A History of the Theories of Aether and Electricity (1910) — part 25 of 29

But the hypothesis of Varley and Crookes was before long involved in difficulties. Taitf in 1880 remarked that if the particles are moving with great velocities, the periods of the luminous vibrations received from them should be affected to a measurable extent in accordance with Doppler's principle. Tait tried to obtain this effect, but without success. It may, however, be argued that if, as Crookes supposed, the particles become luminous only when they have collided with other particles, and have thereby lost part of their velocity, the phenomenon in question is not to be expected.

• Gott. Nach., 2 February, 1881; reprinted, Ann. d. Phys. xiii (1881), p. 191. t Proc. Roy. Soc. Edinb. x (1880). p. 430.

396 Conduction in Solutions and Gases ,

The alternative to the molecular-torrent theory is to suppose that the cathode radiation is a disturbance of the aether. This view was maintained by several physicists,* and notably by Hertz,f who rejected Varley's hypothesis when he found experimentally that the rays did not appear to produce any external electric or magnetic force, and were apparently not affected by an electrostatic field. It was, however, pointed out by Fitz Gerald* that external space is probably screened from the effects of the rays by other electric actions which take place in the discharge tube.

It was further urged against the charged-particle theory that cathode rays are capable of passing through films of metal which are so thick as to be quite opaque to ordinary light ;§ it seemed inconceivable that particles of matter should not be stopped by even the thinnest gold-leaf. At the time of Hertz's experiments on the subject, an attempt to obviate this difficulty was made by J.-J. Thomson,! | who suggested that the metallic film when bombarded by the rays might itself acquire the property of emitting charged particles, so that the rays which were observed on the further side need not have passed through the film. It was Thomson who ultimately found the true explanation ; but this depended in part on another order of ideas, whose introduction and development must now be traced.

The tendency, which was now general, to abandon the electron-theory of Weber in favour of Maxwell's theory involved certain changes in the conceptions of electric charge.

• E.g. E. Wiedemann, Ann. d. Phys. x (1880), p. 202; translated, Phil. Mag. x (1880), p. 357. E. Goldstein, Ann. d. Phys. xii (1881), p. 249.

t Ann. d. Phys. xix (1883), p. 782.

J Nature, November 5, 1896 ; Fitz Gerald's Scientific Writings, p. 433.

§ The penetrating power of the rays had been noticed by Hittorf, and by E. "Wiedemann and Ebert, Sitzber. d. phys.-med. Soc. zu Erlangen, llth December, 1891. It was investigated more thoroughly by Hertz, Ann. d. Phys. xlv (1892), p. 28, and by Philipp Lenard, of Bonn, Ann. d. Phys. li (1894), p. 225 ; lii (1894), p. 23, who conducted a series of experiments on cathode rays which had passed out of the discharge tube through a thin window of aluminium.

|| J. J. Thomson, Recent Researches, p. 126.

from Faraday to J . J . Thomson. 397

•

In the theory of Weber, electric phenomena were attributed to the agency of stationary or moving charges, which could most readily be pictured as having a discrete and atom-like existence. The conception of displacement, on the other hand, which lay at the root of the Maxwellian theory, was more in harmony with the representation of electricity as something of a continuous nature; and as Maxwell's views met with increasing acceptance, the atomistic hypothesis seemed to have entered on a period of decay. Its revival was due largely to the advocacy of Helmholtz,* who, in a lecture delivered to the Chemical Society of London in 1881, pointed outf that it was thoroughly in accord with the ideas of Faraday,J on which Maxwell's theory was founded. " If," he said, " we accept the hypothesis that the elementary substances are composed of atoms, we cannot avoid concluding that electricity also, positive as well as negative, is divided into definite elementary portions which behave like atoms of electricity."

When the conduction of electricity is considered in the light of this hypothesis, it seems almost inevitable to conclude that the process is of much the same character in gases as in electrolytes ; and before long this view was actively maintained. It had indeed long been known that a compound gas might be decomposed by the electric discharge ; and that in some cases the constituents are liberated at the electrodes in such a way as to suggest an analogy with electrolysis. The question had been studied in 1861 by Adolphe Perrot, who examined § the gases liberated by the passage of the electric spark through steam. He found that while the product of this action was a detonating mixture of hydrogen and oxygen, there was a decided preponderance of hydrogen at one pole and of oxygen at the other.

