A History of the Theories of Aether and Electricity (1910) — part 1 of 29

DUBLIN UNIVERSITY PRESS SERIES.

A HISTORY

OF THE

THEORIES OF AETHER AND ELECTRICITY

FKOM THE AGE OF DESCAKTES TO THE CLOSE OF THE NINETEENTH CENTURY.

BY

E. T. WH1TTAKER,

Hon. Sc.D. (DubL}; I.E.S.; Roy at Astronomer of Ireland.

LONGMANS, GREEN, AND CO.,

39 PATERNOSTER ROW, LONDON, NEW YORK, BOMBAY, AND CALCUTTA.

HODGES, FIGGIS, & CO., LTD., DUBLIN. 1910.

MM*

DUBLIN :

PRINTED AT UHE UNIVERSITY PRESS, BY PONSONBY AND OIBRS.

THE author desires to record his gratitude to Mr. W. W. EOUSE BALL, Fellow of Trinity College, Cambridge, and to Professor W. McF. ORR, F.R.S., of the Royal College of Science for Ireland ; these friends have read the proof-sheets, and have made many helpful suggestions and criticisms.

Thanks are also 'due to the BOARD OF TRINITY COLLEGE, DUBLIN, for the financial assistance which made possible the publication of the work.

236360

CONTENTS.

CHAPTEK I.

y THE THEORY OF THE AETHER IN THE SEVENTEENTH CENTURY.

Page

Matter and aether, . . . . . . .1

The physical writings of Descartes, ..... 2

Early history of magnetism : Petrus Peregrinus, Gilbert, Descartes, 7 Fermat attacks Descartes' theory of light : the principle of least

time, ........ 10

Hooke's undulat>ry theory : the advance of wave -fronts, . . 11

Newton overthrows Hooke's theory of colours, . . .15

Conception of the aether in the writings of Newton, . . 17

Newton's theories of the periodicity of homogeneous light, and of

fits of easy transmission, • • . . ,20

The velocity of light : Galileo, Roemer, . . . .21

Huygens' Traite de la lumiere : his theories of the propagation of

waves, and of crystalline optics, • . . .22

Newton shows that rays obtained by double refraction have sides :

his objections to the undulatory theory, . . .28

X

CHAPTER II.

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

The electrical researches of Gilbert : the theory of emanations, . 29 State of physical science in the first half of the eighteenth century, 32 Gray discovers electric conduction : Desaguliers, . . • 37

The electric fluid, ....... 38

Du Fay distinguishes vitreous and resinous electricity, . .39

Xollet's effluent and affluent streams, . . . .40

The Leyden phial, ..... . . 41

The one-fluid theory : ideas of Watson and Franklin, . . 42

Final overthrow by Aepinus of the doctrine of effluvia, . . 48

Priestley discovers the law of electrostatic force, . . .50

viii Contents.

Page

Cavendish, . ... 51

Michell discovers the law of magnetic force, . . . .54

The two-fluid theory : Coulomb, . . . . .56

Limited mobility of the magnetic fluids, . . .58

Poisson's mathematical theory of electrostatics, . . .59

The equivalent surface- and volume-distributions of magnetism :

Poisson's theory of magnetic induction, . . .64

Green's Nottingham memoir, . . . . .65

CHAPTER III. GALVANISM, FROM GALVANI TO OHM.

Sulzer's discovery, ... . .67

Galvanic phenomena, ....... 68

Rival hypotheses regarding the galvanic fluid, , . .70

The voltaic pile, ....... 72

Nicholson and Carlisle decompose water voltaically, . . 75

Davy's chemical theory of the pile, ..... 76

Grothuss' chain, . . . . . . .78

De La Rive's hypothesis, . . . . . .79

Berzelius' scheme of electro-chemistry, . . . .80

Early attempts to discover a connexion between electricity and

magnetism, . ... 83

Oersted's experiment : his explanation of it, . . .85

The law of Biot and Savart, . . . . . .86

The researches of Ampere on electrodynamics, . . 87

Seebeck's phenomenon, . . . . . .90

Davy's researches on conducting power, . . . .94

Ohm's theory : electroscopic force, . . . . .95

CHAPTER IV. THE LUMINIFEBOUS MEDIUM, FROM BRADLEY TO FRESNEL.

