| Whittaker | |||||||||
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| History of Theories of Aether and Electricity | |||||||||
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Electric and magnetic Science prior to the introduction of the potentials
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THE magnetic discoveries of Peregrinus and Gilbert, and the
vortex-hypothesis by which Descartes had attempted to explain
them,[1] had raised magnetism to the rank of a separate science
by the middle of the seventeenth century. The kindred science
of electricity was at that time in a less developed state ; but it
had been considerably advanced by Gilbert, whose researches in
this direction will now be noticed. For two thousand years the attractive power of amber had been regarded as a virtue peculiar to that substance, or possessed by at most one or two others. Gilbert proved [2] this view to be mistaken, showing that the same effects are induced by friction in quite a large class of bodies ; among which he mentioned glass, sulphur, sealing-wax, and various precious stones. A force which was manifested by so many different kinds of matter seemed to need a name of its own; and accordingly Gilbert gave to it the name electric, which it has ever since retained. Between the magnetic and electric forces Gilbert remarked many distinctions. The lodestone requires no stimulus of friction such as is needed to stir glass and sulphur into activity. The lodestone attracts only magnetizable substances, whereas electrified bodies attract everything. The magnetic attraction between two bodies is not affected by interposing a sheet of paper, or a linen cloth, or by immersing the bodies in water ; whereas the electric attraction is readily destroyed by screens. Lastly, the magnetic force tends to arrange bodies in definite orientations ; while the electric force merely tends to heap them together in shapeless clusters. These facts appeared to Gilbert to indicate that electric phenomena are due to something of a material nature, which under the influence of friction is liberated from the glass or amber in which under ordinary circumstances it is imprisoned. In support of this view he adduced evidence from other quarters. Being a physician, he was well acquainted with the doctrine that the human body contains various humours or kinds of moisture-phlegm, blood, choler, and melancholy, - which, as they predominated, were supposed to determine the temper of mind; and when he observed that electrifiable bodies were almost all hard and transparent, and therefore (according to the ideas of that time) formed by the consolidation of watery liquids, he concluded that the common menstruum of these liquids must be a particular kind of humour, to the possession of which the electrical properties of bodies were to be referred. Friction might be supposed to warm or otherwise excite or liberate the humour, which would then issue from the body as an effluvium and form an atmosphere around it. The effluvium must, he remarked, be very attenuated, for its emission cannot be detected by the senses. The existence of an atmosphere of effluvia round every electrified body might indeed have been inferred, according to Gilbert's ideas, from the single fact of electric attraction. For he believed that matter cannot act where it is not ; and hence if a body acts on all surrounding objects without appearing to touch them, something must have proceeded out of it unseen. The whole phenomenon appeared to him to be analogous to the attraction which is exercised by the earth on falling bodies. For in the latter case he conceived of the atmospheric air as the effluvium by which the earth draws all things downwards to itself. Gilbert's theory of electrical emanations commended itself generally to such of the natural philosophers of the seventeenth century as were interested in the subject ; among whom were numbered Niccolo Cabeo (b. 1585, d. 1650), an Italian Jesuit who was perhaps the first to observe that electrified bodies repel as well as attract ; the English royalist exile, Sir Kenelm Digby (b. 1603, d. 1665); and the celebrated Robert Boyle (b. 1627, d. 1691). There were, however, some differences of opinion as to the manner in which the effluvia acted on the small bodies and set them in motion towards the excited electric; Gilbert himself had supposed the emanations to have an inherent tendency to reunion with the parent body ; Digby likened their return to the condensation of a vapour by cooling ; and other writers pictured the effluvia as forming vortices round the attracted bodies in the Cartesian fashion. There is a well-known allusion to Gilbert's hypothesis in Newton's Opticks. [3] " Let him also tell me, how an electrick body can by friction emit an exhalation so rare and subtle,[4] and yet so potent, as by its emission to cause no sensible diminution of the weight of the electrick body, and to be expanded through a sphere, whose diameter is above two feet, and yet to be able to agitate and carry up leaf copper, or leaf gold, at a distance of above a foot from the electrick body ? " It is, perhaps, somewhat surprising that the Newtonian doctrine of gravitation should not have proved a severe blow to the emanation theory of electricity ; but Gilbert's doctrine was now so firmly established as to be unshaken by the overthrow of the analogy by which it had been originally justified. It was, however, modified in one particular about the beginning of the eighteenth century. In order to account for the fact that electrics are not perceptibly wasted away by excitement, the earlier writers had supposed all the emanations to return ultimately to the body which had emitted them ; but the corpuscular theory of light accustomed philosophers to the idea of emissions so subtle as to cause no perceptible loss ; and after the time of Newton the doctrine of the return of the electric effluvia gradually lost credit. Newton died in 1727. Of the expositions of his philosophy which were published in his lifetime by his followers, one at least deserves to be noticed for the sake of the insight which it affords into the state of opinion regarding light, heat, and electricity in the first half of the eighteenth century. This was the Physices elementa mathematica experimentis confirmata of Wilhelm