The analogy of gaseous conduction to electrolysis was applied by W. Giese,|| of Berlin, in 1882, in order to explain

• Cf. also G. Johnstone Stoney, Phil. Mag., May, 1881.

f Journ. Chem. Soc. xxxix (1881), p. 277. \ Cf. p. 200.

§ Annales de Chimie (3), Ixi, p. 161.

|| Ann. d. Phys. xvii (1882), pp. 1, 236, 519.

398 Conduction in Solutions and Gases ,

the conductivity of the hot gases of flames. " It is assumed," he wrote, " that in electrolytes, even before the application of an external electromotive force, there are present atoms or atomic groups — the ions, as they are called — which originate when the molecules dissociate ; hy these the passage of electri- city through the liquid is effected, for they are set in motion by the electric field and carry their charges with them. We shall now extend this hypothesis by assuming that in gases also the property of conductivity is due to the presence of ions. Such ions may be supposed to exist in small numbers in all gases at the ordinary temperature and pressure ; and as the temperature rises their numbers will increase."

Ideas similar to this were presented in a general theory of the discharge in rarefied gases, which was devised two years later by Arthur Schuster, of Manchester.* Schuster remarked that when hot liquids are maintained at a high potential, the vapours which rise from them are found to be entirely free from electrification ; from which he inferred that a molecule striking an electrified surface in its rapid motion cannot carry away any part of the charge, and that one molecule cannot communicate electricity to another in an encounter in which both molecules remain intact. Thus he was led to the con- clusion that dissociation of the gaseous molecules is necessary for the passage of electricity through gases. f

Schuster advocated the charged-particle theory of cathode rays, and by extending and interpreting an experiment of Hittorf s was able to adduce strong evidence in its favour. He placed the positive and negative electrodes so close to each other that at very low pressures the Crookes' dark space extended from the cathode to beyond the anode. In these circumstances it was found that the discharge from the positive electrode always passed to the nearest point of the inner boundary of the Crookes' dark space — which, of course, was in

• Proc. Roy. Soc. xxxvii (1884), p. 317.

t In the case of an elementary gas, this would imply dissociation of the molecule into two atoms chemically alike, but oppositely charged ; in electrolysis the .dissociation is into two chemically unlike ions.

Jrom Faraday to J . J . Thomson* 399

the opposite direction to the ijathode. Thus, in the neighbour- hood of the positive discharge, the current was flowing in two opposite directions at closely adjoining places ; which could scarcely happen unless the current in one direction were carried by particles moving against the lines of force by virtue of their inertia.

Continuing his researches, Schuster* showed in 1887 that a steady electric current may be obtained in air between electrodes whose difference of potential is but small, provided that an independent current is maintained in the same vessel ; that is to say, a continuous discharge produces in the air such a condition that conduction occurs with the smallest electromotive forces. This effect he explained by aid of the hypothesis previously advanced ; the ions produced by the main discharge become diffused throughout the vessel, and, coming under the influence of the field set up by the auxiliary electrodes, drift so as to carry a current between the latter.

A discovery related to this was made in the same year by Hertz,f in the course of the celebrated researches? which have been already mentioned. Happening to notice that the passage of one spark is facilitated by the passage of another spark in its neighbourhood, he followed up the observation, and found the phenomenon to be due to the agency of ultra-violet light emitted by the latter spark. It appeared in fact that the distance across which an electric spark can pass in air is greatly increased when light of very short wave-length is allowed to fall on the spark-gap. It was soon found§ that the effective light is that which falls on the negative electrode of the gap ; and Wilhelm Hallwachs|| extended the discovery

• Proc. Roy. Soc. xlii (1887), p. 371. Hittorf had discovered that very small electromotive forces are sufficient to cause a discharge across a space through which the cathode radiation is passing.

t Berlin Ber., 1887, p. 487 ; Ann. d. Phys. xxxi (1887), p. 983 ; Electric Waves (English ed.), p. 63.

I Cf.p. 357.