Bradley discovers aberration, . . . . .99

John Bernoulli's model of the aether, .... 100

Maupertuis and the principle of least action, . . . 102

Views of Euler, Courtivron, Melvill, .... 104

Young defends the undulatory theory, and explains the colours of

thin plates, ... ... 105

Laplace supplies a corpuscular theory of double refraction, . . 109

Contents. ix

Page

Young proposes a dynamical theory of light in crystals, . . 110

Researches of Malus on polarization, .... Ill

Recognition of biaxal crystals, ... . 113

Fresnel successfully explains diffraction, . . . 114

His theory of the relative motion of aether and matter, . . 115

Young suggests the transversality of the vibrations of light, . 121

Fresnel discusses the dynamics of transverse vibrations, . . 123

Fresnel's theory of the propagation of light in crystals, . • . 125

Hamilton predicts conical refraction, . . . ] 31

Fresnel's theory of reflexion, ..... 133

CHAPTER V. I,THE AETHER AS AN ELASTIC SOLID.

Astronomical objection to the elastic-solid theory : Stokes'

hypothesis. . . . . . . .137

Navier and Cauchy discover the equation of vibration of an elastic

solid, 139

Poisson distinguishes condensational and distortional waves, . 141

Cauchy's first and second theories of light iq, crystals, . . 143

Cauchy's first theory of reflexion, ..... 145

His second theory of reflexion, ..... 147

The theory of reflexion of MacCullagh and Neumann, . . 148

Green discovers the correct conditions at the boundaries, . . 151

Green's theory of reflexion : objections to it, . . . 152

MacCullagh introduces a new type of elastic solid, . . . 154

W. Thomson's model of a rotationally-elastic body, . . 157

Cauchy's third theory of reflexion : the contractile aether, . . 158

Later work of W. Thomson and others on the contractile aether, . 159

Green's first and second theories of light in crystals, . . 161

Influence of Green, ....... 167

Researches of Stokes on the relation of the direction of vibration of

light to its plane of polarization, .... 168

The hypothesis of aeolotropic inertia, .... 171

Rotation of the plane of polarization of light by active bodies, . 173

MacCullagh's theory of natural rotatory power, . . 175

MacCullagh's and Cauchy's theory of metallic reflexion, . . 177

Extension of the elastic -solid theory to metals, . . 179

Lord Rayleigh's objection, .... . 181

Cauchy's theory of dispersion, • . . 182

Boussinesq's elastic-solid theory, ..... 185

x Contents.

CHAPTEE VI.

FARADAY.

Page

Discovery of induced currents : lines of magnetic force, . . 189

Self-induction, . . . . . . .193

Identity of frictional and voltaic electricity : Faraday's views on the

nature of electricity, . . . . . 194

Electro-chemistry, . . ".. • . • *. . . . 197

Controversy between the adherents of the chemical and contact

hypotheses, . » . . . . . 201

The properties of dielectrics, . . . . . 206

Theory of dielectric polarization : Faraday, W. Thomson, and

Mossotti, . . : . . . . .211

The connexion between magnetism and light, . . . 213

Airy's theory of magnetic rotatory polarization, . « . 214

Faraday's Thoughts on Ray -Vibrations, . ..''-. . . 217

Researches of Faraday and Pliicker on diamagnetism, . . 218

CHAPTER VII.

THE MATHEMATICAL ELECTRICIANS OF THE MIDDLE OF THE NINETEENTH

CENTURY.

F. Neumann's theory of induced currents : the electrodynamic

potential, . . . . . ; . . 222

W. Weber's theory of electrons, . . . . .225

Riemann's law, . . . ... . 231

v-Proposals to modify the law of gravitation, . .. . . 232

Weber's theory of paramagnetism and diamagnetism : later theories, 234

Joule's law : energetics of the voltaic cell, .... 239

Researches of Helmholtz on electrostatic and electrodynamic energy, 242 W. Thomson distinguishes the circuital and irrotational magnetic

vectors, ........ 244

His theory of magnecrystallic action, .... 245

His formula for the energy of a magnetic field, . . . 247

Extension of this formula to the case of fields produced by currents, 249 Kirchhoff identifies Ohm's electroscopic force with electrostatic

potential, . . . . . . / 251

The discharge of a Leyden jar : W. Thomson's theory, . . 253

The velocity of electricity and the propagation of telegraphic signals, 254

Clausius' law of force between electric charges : crucial experiments, 261

Nature of the current, ...... 263

The thermo-electric researches of Peltier and W. Thomson, • 264

Contents. xi

CHAPTER VIII. MAXWELL.

