A History of the Theories of Aether and Electricity (1910) — part 3 of 29

simultaneously. In this idea he found no difficulty ; as he says : " It is certain that a space occupied by more than one kind of matter may permit the propagation of several kinds of waves, different in velocity; for this actually happens in air mixed with aethereal matter, where sound-waves and light- waves are propagated together."

Accordingly he supposed that a light-disturbance generated at any spot within a crystal of Iceland spar spreads out in the form of a wave-surface, composed of a sphere and a spheroid having the origin of disturbance as centre. The spherical wave- front corresponds to the ordinary ray, and the spheroid to the extraordinary ray ; and the direction in which the extraordinary ray is refracted may be determined by a geometrical construc- tion, in which the spheroid takes the place which in the ordinary construction is taken by the sphere.

Thus, let the plane of the figure be at right angles to the intersection of the wave-front with the surface of the crystal ; let AB represent the trace of the incident wave-front ; and suppose that in unit time the disturbance from B reaches the interface at T. In this unit-interval of time the disturbance from A will have spread out within the crystal into a sphere and spheroid : so the wave-front corresponding to the

ordinary ray will be the tangent-plane to the sphere through the line whose trace is T, while the wave-front corresponding to the extraordinary ray will be the tangent-plane to the spheroid through the same line. The points of contact N

in the Seventeenth Century. 27

and M will determine the directions AN and A M of the two- refracted rays* within the crystal.

Huygens did not in the Thtoi-ie de la lumiere attempt a detailed physical explanation of the spheroidal wave, but communicated one later in a letter to Papin,f written in December, 1690. " As to the kinds of matter contained in Iceland crystal," he says, " I suppose one composed of small spheroids, and another which occupies the interspaces around these spheroids, and which serves to bind them together. Besides these, there is the matter of aether permeating all the crystal, both between and within the parcels of the two kinds of matter just mentioned ; for I suppose both the little spheroids, and the matter which occupies the intervals around them, to be composed of small fixed particles, amongst which are diffused in perpetual motion the still finer particles of the aether. There is now no reason why the ordinary ray in the crystal should not be due to waves propa- gated in this aethereal matter. To account for the extraordinary refraction, I conceive another kind of waves, which have for vehicle both the aethereal matter and the two other kinds of matter constituting the crystal. Of these latter, I suppose that the matter of the small spheroids transmits the waves a little more quickly than the aethereal matter, while that around the spheroids transmits these waves a little more slowly than the same aethereal matter. . . . These same waves, when they travel in the direction of the breadth of the spheroids, meet with more of the matter of the spheroids, or at least pass with less obstruction, and so are propagated a little more quickly in this sense than in the other ; thus the light-disturbance is propagated as a spheroidal sheet."

Huygens made another disco veryj of capital importance when

• The word ray in the wave-theory is always applied to the line which goes from the centre of a wave (i.e. the origin of the disturbnnce) to a point on its surface, whatever may be the inclination of this line to the surface-element on which it abuts; for this line has the optical properties of the "rays" of the emission theory.

t Huygens' (Envres, ed. 1905, x., p. 177.

• T/ieorie de la lumiere, p. 89.

28 Theory of the Aether in the Seventeenth Century.

experimenting with the Iceland crystal. He observed that the two rays which are obtained by the double refraction of a single ray afterwards behave in a way different from ordinary light which has not experienced double refraction ; and in particular, if one of these rays is incident on a second crystal of Iceland spar, it gives rise in some circumstances to two, and in others to only one, refracted ray. The behaviour of the ray at this second refraction can be altered by simply rotating the second crystal about the direction of the ray as axis ; the ray under- going the ordinary or extraordinary refraction according as the principal section of the crystal is in a certain direction or in the direction at right angles to this.

The first stage in the explanation of Huygens' observation was reached by Newton, who in 1717 showed* that a ray obtained by double refraction differs from a ray of ordinary light in the same way that a long rod whose cross-section is a rectangle differs from a long rod whose cross-section is a circle : in other words, the properties of a ray of ordinary light are the same with respect to all directions at right angles to its direction of propagation, whereas a ray obtained by double refraction must be supposed to have sides, or properties related to special directions at right angles to its own direction. The refraction of such a ray at the surface of a crystal depends on the relation of its sides to the principal plane of the crystal.

That a ray of light should possess such properties seemed to Newton f an insuperable objection to the hypothesis which regarded waves of light as analogous to waves of sound. On this point he was in the right : his objections are perfectly valid against the wave-theory as it was understood by his contemporaries J, although not against the theory § which was put forward a century later by Young and Fresnel.

• The second edition of Newton's Opticks, Query 26. t Opticks, Query 28.

J In which the oscillations are performed in the direction in which the wave advances.

§ In which the oscillations are performed in a direction at right angles to that in which the wave advances.