§ By E. Wiedemann and Ebert, Ann. d. Phys. xxxiii (1888), p. 241.

|| Ann. d. Phys. xxxiii (1888), p. 301.

400 Conduction in Solutions and Gases,

by showing that when a sheet of metal is negatively electrified and exposed to ultra-violet light, the adjacent air is thrown into a state which permits the charge to leak rapidly away.

Interest was now thoroughly aroused in the problem of conductivity in gases ; and it was generally felt that the best hope of divining the nature of the process lay in studying the discharge at high rarefactions. " If a first step towards under- standing the relations between aether and ponderable matter is to be made," said Lord Kelvin in 1893,* " it seems to me that the most hopeful foundation for it is knowledge derived from experiments on electricity in high vacuum."

Within the two following years considerable progress was effected in this direction. J. J. Thomson,^ by a rotating-mirror method, succeeded in measuring the velocity of the cathode rays, finding it to be| 1*9 x 107 cm./sec. ; a value so much smaller than that of the velocity of light that it was scarcely possible to conceive of the rays as vibrations of the aether. A further blow was dealt at the latter hypothesis when Jean Perrin,§ having received the rays in a metallic cylinder, found that the cylinder became charged with resinous electricity. When the rays were deviated by a magnet in such a way that they could no longer enter the cylinder, it no longer acquired a charge. This appeared to demonstrate that the rays transport negative electricity.

With cathode rays is closely connected another type of radiation, which was discovered in December, 1895, by W. C. K6ntgen.ll The discovery seems to have originated in an accident : a photographic plate which, protected in the usual way, had been kept in a room in which vacuum-tube experiments were carried on, was found on development to show distinct markings. Experiments suggested by this showed

• Proc. Roy. Soc. liv (1893), p. 389. t Phil. Mag. xxxviii (1894), p. 358.

£ The value found by the same investigator in 1897 was much larger than this. $ Cornptes Rendus, cxxi (1895), p. 1130.

|| Sitzungsber. der Wiirzburger Physikal. -Medic. Gesellschaft, 1895 ; reprinted, Ann. d. Phys. Ixiv (1898), pp. 1, 12; translated, Nature, liii (1896), p. 274.

Jrom Faraday to J. J . Thomson. 401

that radiation, capable of affecting sensitive plates and of causing fluorescence in certain substances, is emitted by tubes in which the electric discharge is passing; and that the radia- tion proceeds from the place where the cathode rays strike the glass walls of the tube. The X-rays, as they were called by their discoverer, are propagated in straight lines, and can neither be refracted by any of the substances which refract light, nor deviated from -their course by a magnetic field ; they are moreover able to pass with little absorption through many substances which are opaque to ordinary and ultra-violet light — a property of which considerable use has been made in surgery.

The nature of the new radiation was the subject of much speculation. Its discoverer suggested that it might prove to represent the long-sought-for longitudinal vibrations of the aether ; while other writers advocated the rival claims of aethereal vortices, infra-red light, and "sifted" cathode rays. The hypothesis which subsequently obtained general acceptance was first propounded by Schuster* in the month following the publication of Kontgen's researches. It is, that the X-rays are transverse vibrations of the aether, of exceedingly small wave- length. A suggestion which was put forward later in the year by E. Wiechertf and Sir George StokesJ to the effect that the rays are pulses generated in the aether when the glass of the discharge tube is bombarded by the cathode particles, is not really distinct from Schuster's hypothesis ; for ordinary white light likewise consists of pulses, as Gouy§ had shown, and the essential feature which distinguishes the Eontgen pulses is that the harmonic vibrations into which they can be resolved by Fourier's analysis are of very short period.

• Nature, January 23, 1896, p. 268. Fitz Gerald independently made the same suggestion in a letter to O. J. Lodge, printed in the Electrician xxxvii, p. 372.

t Ann. d. Phys. lix (1896), p. 321.

I Xature, September 3, 1896, p. 427 : Proc. Canib. Phil. Soc. ix (1896), p. 215 ; Mem. Manchester Lit. & Phil. Soc. xli (1896-7).

$ Journ. de Phys. v (1886), p. 354.