Page

Gauss and Riemann on the propagation of electric actions, . . 268

Analogies suggested by W. Thomson, .... 269

Maxwell's hydrodynamical analogy, ..... 271

The vector potential, ...... 273

Linear and rotatory interpretations of magnetism, . . . 274

Maxwell's mechanical model of the electromagnetic field, . . 276

Electric displacement, ...... 279

Similarity of electric vibrations to those of light, . . . 281

Connexion of refractive index and specific inductive capacity, . 283 Maxwell's memoir of 1864, . . ... .284

The propagation of electric disturbances in crystals and in metals, . 288

Anomalous dispersion, ...... 291

The Max well -Sellmeier theory of dispersion, . . . 292

Imperfections of the electromagnetic theory of light, . . 295

The theory of L. Lorenz, ...... 297

Maxwell's theory of stress in the electric field, . . . 300

The pressure of radiation, ...... 303

Maxwell's theory of the magnetic rotation of light, . . . 307

CHAPTER IX.

MODELS OF THE AETHER.

Analogies in which a rotatory character is attributed to magnetism, 310

Models in which magnetic force is represented as a linear velocity, 311 Researches of W. Thomson, Bjerknes, and Leahy, on pulsating and

oscillating bodies, ...... 316

MacCullagh's quasi-elastic solid as a model of the electric medium, 318 The Hall effect, . . . . . .320

Models of Riemann and Fitz Gerald, . . . . 324

Vortex-atoms, . . . . . . .326

The vortex-sponge theory of the aether : researches of W. Thomson,

Fitz Gerald, and Hicks, , . . . . .327

CHAPTER X.

THE FOLLOWERS OF MAXWELL.

Helmholtz and H. A. Lorentz supply an electromagnetic theory of

reflexion, ....... 337

Crucial experiments of Helmholtz and Schiller, . . . 338

xii Contents.

Page

Convection -currents : Rowland's experiments, . . . 339 The moving charged sphere : researches of J. J. Thomson, Fitz Gerald,

and Heaviside, . . . . . . . 340

Conduction of rapidly -alternating currents, .... 344

Fitz Gerald devises the magnetic radiator, .... 345

Poynting's theorem, ....... 347

Poynting and J. J. Thomson develop the theory of moving lines of

force, . . . . . . . 349

Mechanical momentum in the electromagnetic field, . . 352

New derivation of Maxwell's equations by Hertz, . . . 353

Hertz's assumptions and Weber's theory, .... 356

Experiments of Hertz on electric waves, .... 357

The memoirs of Hertz and Heaviside on fields in which material

bodies are in motion, ...... 365

The current of dielectric convection, ..... 367

Kerr's magneto-optic phenomenon, . ... 368

Rowland's theory of magneto-optics, .... 369

The rotation of the plane of polarization in naturally active bodies, 370

CHAPTER XI.

CONDUCTION IN SOLUTIONS AND GASES, FROM FARADAY TO J. J. THOMSON.

The hypothesis of Williamson and Clausius, . . . 372

Migration of the ions, ...... 373

The researches of Hittorf and Kohlrausch, .... 374

Polarization of electrodes, ...... 375

Electrocapillarity, .... . 376

Single differences of potential, . . . . . 379

Helmholtz' theory of concentration-cells, .... 381

Arrhenius' hypothesis, ... ... 383

The researches of Nernst, ... . 386

Earlier investigations of the discharge in rarefied gases, . . 390

Faraday observes the dark space, ..... 391

Researches of Pliicker, Hittorf, Goldstein, and Varley, on the

cathode rays, .... . 393

Crookes and the fourth state of matter, .... 394 Objections and alternatives to the charged-particle theory of

cathode rays, ....... 395

Giese's and Schuster's ionic theory of conduction in gases, . . 397

J. J. Thomson measures the velocity of cathode rays, . . 400

Contents. xiii

Page

Discovery of X-rays : hypotheses regarding them, . . 401

Further researches of J. J. Thomson on cathode rays : the ratio m/e, 404

Vitreous and resinous electricity, . . . 406

Determination of the ionic charge by J. J. Thomson, . . 407

Becquerel's radiation : discovery of radio-active substances, . 408

CHAPTER XII.