29 )

CHAPTEE II.

ELECTRIC AND MAGNETIC SCIENCE PRIOR TO THE INTRODUCTION OF THE POTENTIALS.

THE magnetic discoveries of Peregrinus and Gilbert, and the vortex-hypothesis by which Descartes had attempted to explain them,* 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 provedf 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 j whereas the electric attraction is readily destroyed by screens. Lastly, the magnetic force tends to arrange bodies in definite

*Cf. pp. 7-9. t De Magnete, lib. ii., cap. 2.

30 Electric and Magnetic Science

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

prior to the Introduction of the Potentials. 31

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.*

" Let him also tell me, how an electrick body can by friction emit an exhalation so rare and subtle,t 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

• Query 22.

t " Subtlety," says Johnson, " which in its original import means exility of particles, is taken in its metaphorical meaning for nicety of distinction."

32 Electric and Magnetic Science

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 matlwmatica 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* of the metals figured in the list, while almost all the chemical elements now recognized were

prior to the Introduction oj the Potentials. 33

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.* That some- thing 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 niotus expansivus, cohibitus, et nitens per partes minores."f Boyle+ affirmed that the " Nature of Heat " consists in " a various, vehement, and intestine commotion of the Parts among themselves." Hooke§ declared that " Heat is a property of a body arising from the motion or agitation of its parts." And Newton|| asked : " Do not

• 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.

t Nov. Org., Lib. n., Aphor. xx. J Mechanical Production of Heat and Cold.

§ Micrographia, p. 37. || Opticks.

D

34 Electric and Magnetic Science

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 main- tained 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* 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 inter- planetary spaces. Between this view and that of Newton it might at first seem as if nothing but sharp opposition was to be expected, j- 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-corpusclesj 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

• Mem. del'Acad., 1705, p. 88.

t 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? "

J 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 gttein fibris in fundo oculi cont/tninicai ideam luminis excitat.

prior to the Introduction of the Potentials. 35

rapid progress. It was frankly advocated by another member of the Dutch school, Hermann Boerhaave* (6. 1668, d. 1738), Professor in the University of Leyden, whose treatise on chemistry was translated into English in 1727.

Somewhat later it was found that the heating effects of the rays from incandescent bodies may be separated from their luminous effects by passing the rays through a plate of glass, which transmits the light, but absorbs the heat. After this discovery it was no longer possible to identify the matter of heat with the corpuscles of light ; and the former was consequently accepted as a distinct element, under the name of caloric.^ In the latter part of the eighteenth and early part of the nineteenth centuries} caloric was generally conceived as occupying the interstices between the particles of ponderable matter — an idea which fitted in well with the observation that bodies commonly expand when they are absorbing heat, but which was less com- petent to explain the fact§ that water expands when freezing. The latter difficulty was overcome by supposing the union between a body and the caloric absorbed in the process of melting to be of a chemical nature; so that the consequent changes in volume would be beyond the possibility of prediction.

As we have already remarked, the imponderability of heat did not appear to the philosophers of the eighteenth century to be a sufficient reason for excluding it from the list of chemical elements ; and in any case there was considerable doubt as to whether caloric was ponderable or not. Some experimenters believed that bodies were heavier when cold than when hot; others that they were heavier when hot than when cold. The century was far advanced before Lavoisier and Eumford finally

• Boerhaave followed Homberg in supposing the matter of heat to be present ia all so-called vacuous spaces.

t Scheele in 1777 supposed caloric to be a compound of oxygen and phlogiston, and light to be oxygen combined with a greater proportion of phlogiston.

J In suite of the experiments of Benjamin Thompson, Count Eumford (b. 1753, .d. 1814), in the closing years of the eighteenth century. These should have -sufficed to re-establish the older conception of heat.

§ This had been known since the time of Boyle.

D 2

36 Electric and Magnetic Science

proved that the temperature of a body is without sensible influence on its weight.

Perhaps nothing in the history of natural philosophy is more amazing than the vicissitudes of the theory of heat. The true hypothesis, after having met with general acceptance throughout a century, and having been approved by a succession of illus- trious men, was deliberately abandoned by their successors in favour of a conception utterly false, and, in some of its developments, grotesque and absurd.