2 D

402 Conduction in Solutions and Gases,

The rapidity of the vibrations explains the failure of all attempts to refract the X-rays. For in the formula

«• = !- "**

of the Maxwell-Sellmeier theory,* n denotes the frequency, and so is in this case extremely large ; whence we have

/*' = !,

i.e., the refractive index of all substances for the X-rays is unity. In fact, the vibrations alternate too rapidly to have an effect on the sluggish systems which are concerned in refraction. Some years afterwards H. Haga and C. H. Wind,f having measured the diffraction-patterns produced by X-rays, concluded that the wave-length of the vibrations concerned was of the

o

order of one Angstrom unit, that is about 1/6000 of the wave- length of the yellow light of sodium.

One of the most important properties of X-rays was discovered, shortly after the rays themselves had become known, by J. J. Thomson,]: who announced that when they pass through any substance, whether solid, liquid, or gaseous, they render it conducting. This he attributed, in accordance with the ionic theory of conduction, to " a kind of electrolysis, the molecule of the non-conductor being split up, or nearly split up, by the Kb'ntgen rays."

The conductivity produced in gases by this means was at once investigated! more closely. It was found that a gas which had acquired conducting power by exposure to X-rays lost this quality when forced through a plug of glass-wool; whence it was inferred that the structure in virtue of which the gas conducts is of so coarse a character that it is unable to survive the passage through the fine pores of the plug. The

• Cf. p. 293.

t Proceedings of the Amsterdam Acad., March 25th, 1899 (English edition, i, p. 420), and September 27th, 1902 (English edition, v, p. 247). % Nature, February 27, 1896, p. 391. § J. J. Thomson and E. Rutherford, Phil. Mag. xlii (1896), p. 392.

from Faraday to J. J. Thomson. 403

conductivity was also found to be destroyed when an electric current was passed through the gas — a phenomenon for which a parallel may be found in electrolysis. For if the ions were removed from an electrolytic solution by the passage of a current, the solution would cease to conduct as soon as sufficient electricity had passed to remove them all ; and it may be supposed that the conducting agents which are produced in a gas by exposure to X-rays are likewise abstracted from it when they are employed to transport charges.

The same idea may be applied to explain another property of gases exposed to X-rays. The strength of the current through the gas depends both on the intensity of the radiation and also on the electromotive force ; but if the former factor be constant, and the electromotive force be increased, the current does not increase indefinitely, but tends to attain a certain " saturation " value. The existence of this saturation value is evidently due to the inability of the electromotive force to do more than to remove the ions as fast as they are produced by the rays.

Meanwhile other evidence was accumulating to show that the conductivity produced in gases by X-rays is of the same nature as the conductivity of the gases from flames and from the path of a discharge, to which the theory of Giese and Schuster had already been applied. One proof of this identity was supplied by observations of the condensation of water- vapour into clouds. It had been noticed long before by John Aitken* that gases rising from flames cause precipita- tion of the aqueous vapour from a saturated gas; and E. von Helmholtzf had found that gases through which an electric discharge has been passed possess the same property. It was now shown by C. T. E. Wilson, % working in the Cavendish Laboratory at Cambridge, that the same is true of gases which have been exposed to X-rays. The explanation

• Trans. R. S. Edinb. xxx (1880), p. 337.

t Ann. d. Phys. xxxii (1887), p. 1.

% Proc. Roy. Soc., March 19, 1896 ; Phil. Trans., 1897, p. 265.

2 T> 2

404 Conduction in Solutions and Gases,

furnished by the ionic theory is that in all three cases the gas contains ions which act as centres of condensation for the vapour.

During the year which followed their discovery, the X-rays were so thoroughly examined that at the end of that period they were almost better understood than the cathode rays from which they derived their origin. But the obscurity in which this subject had been so long involved was now to be dispelled.

Lecturing at the Eoyal Institution on April 30th, 1897, J. J. Thomson advanced a new suggestion to reconcile the molecular- torrent hypothesis with Lenard's observations of the passage of cathode rays through material bodies. " We see from Lenard's table," he said, " that a cathode ray can travel through air at atmospheric pressure a distance of about half a centimetre before the brightness of the phosphorescence falls to about half its original value. Now the mean free path of the molecule of air at this pressure is about 10~5 cm., and if a molecule of air were projected it would lose half its momentum in a space comparable with the mean free path. Even if we suppose that it is not the same molecule that is carried, the effect of the obliquity of the collisions would reduce the momentum to half in a short multiple of that path.