THE THEORY OF AETHER AND ELECTRONS IN THE CLOSING YEARS OF THE NINETEENTH CENTURY.

Stokes' theory of aethereal motion near moving bodies, . . 411

Astronomical phenomena in which the velocity of light is involved, 413

Crucial experiments relating to the optics of moving bodies, . 416

Lorentz' theory of electrons, ...... 419

The current of dielectric convection : Rontgen's experiment, . 426

The electronic theory of dispersion, ..... 428

Deduction of Fresnel's formula from the theory of electrons, . 430

Experimental verification of Lorentz' hypothesis, . . . 431

Fitz Gerald's explanation of Michelson's experiment, . . 432 Lorentz' treatise of 1895, . . . . . . . 433

Expression of the potentials in terms of the electronic charges, . 436

Further experiments on the relative motion of earth and aether, . 437 Extension of Lorentz' transformation : Larmor discovers its

connexion with Fitz Gerald's hypothesis of contraction, . 440 Examination of the supposed primacy of the original variables :

fixity relative to the aether : the principle of relativity, . 444

The phenomenon of Zeeman, ..... 449

Connexion of Zeeman's effect with the magnetic rotation of light, . 452

The optical properties of metals, ..... 454

The electronic theory of metals, ..... 456

Thermionics, ........ 464

INDEX, . 470

MEMOKANDUM ON NOTATION.

VECTORS are denoted by letters in clarendon type, as E.

The three components of a vector E are denoted by Ex, Ey, Ez ; and the magnitude of the vector is denoted by E, so that

The vector product of two vectors E and H, which is denoted by [E . H], is the vector whose components are (EyHz - E^H^ EZHX - E*HZ, EtHy - EyHx}. Its direction is at right angles to the direction of E and H, and its magnitude is represented by twice the area of the triangle formed by them.

The scalar product of E and H is EXHX + EyEy + E^. It is denoted by (E . H).

OJ^j (1 jjj O Jjj

The quantity — -f — y -I- — is denoted by div E.

The vector whose components are

J — f *t — * ^ . — y _ *\

is denoted by curl E.

If V denote a scalar quantity, the vector whose components are 8F 8F 9F\

• 5T» * ^7' - -5T 1S denoted b7 grad ^

The symbol V is used to denote the vector operator whose

898

components are — , — , — . dx dy 82

Differentiation with respect to the time is frequently indicated by a dot placed over the symbol of the variable which is differentiated.

THEORIES OF AETHER AND ELECTRICITY.

CHAPTEK I.

THE THEORY OF THE AETHER IN THE SEVENTEENTH CENTURY.

THE observation of the heavens, which has been pursued con- tinually from the earliest ages, revealed to the ancients the regularity of the planetary motions, and gave rise to the conception of a universal order. Modern research, building on this foundation, has shown how intimate is the connexion between the different celestial bodies. They are formed of the same kind of matter ; they are similar in origin and history ; and across the vast spaces which divide them they hold perpetual intercourse.

Until the seventeenth century the only influence which was known to be capable of passing from star to star was that of light. Newton added to this the force of gravity ; and it is now recognized that the power of communicating across vacuous regions is possessed also by the electric and magnetic attractions.

It is thus erroneous to regard the heavenly bodies as isolated in vacant space; around and between them is an incessant conveyance and transformation of energy. To the vehicle of this activity the name aetlier has been given.

The aether is the solitary tenant of the universe, save for that infinitesimal fraction of space which is occupied by ordinary matter. Hence arises a problem which has long engaged attention, and is not yet completely solved : What relation subsists between the medium which fills the interstellar void and the condensations of matter that are scattered throughout it?

B

$5 l ' r The ^Theory of the • -Aether

The history of this problem may be traced back continuously to the earlier half of the seventeenth century. It first emerged clearly in that reconstruction of ideas regarding the physical universe which was effected by Eene Descartes.