We must now return to s'Gravesande's book. The pheno- mena of combustion he explained by assuming that when a body is sufficiently heated the light-corpuscles interact with the material particles, some constituents being in consequence sepa- rated and carried away with the corpuscles as flame and smoke. This view harmonizes with the theory of calcination which had been developed by Becher and his pupil Stahl at the end of the- seventeenth century, according to which the metals were sup- posed to be composed of their calces and an element phlogiston. The process of combustion, by which a metal is changed into its- calx, was interpreted as a decomposition, in which the phlogiston separated from the metal and escaped into the atmosphere ; while the conversion of the calx into the metal was regarded as a union with phlogiston.*

s'Gravesande attributed electric effects to vibrations induced in effluvia, which he supposed to be permanently attached to such bodies as amber. " Glass," he asserted, " contains in it, and has about its surface, a certain atmosphere, which is excited by Friction and put into a vibratory motion ; for it attracts and

• The correct idea of combustion had been advanced by Hooke. "The disso- lution of inflammable bodies," he asserts in the Micrographia, " is performed by a substance inherent in and mixed with the air, that is like, if not the very same with, that which is fixed in saltpetre." But this statement met with little favour at the time, and the doctrine of the compound nature of metals survived in full vigour until the discovery of oxygen by Priestley and Scheele in 1771-5. In 1775 Lavoisier reaffirmed Hooke's principle that a metallic calx is not the metal minus phlogiston, but the metal plus oxygen; and this idea, which carried with it the recognition of the elementary nature of metals, was generally accepted by the end' of the eighteenth century.

prior to the Introduction of the Potentials. 37

repels light Bodies. The smallest parts of the glass are agitated by the Attrition, and by reason of their elasticity, their motion is vibratory, which is communicated to the Atmosphere above- mentioned : and therefore that Atmosphere exerts its action the further, the greater agitation the Parts of the Glass receive when a greater attrition is given to the glass."

The English translator of s'Gravesande's work was himself destined to play a considerable part in the history of electrical science. Jean Theophile Desaguliers (b. 1683, d. 1744) was an Englishman only by adoption. His father had been a Huguenot pastor, who, escaping from France after the revocation of the Edict of Nantes, brought away the boy from La Kochelle, concealed, it is said, in a tub. The young Desaguliers was afterwards ordained, and became chaplain to that Duke of Chandos who was so ungratefully ridiculed by Pope. In this situation he formed friendships with some of the natural philosophers of the capital, and amongst others with Stephen Gray, an experimenter of whom little is known* beyond the fact that he was a pensioner of the Charterhouse.

In 1729 Gray communicated, as he says,f " to Dr. Desaguliers and some other Gentlemen " a discovery he had lately made, " showing that the Electrick Vertue of a Glass Tube may be •conveyed to any other Bodies so as to give them the same Property of attracting and repelling light Bodies as the Tube does, when excited by rubbing : and that this attractive Vertue might be carried to Bodies that were many Feet distant from the Tube."

This was a result of the greatest importance, for previous workers had known of no other way of producing the attractive emanations than by rubbing the body concerned.* It was found

• Those M*ho are interested in the literary history of the eighteenth century will recall the controversy as to whether the verses on the death of Stephen Gray were written hy Anna "Williams, whose name they bore, or by her patron Johnson.

| Phil. Trans, xxxvii (1731), pp. 18, 227, 285, 397.

j Otto von Guericke (b. 1602, d. 1686) bad, as a matter of fact, observed the conduction of electricity along a linen thread ; but this experiment does not seem to have been followed up. Cf. Experimenta novamagdeburgica, 1672.

38 Electric and Magnetic Science

o

that only a limited class of substances, among which the metals were conspicuous, had the capacity of acting as channels for the transport of the electric power ; to these Desaguliers, who. con- tinued the experiments after Gray's death in 1736, gavfc^ the name non-electrics or conductors.

After Gray's discovery it was no longer possible to believe that the electric effluvia are inseparably connected with the bodies from which they are evoked by rubbing ; and it became necessary to admit that these emanations have an independent existence, and can be transferred from one body to another. Accordingly we find them recognized, under the name of the electric fluidft as one of the substances of which the world is constituted. The imponderability of this fluid did not, for the reasons already mentioned, prevent its admission by the side of light and caloric into the list of chemical elements.

The question was actively debated as to whether the electric fluid was an element sui generis, or, as some suspected, was another manifestation of that principle whose operation is seen in the phenomena of heat. Those who held the latter view urged that the electric fluid and heat can both be induced by friction, can both induce combustion, and can both be transferred from one body to another by mere contact ; and, moreover, that the best conductors of heat are also in general the best con- ductors of electricity. On the other hand it was contended that the electrification of a body does not cause any appreciable rise in its temperature; and an experiment of Stephen Gray's brought to light a yet more striking difference. Gray,J in 1729,. made two oaken cubes, one solid and the other hollow, and showed that when electrified in the same way they produced exactly similar effects ; whence he concluded that it was only the surfaces which had taken part in the phenomena. Thus while heat is disseminated throughout the substance of a body, the electric fluid resides at or near its surface. In the middle of

• Phil. Trans, xli. (1739), pp. 186, 193, 200, 209: Dissertation concerning Electricity, 1742.