" Thus, from Lenard's experiments on the absorption of the rays outside the tube, it follows on the hypothesis that the cathode rays are charged particles moving with high velocities that the size of the carriers must be small compared with the dimensions of ordinary atoms or molecules.* The assumption of a state of matter more finely subdivided than the atom of an element is a somewhat startling one ; but a hypothesis that would involve somewhat similar consequences — viz. that the so-called elements are compounds of some primordial element — has been put forward from time to time by various chemists."

• A similar suggestion was made by E. Wiechert, Verhandl. d. physik.-ocon. Gesellscb. in Konigsberg, Jan. 1897.

from Faraday tcr J. J . Thomson. 405

Thomson's lecture drew from Fitz Gerald* the suggestion that " we are dealing with free electrons in these cathode rays " — a remark the point of which will become more evident when we come to consider the direction in which the Maxwellian theory was being developed at this time.

Shortly afterwards Thomson himself published an accountf of experiments in which the only outstanding objections to the charged-particle theory were removed. The chief of these was Hertz' failure to deflect the cathode rays by an electrostatic field. Hertz had caused the rays to travel between parallel plates of metal maintained at different potentials ; but Thomson now showed that in these circumstances the rays generate ions in the rarefied gas, which settle on the plates, and annul the electric force in the intervening space. By carrying the exhaustion to a much higher degree, he removed this source of confusion, and obtained the expected deflexion of the rays.

The electrostatic and magnetic deflexions taken together suffice to determine the ratio of the mass of a cathode particle to the charge which it carries. For the equation of motion of the particle is

rar = eE .+ e[v. H],

where r denotes the vector from the origin to the position of the particle ; E and H denote the electric and magnetic forces ; e the charge, m the mass, and v the velocity of the particle. By observing the circumstances in which the force #E, due to the electric field, exactly balances the force e [v . H], due to the magnetic field, it is possible to determine v ; and it is readily seen from the above equation that a measurement of the deflexion in the magnetic field supplies a relation between v and m/e ; so both v and m/e may be determined. Thomson found the value of m/e to be independent of the nature of the rarefied gas : its amount was 10~7 (grammes/electromagnetic units of charge), which is only about the thousandth part of the value of m/e for the hydrogen atom in electrolysis. If the charge

• Electrician, May 21, 1897. t Phil. Mag. xliv (1897), p. 298.

406 Conduction in Solutions and Gases,

were supposed to be of the same order of magnitude as that on an electrolytic ion, it would be necessary to conclude that the particle whose mass was thus measured is much smaller than the atom, and the conjecture might be entertained that it is the primordial unit or corpuscle of which all atoms are ultimately composed.*

The nature of the resinously charged corpuscles which constitute cathode rays being thus far determined, it became of interest to inquire whether corresponding bodies existed carrying charges of vitreous electricity — a question to which at any rate a provisional answer was given by W. Wienf of Aachen in the same year. More than a decade previously E. Goldstein^ had shown that when the cathode of a discharge-tube is perforated, radiation of a certain type passes outward through the per- forations into the part of the tube behind the cathode. To this radiation he had given the name canal rays. Wien now showed that the canal rays are formed of positively charged particles, obtaining a value of m/e immensely larger than Thomson had obtained for the cathode rays, and indeed of the same order of magnitude as the corresponding ratio in electrolysis.

The disparity thus revealed between the corpuscles of cathode rays and the positive ions of Goldstein's rays excited great interest ; it seemed to offer a prospect of explaining the curious differences between the relations of vitreous and of resinous electricity to ponderable matter. These phenomena had been studied by many previous investigators ; in particular Schuster,§ in the Bakerian lecture of 1890, had remarked that " if the law of impact is different between the molecules of the gas and the positive and negative ions respectively, it follows that the rate of diffusion of the two sets of ions will in general be different," and had inferred from his theory of the discharge

• The value of m/e for cathode rays was determined also in the same year by W. Kaufmaim, Ann. d. Phys. Ixi, p. 544.

t Verh;»ndl. der physik. Gesells. zu Berlin, xvi (1897), p. 165; Ann. d. Phys. Ixv (1898), {>. 440.