Descartes was born in 1596, the son of Joachim Descartes, Counsellor to the Parliament of Brittany. As a young man he followed the profession of arms, and served in the campaigns of Maurice of Nassau, and the Emperor ; but his twenty-fourth year brought a profound mental crisis, apparently not unlike those which have been recorded of many religious leaders ; and he resolved to devote himself thenceforward to the study of philosophy.

The age which preceded the birth of Descartes, and that in which he lived, were marked by events which greatly altered the prevalent conceptions of the world. The discovery of America, the circumnavigation of the globe by Drake, the over- throw of the Ptolemaic system of astronomy, and the invention of the telescope, all helped to loosen the old foundations and to make plain the need for a new structure. It was this that Descartes set himself to erect. His aim was the most ambitious that can be conceived ; it was nothing less than to create from the beginning a complete system of human knowledge.

Of such a system the basis must necessarily be metaphysical ; and this part of Descartes' work is that by which he is most widely known. But his efforts were also largely devoted to the mechanical explanation of nature, which indeed he regarded as one of the chief ends of Philosophy.*

The general character of his writings may be illustrated by a comparison with those of his most celebrated contemporary, f Bacon clearly defined the end to be sought for, and laid down the method by which it was to be attained; then, recognizing that to discover all the laws of nature is a task beyond the

• Of the works M'hich bear on our present subject, the Dioptrique and the Me'teores were published at Leyden in 1638, and the Principia Philosophiae at Amsterdam in 1644, six years before the death of its author.

t The principal philosophical works of Bacon were written about eighteen years before those of Descartes.

in the SeventeentJi Century. 3

powers of one man or one generation, he left to posterity the work of filling in the framework which he had designed. Descartes, on the other hand, desired to leave as little as possible for his successors to do ; his was a theory of the universe, worked out as far as possible in every detail. It is, however, impossible to derive such a theory inductively unless there are at hand sufficient observational data on which to base the induction ; and as such data were not available in the age of Descartes, he was compelled to deduce phenomena from preconceived principles and causes, after the fashion of the older philosophers. To the inherent weakness of this method may be traced the errors that at last brought his scheme to ruin.

The contrast between the systems of Bacon and Descartes is not unlike that between the Eoman republic and the empire of Alexander. In the one case we have a career of aggrandizement pursued with patience for centuries ; in the other a growth of fungus-like rapidity, a speedy dissolution, and an immense influence long exerted by the disunited fragments. The grandeur of Descartes' plan, and the boldness of its execution, stimulated scientific thought to a degree before unparalleled ; and it was largely from its ruins that later philosophers constructed those more valid theories which have endured to our own time.

Descartes regarded the world as an immense machine, operating by the motion and pressure of matter. " Give me matter and motion," he cried, " and I will construct the universe." A peculiarity which distinguished his system from that which afterwards sprang from its decay was the rejection of all forms of action at a distance ; he assumed that force cannot be com- municated except by actual pressure or impact. By this assumption he was compelled to provide an explicit mechanism in order to account for each of the known forces of nature — a task evidently much more difficult than that which lies before those who are willing to admit action at a distance as an ultimate property of matter.

Since the sun interacts with the planets, in sending them

B 2

4 The Theory of the Aether

light and heat and influencing their motions, it followed from Descartes' principle that interplanetary space must be a plenum,, occupied by matter imperceptible to the touch but capable of serving as the vehicle of force and light. This conclusion in turn determined the view which he adopted on the all- important question of the nature of matter.

Matter, in the Cartesian philosophy, is characterized not by impenetrability, or by any quality recognizable by the senses,, but simply by extension ; extension constitutes matter, and matter constitutes space. The basis of all things is a primitive,, elementary, unique type of matter, boundless in extent and infinitely divisible. In the process of evolution of the universe three distinct forms of this matter have originated, correspond- ing respectively to the luminous matter of the sun, the transparent matter of interplanetary space, and the dense, opaque matter of the earth. " The first is constituted by what has been scraped off the other particles of matter when they were rounded ; it moves with so much velocity that when it meets other bodies the force of its agitation causes it to be broken and divided by them into a heap of small particles that are of such a figure as to fill exactly all the holes and small interstices which they find around these bodies. The next type includes most of the rest of matter ; its particles are spherical, and are very small compared with the bodies we see on the earth ; but nevertheless they have a finite magnitude, so that they can be divided into others yet smaller. There exists in addition a third type exemplified by some kinds of matter — namely, those which, on account of their size and figure, cannot be so easily moved as the preceding. I will endeavour to show that all the bodies of the visible world are composed of these three forms of matter, as of three distinct elements ; in fact, that the sun and the fixed stars are formed of the first of these elements, the interplanetary spaces of the second, and the earth, with the planets and comets, of the third. For, seeing that the sun and the fixed stars emit light, the heavens transmit it, and the earth, the planets, and the comets reflect it, it appears to me that there