t The Cartesians defined a fluid to be a body whose minute parts are in a continual agitation. J Phil. Trans, xxxvii., p. 35.

prior to the Introduction of the Potentials. 39

the eighteenth century it was generally compared to an envelop- ing atmosphere. " The electricity which a non-electric of great length (for example, a hempen string 800 or 900 feet long) receives, runs from one end to the other in a sphere of electrical Effluvia" says Desaguliers in 1740 ^and a report of the French Academy in 1733 says :f " Around an electrified body there is formed a vortex of exceedingly fine matter in a state of agitation,, which urges towards the body such light substances as lie within its sphere of activity. The existence of this vortex is more than a mere conjecture ; for when an electrified body i& brought close to the face it causes a sensation like that of encountering a cobweb. "J

The report from which this is quoted was prepared in connexion with the discoveries of Charles-Francois du Fay (b. 1698, d. 1739), superintendent of gardens to the King of France. Du Fay§ accounted for the behaviour of gold leaf when brought near to an electrified glass tube by supposing that at first the vortex of the tube envelopes the gold-leaf, and so attracts it towards the tube. But when contact occurs, the gold-leaf acquires the electric virtue, and so becomes surrounded by a vortex of its own. The two vortices, striving to extend in contrary senses, repel each other, and the vortex of the tube, being the stronger, drives away that of the gold-leaf. " It is then certain/' says du Fay,H " that bodies which have become electric by contact are repelled by those which have rendered them electric ; but are they repelled likewise by other electrified bodies of all kinds ? And do electrified bodies differ from each other in no respect save their intensity of electrification ? An examination of this matter has led me to a discovery which I should never have foreseen, and of which I believe no one hitherto has had the least idea."

• Phil. Trans, xli., p. 636. t Hist, de 1'Acad., 1733, p. 6.

t This observation had been made first by Hawksbee at the beginning of the century.

§ Mem. de 1'Acad. des Sciences, 1733, pp. 23, 73, 233, 457 ; 1734, pp. 341, 503; 1737, p. 86 ; Phil. Trans, xxxviii. (1734), p. 258.

|| Mem. de 1'Acad., 1733, p. 464.

40 Electric and Magnetic Science

He found, in fact, that when gold-leaf which had been electrified by contact with excited glass was brought near to an excited piece of copal,* an attraction was manifested between them. " I had expected," he writes, " quite the opposite effect, since, according to my reasoning, the copal and gold-leaf, which were both electrified, should have repelled each other." Proceeding with his experiments he found that the gold-leaf, when electrified and repelled by glass, was attracted by all electrified resinous substances, and that when repelled by the latter it was attracted by the glass. " We see, then," he continues, " that there are two electricities of a totally different nature — namely, that of transparent solids, such as glass, crystal, &c., and that of bituminous or resinous bodies, such as amber, copal, sealing-wax, &c. Each of them repels bodies which have contracted an electricity of the same nature as its own, and attracts those whose electricity is of the contrary nature. We see even that bodies which are not themselves electrics can acquire either of these electricities, and that then their effects are similar to those of the bodies which have communicated it to them."

To the two kinds of electricity whose existence was thus demonstrated, du Fay gave the names vitreous and resinous, by which they have ever since been known.

An interest in electrical experiments seems to have spread from du Fay to other members of the Court circle of Louis XV ; and from 1745 onwards the Memoirs of the Academy contain a series of papers on the subject by the Abbe Jean-Antoine Nollet {&. 1700, d. 1770), afterwards preceptor in natural philosophy to the Koyal Family. Nollet attributed electric phenomena to the movement in opposite directions of two currents of a fluid, " very subtle and inflammable," which he supposed to be present in all bodies under all circumstances.f When an electric is excited by friction, part of this fluid escapes from its pores, forming an effluent stream; and this loss is repaired by an

• A hard transparent resin, used in the preparation of varnish. t Cf. Nollet' s lieeherchet, 1749, p. 245.

prior to the Introduction of the Potentials. 41

dtfiucnt stream of the same fluid entering the body from outside. Light bodies in the vicinity, being caught in one or other of these streams, are attracted or repelled from the excited electric.

Nollet's theory was in great vogue for some time ; but six or seven years after its first publication, its author came across a work purporting to be a French translation of a book printed originally in England, describing experiments said to have been made at Philadelphia, in America, by one Benjamin Franklin. "He could not at first believe," as Franklin tells us in his AutobiograpJvy, " that such a work came from America, and said it must have been fabricated by his enemies at Paris to decry his system. Afterwards, having been assured that there really existed such a person as Franklin at Philadelphia, which he had doubted, he wrote and published a volume of letters, chiefly addressed to me, defending his theory, and denying the verity of my experiments, and of the positions deduced from them."

We must now trace the events which led up to the discovery which so perturbed Nollet.