J Berlin Sitzungsber., 1886, p. 691. § Proc. R.S. xlvii (1890), p. 526.

Jrom Faraday to J . J . Thomson. 407

that " the negative ions diffuse more rapidly." This inference was confirmed in 1898 by John Zeleny,* who showed that of the ions produced in air by exposure to X-rays, the positive are decidedly less mobile than the negative.

The magnitude of the electric charge on the ions of gases was not known with certainty until 1898, when a plan for determining it was successfully executed by J. J. Thomson.f The principles on which this celebrated investigation was based are very ingenious. By measuring the current in a gas which is exposed to Rontgen rays and subjected to a known electro- motive force, it is possible to determine the value of the product nev, where n denotes the number of ions in unit volume of the gas, e the charge on an ion, and v the mean velocity of the positive and negative ions under the electromotive force. As v had been already determined ,J the experiment led to a determination of ne ; so if n could be found, the value of e might be deduced.

The method employed by Thomson to determine n was founded on the discovery, to which we have already referred, that when X-rays pass through dust-free air, saturated with aqueous vapour, the ions act as nuclei around which the water condenses, so that a cloud is produced by such a degree of saturation as would ordinarily be incapable of producing con- densation. The size of the drops was calculated from measure- ments of the rate at which the cloud sank ; and, by comparing this estimate with the measurement of the mass of water deposited, the number of drops was determined, and hence the number n of ions. The value of e consequently deduced was found to be independent of the nature of the gas in which the ions were produced, being approximately the same in hydrogen as in air, and being apparently in both cases the same as for the charge carried by the hydrogen ion in electrolysis.

Since the publication of Thomson's papers his general conclusions regarding the magnitudes of e and m/e for gaseous

• Phil. Mag. xlvi (1898;, p. 120. t Phil. Mag. xlvi (1898), p. 528.

• By E. Rutherford, Phil. Mag. xliv (1897), p. 422.

408 Conduction in Solutions and Gases,

ions have been abundantly confirmed. It appears certain that electric charge exists in discrete units, vitreous and resinous, each of magnitude 15 x 10~19 coulombs approximately. Each ion, whether in an electrolytic liquid or in a gas, carries one (or an integral number) of these charges. An electrolytic ion also contains one or more atoms of matter; and a positive gaseous ion has a mass of the same order of magnitude as that of an atom of matter. But it is possible in many ways to produce in a gas negative ions which are not attached to atoms of matter ; for these the inertia is only about one- thousandth of the inertia of an atom; and there is reason for believing that even this apparent mass is in its origin purely electrical.

The closing years of the nineteenth century saw the founda- tion of another branch of experimental science which is closely related to the study of conduction in gases. When Rontgen announced his discovery of the X-rays, and described their power of exciting phosphorescence, a number of other workers commenced to investigate this property more completely. In particular, Henri Becquerel resolved to examine the radiations which are emitted by the phosphorescent double sulphate of uranium and potassium after exposure to the sun. The result was communicated to the French Academy on February 24th, 1896.f " Let a photographic plate," he said, " be wrapped in two sheets of very thick black paper, such that the plate is not affected by exposure to the sun for a day. Outside the paper place a quantity of the phosphorescent substance, and expose the whole to the sun for several hours. When the plate is developed, it displays a silhouette of the phosphorescent substance. So the latter must emit radiations which are capable of passing through paper opaque to ordinary light, and of reducing salts of silver."

At this time Becquerel supposed the radiation to have been excited by the exposure of the phosphorescent substance to the sun ; but a week later he announced^ that it persisted for an

• Cf. p. 343. f Comptes Rendus, cxxii (1890), p. 420.