in the Seventeenth Century. 5

is ground for using these three qualities of luminosity, trans- parence, and opacity, in order to distinguish the three elements of the visible world.*

According to Descartes' theory, the sun is the centre of an immense vortex formed of the first or subtlest kind of inatter.f The vehicle of light in interplanetary space is matter of the second kind or element, composed of a closely packed assemblage of globules whose size is intermediate between that of the vortex-matter and that of ponderable matter. The globules of the second element, and all the matter of the first element, are constantly straining away from the centres around which they turn, owing to the centrifugal force of the vortices ;J so that the globules are pressed in contact with each other, and tend to move outwards, although they do not actually so move.§ It is the transmission of this pressure which constitutes light ; the action of light therefore extends on all sides round the sun and fixed stars, and travels instantaneously to any distance. |j In the Dwptrique$ vision is compared to the perception of the presence of objects which a blind man obtains by the use of his stick ; the transmission of pressure along the stick from the object to the hand being analogous to the transmission of pressure from a luminous object to the eye by the second kind of matter.

Descartes supposed the " diversities of colour and light " to he due to the different ways in which the matter moves.** In the Meteores,^ the various colours are connected with different rotatory velocities of the globules, the particles winch rotate most rapidly giving the sensation of red, the slower ones of yellow, and the slowest of green and blue — the order of colours being taken from the rainbow. The assertion of the dependence of colour

• Principia, Part iii, § 52.

t It is curious to speculate on the impression which would have been produced had the spirality of nehulse heen discovered hefore the overthrow of the Cartesian theory of vortices.

J Ibid., §§ 55-59. § Ibid., § 63. || Ibid., § 64. IT Discours premier.

** Principia, Part iv, § 195. ft Discours Huitieme.

6 The Theory of the Aether

on periodic time is a curious foreshadowing of one of the great discoveries of Newton.

The general explanation of light on these principles was amplified by a more particular discussion of reflexion and refraction. The law of reflexion— that the angles of incidence and refraction are equal — had been known to the Greeks ; but the law of refraction — that the sines of the angles of incidence and refraction are to each other in a ratio depending on the media — was now published for the first time.* Descartes gave it as his own ; but he seems to have been under considerable obligations to Willebrord Snell (b. 1591, d. 1626), Professor of Mathematics at Leyden, who had discovered it experimentally (though not in the form in which Descartes gave it) about 1621. Snell did not publish his result, but communicated it in manuscript to several persons, and Huygens affirms that this manuscript had been seen by Descartes.

Descartes presents the law as a deduction from theory. This, however, he is able to do only by the aid of analogy ;. when rays meet ponderable bodies, " they are liable to be deflected or stopped in the same way as the motion of a ball or a stone impinging 011 a body " ; for " it is easy to believe that the action or inclination to move, which I have said must be taken for light, ought to follow in this the same laws as motion."f Thus he replaces light, whose velocity of propagation he believes to be always infinite, by a projectile whose velocity varies from one medium to another. The law of refraction is then proved as follows J : —

Let a ball thrown from A meet at B a cloth CBE, so weak that the ball is able to break through it and pass beyond, but with its resultant velocity reduced in some definite proportion,, say 1 : k.

Then if BI be a length measured on the refracted ray equal to AB, the projectile will take k times as long to describe BI as it took to describe AB. But the component

• Dioptrique, Discount second. t Jbid., Discows premier.

