and in the terrella the terrella’s centre. All loadstones alike, whether spherical or oblong, have the selfsame mode of turning to the poles of the world ... whatever the shape, verticity is present and there are poles.
Chap. VII. “Of the potency of the magnetic force, and of its spherical extension.” The magnetic energy is not hindered by any dense or opaque body, but goes out freely and diffuses its force every whither: in the case of the terrella, and in a spherical loadstone, it extends outside the body in a circle, but, in the case of an oblong loadstone, it extends into an area of form determined by the shape of, and is everywhere equidistant from, the stone itself.
Chap. XIII. “Of the magnetic axis and poles.”
Chap. XV. “The magnetic force imparted to iron is more apparent in an iron rod than in an iron sphere or cube, or iron in any other shape.”
Chap. XVI. “Motion is produced by the magnetic force through solid bodies interposed: of the interposition of a plate of iron.”
Chaps. XVII-XXII. Herein are detailed as many as twelve different experiments to prove the increased efficiency of armed loadstones.
Chap. XXV. “Intensifying the loadstone’s forces.” Magnetic bodies can restore soundness (when not totally lost) to magnetic bodies, and can give to some of them powers greater than they originally had; but to those that are by their nature in the highest degree perfect, it is not possible to give further strength.
Chap. XXVIII. “A loadstone does not attract to a fixed point or pole only, but to every part of a terrella, except the equinoctial line.”
Chap. XXIX. “Of differences of forces dependent on quantity or mass.” Four experiments.
Chaps. XXXVIII and XXXIX are the last, and they treat of the attractions of other bodies and of mutually repellant bodies. All electrics attract objects of every kind: they never repel or propel.
In the preceding Chapter XXXV, Gilbert had alluded to the perpetual-motion engine actuated by the attraction of a loadstone, which we have given an account of at Peter Peregrinus, A.D. 1269.