I Ibid., cxxii (March 2nd, 1896), p. 501.

from Faraday to J'. J. Thomson. 409

indefinite time after the substance had been removed from the sunlight, and after the luminosity which properly constitutes phosphorescence had died away ; and he was thus led to con- clude that the activity was spontaneous and permanent. It was soon found that those salts of uranium which do not phosphoresce — e.g., the uranous salts, — and the metal itself, all emit the rays ; and it became evident that what Becquerel had discovered was a radically new physical property, possessed by the element uranium in all its chemical compounds.

Attempts were now made to trace this activity in other substances. In 1898 it was recognized in thorium and its compounds;* and in the same year P. Curie and Madame Sklodowska Curie announced to the French Academy the separation from the mineral pitchblende of two new highly active elements, to which they gave the names of poloniumf and radium.}: A host of workers was soon engaged in studying the properties of the Becquerel rays. The discoverer himself had shown§ in 1896 that these rays, like the X- and cathode rays, impart conductivity to gases. It was found in 1899 by Kutherfordll that the rays from uranium are not all of the same kind, biit that at least two distinct types are present ; one of these, to which he gave the name a-rays, is readily absorbed ; while another, which he named /3-radiation, has a greater penetrating power. It was then shown by Giesel, Becquerel, and others, that part of the radiation is deflected by a magnetic field,1T and part is not.** After this Monsieur and Madame Curieft found that the deviable rays carry negative electric charges,

• By Schmidt, Ann. d. Phys., Ixv (1898), p. 141 ; and by Ma.iame Curie, Comptes Rendus, cxxvi (1898), p. 1101.

t Comptes Rendus, cxxvii (1898), p. 175. % Ibid., cxxvii (1898), p. 1215.

§ Ibid., cxxii (1896), p. 559. || Phil. Mag. (5), xlvii (1899), p. 109.

H Giesel, Ann. d. Phys. Ixix (1899), p. 834 (working with polonium); Becquerel, Comptes Rendus, cxxix (1899), p. 996 (working with radium) ; Meyer andv. Schweidler, Phys. Zeitschr. i (1899), p. 113 (working with polonium and radium).

** Bc-cquerel, Comptes Rendus, cxxix (1889), p. 1205); cx\x (1900), pp. 206, 372. Curie, ibid, exxx (1900), p. 73.

ft Comptes Rendus, cxxx (1900), p. 647.

410 Conduction in Solutions and Gases.

and Becquerel* succeeded in deviating them by an electrostatic field. The deviable or j3- rays were thus clearly of the same nature as cathode rays ; and when measurements of the electric and magnetic deviations gave for the ratio m/e a value of the order 10~7, the identity of the /3-particles with the cathode-ray corpuscles was fully established.

The subsequent history of the new branch of physics thus created falls outside the limits of the present work. We must now consider the progress which was achieved in the general theory of aether and electricity in the last decade of the nineteenth century.

• Comptes Eendus, c>xx (1900), p. 809.

CHAPTEE XII.

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

THE attempts of Maxwell* and of Hertzf to extend the theory of the electromagnetic field to the case in which ponderable bodies are in motion had not been altogether successful. Neither writer had taken account of any motion of the material particles relative to the aether entangled with them, so that in both investigations the moving bodies were regarded simply as homogeneous portions of the medium which fills all space, distinguished only by special values of the electric and magnetic constants. Such an assumption is evidently incon- sistent with the admirable theory by which FresnelJ had explained the optical behaviour of moving transparent bodies ; it was therefore not surprising that writers subsequent to Hertz should have proposed to replace his equations by others designed to agree with Fresnel's formulae. Before discussing these, however, it may be well to review briefly the evidence for and against the motion of the aether in and adjacent to moving ponderable bodies, as it appeared in the last decade of the nineteenth century.

The phenomena of aberration had been explained by Young§ on the assumption that the aether around bodies is unaffected by their motion. But it was shown by Stokes|| in 1845 that this is not the only possible explanation. For suppose that the motion of the earth communicates motion to the neighbour- ing portions of the aether ; this may be regarded as superposed on the vibratory motion which the aethereal particles have

• Cf p. 288. t Cf. p. 365. I Cf. p. 116. $ Cf. p. 115.

|| Phil. Mag. xxvii (1845), p. 9 ; xxviii (1846), p. 76; xxix (1846), p. 6.

-412 The Theory of Aether and Electrons in the