Jacob s'Gravesande (b. 1688, d. 1742), published at Ley den in 1720. The Latin edition was afterwards reprinted several times, and was, moreover, translated into French and English : it seems to have exercised a considerable and, on the whole, well-deserved influence on contemporary thought. s'Gravesande supposed light to consist in the projection of corpuscles from luminous bodies to the eye ; the motion being very swift, as is shown by astronomical observations. Since many bodies, e.g. the metals, become luminous when they are heated, he inferred that every substance possesses a natural store of corpuscles, which are expelled when it is heated to incandescence ; conversely, corpuscles may become united to a material body ; as happens, for instance, when the body is exposed to the rays of a fire. Moreover, since the heat thus acquired is readily conducted throughout the substance of the body, he concluded that corpuscles can penetrate all substances, however hard and dense they be. Let us here recall the ideas then current regarding the nature of material bodies. From the time of Boyle (1626-1691) it had been recognized generally that substances perceptible to the senses may be either elements or compounds or mixtures ; the compounds being chemical individuals, distinct from mere mixtures of elements. But the substances at that time accepted as elements were very different from those which are now known by the name. Air and the calces [5] of the metals figured in the list, while almost all the chemical elements now recognized were omitted from it ; some of them, such as oxygen and hydrogen, because they were as yet undiscovered, and others, such as the metals, because they were believed to be compounds. Among the chemical elements, it became customary after the time of Newton to include light-corpuscles. [6] That something which is confessedly imponderable should ever have been admitted into this class may at first sight seem surprising. But it must be remembered that questions of ponderability counted for very little with the philosophers of the period. Three quarters of the eighteenth century had passed before Lavoisier enunciated the fundamental doctrine that the total weight of the substances concerned in a chemical reaction is the same after the reaction as before it. As soon as this principle came to be universally applied, light parted company from the true elements in the scheme of chemistry. We must now consider the views which were held at this time regarding the nature of heat. These are of interest for our present purpose, on account of the analogies which were set up between heat and electricity. The various conceptions which have been entertained concerning heat fall into one or other of two classes, according as heat is represented as a mere condition producible in bodies, or as a distinct species of matter. The former view, which is that universally held at the present day, was advocated by the great philosophers of the seventeenth century. Bacon maintained it in the Novum Organum : " Calor," he wrote, " est motus expansivus, cohibitus, et nitens per partes minores."[7] Boyle [8] affirmed that the " Nature of Heat " consists in " a various, vehement, and intestine commotion of the Parts among themselves." Hooke [9] declared that " Heat is a property of a body arising from the motion or agitation of its parts." And Newton [10] asked : " Do not all fixed Bodies, when heated beyond a certain Degree, emit light and shine ; and is not this Emission performed by the vibrating Motion of their Parts ? " and, moreover, suggested the converse of this, namely, that when light is absorbed by a material body, vibrations are set up which are perceived by the senses as heat. The doctrine that heat is a material substance was maintained in Newton's lifetime by a certain school of chemists. The most conspicuous member of the school was Wilhelm Homberg (b. 1652, d. 1715) of Paris, who [11] identified heat and light with the sulphureous principle, which he supposed to be one of the primary ingredients of all bodies, and to be present even in the interplanetary spaces. Between this view and that of Newton it might at first seem as if nothing but sharp opposition was to be expected. [12] But a few years later the professed exponents of the Principia and the Opticks began to develop their system under the evident influence of Homberg's writings. This evolution may easily be traced in s'Gravesande, whose starting-point is the admittedly Newtonian idea that heat bears to light a relation similar to that which a state of turmoil bears to regular rectilinear motion ; whence, conceiving light as a projection of corpuscles, he infers that in a hot body the material particles and the light-corpuscles [13] are in a state of agitation, which becomes more violent as the body is more intensely heated. s'Gravesande thus holds a position between the two opposite camps. On the one hand he interprets heat as a mode of motion ; but on the other he associates it with the presence of a particular kind of matter, which he further identifies with the matter of light. After this the materialistic hypothesis made [1] Cf. pp. 7-9. [2] De Magnete, lib. ii., cap. 2. [3] Query 22. [4] " Subtlety," says Johnson, " which in its original import means exility of particles, is taken in its metaphorical meaning for nicety of distinction." [5] i.e. oxides [6] Newton himself (Oplicks, p. 349) suspected that light-corpuscles and ponderable matter might be transmuted into each other : much later, Boscovich (Theoria, pp. 215, 217) regarded the matter of light as a principle or element in the constitution of natural bodies. [7] Nov. Org., Lib. n., Aphor. xx. [8] Mechanical Production of Heat and Cold. [9] Micrographia, p. 37. [10] Opticks. [11] Mem. del'Acad., 1705, p. 88. [12] Though it reminds us of a curious conjecture ofNewtoa'i: "Is not the strength and vigour of the action between light and sulphureous bodies one reason M-liy sulphureous bodies take fire more readily and burn more vehemently than other bodies do? " [13] I have thought it best to translate s'Gravesande's ignis by " light-corpuscles." This is, I think, fully justified by such of his statements as Quando ignis per lineas rectas oculos nostros intrat, ex motu quem fibris in fundo oculi communicat ideam luminis excitat. |