% Ibid., Discotirs second.

in the Seventeenth Century. 7

of velocity parallel to the cloth must be unaffected by the impact; and therefore the projection BE of the refracted ray must be k times as long as the projection BC of the incident

I

ray. So if i and r denote the angles of incidence and refraction, we have

• BE BC

or the sines of the angles of incidence and refraction are in a constant ratio ; this is the law of refraction.

Desiring to include all known phenomena in .his system, Descartes devoted some attention to a class of effects which were at that time little thought of, but which were destined to play a great part in the subsequent development of Physics.

The ancients were acquainted with the curious properties possessed by two minerals, amber (riXtKrpov) and magnetic iron ore (77 \iOos Mayv?}r/e). The former, when rubbed, attracts light bodies : the latter has the power of attracting iron.

The use of the magnet for the purpose of indicating direc- tion at sea does not seem to have been derived from classical antiquity ; but it was certainly known in the time of the Crusades. Indeed, magnetism was one of the few sciences which progressed during the Middle Ages ; for in the thirteenth century Petrus Peregrinus,* a native of Maricourt in Picardy, made a discovery of fundamental importance.

Taking a natural magnet or lodestone, which had been rounded into a globular form, he laid it on a needle, and marked

• His Epistola was written in 1269.

8 The Theory of the Aether

the line along which the needle set itself. Then laying the needle on other parts of the stone, he obtained more lines in the same way. When the entire surface of the stone had been covered with such lines, their general disposition became evident; they formed circles, which girdled the stone in exactly the same way as meridians of longitude girdle the earth ; and there were two points at opposite ends of the stone through which all the circles passed, just as all the meridians pass through the Arctic and Antarctic poles of the earth.* Struck by the analogy, Peregrinus proposed to call these two points the poles of the magnet : and he observed that the way in which magnets set themselves and attract each other depends solely on the position of their poles, as if these were the seat of the magnetic power. Such was the origin of those theories of poles and polarization which in later ages have played so great a part in Natural Philosophy.

The observations of Peregrinus were greatly extended not long before the tune of Descartes by William Gilberd or Gilbertf (6. 1540, d. 1603). Gilbert was born at Colchester: after studying at Cambridge, he took up medical practice in London, and had the honour of being appointed physician to Queen Elizabeth. In 1600 he published a work* on Magnetism and Electricity, with which the modern history of both subjects begins.

Of Gilbert's electrical researches we shall speak later : in magnetism he made the capital discovery of the reason why magnets set in definite orientations with respect to the earth ; which is, that the earth is itself a great magnet, having one of its poles in high northern and the other in high southern latitudes. Thus the property of the compass was seen to be included in the general principle, that the north-seeking pole of

• " Procul dubio oranes lineae hujusmodi in duo puncta concurrent sicut omnes orbes meridian! in duo concurrunt polos mundi oppositos."

t The form in the Colchester records is Gilberd.

J Gulielmi Gilberti de Magnete, Magneticisque corporibus, et de magno magnete tellure : London, 1600. An English translation by P. F. Mottelay was published in 1893.

in the Seventeenth Century. 9

every magnet attracts the south-seeking pole of every other magnet, and repels its north-seeking pole.

Descartes attempted* to account for magnetic phenomena by his theory of vortices. A vortex of fluid matter was postulated round each magnet, the matter of the vortex entering by one pole and leaving by the other : this matter was supposed to act on iron and steel by virtue of a special resistance to its motion afforded by the molecules of those substances.

Crude though the Cartesian system was in this and many other features, there is no doubt that by presenting definite conceptions of molecular activity, and applying them to so wide a range of phenomena, it stimulated the spirit of inquiry, and prepared the way for the more accurate theories that came after. In its own day it met with great acceptance: the confusion which had resulted from the destruction of the old order was now, as it seemed, ended by a reconstruction of knowledge in a system at once credible and complete. Nor did its influence quickly wane ; for even at Cambridge it was studied long after Newton had published his theory of gravitation ;f and in the middle of the eighteenth century Euler and two of the Bernoullis based the explanation of magnetism on the hypothesis of vertices.*

Descartes' theory of light rapidly displaced the conceptions which had held sway in the Middle Ages. The validity of his explanation of refraction was, however, called in question by his fellow-countryman Pierre de Ferinat (b. 1601, d. 1665),§ and a controversy ensued, which was kept up by the Cartesians long after the death of their master. Fermat