Opening a Window into the Atom

The discovery of X rays had directly and immediately led to the discovery of radioactivity, and soon the two provided an important foundation for formulating the concept of the "electron," which in turn stimulated further research into X rays and radioactivity.

Keywords: X rays, radioactivity, electron, the history of science

1. RONTGEN'S DISCOVERY

The centennials of the three important discoveries celebrated in 1995- 1997 provided an opportunity for revising our ideas of these events, and new research shows that the three were more closely linked than has been thought before.

In the summer of 1895, Wilhelm Conrad Rijntgen (1 845-1923) Phys- ics Professor at Wiirzburg University, and an established researcher, decided to switch to a new research area: cathode rays (Fig. 1). For sev- eral decades their nature was a subject of a heated controversy between supporters of the corpuscular theory (mostly British) and of the wave theory (primarily German). In 1894, Philipp Lenard discovered a phe- nomenon that tilted the balance in favor of waves: he provided a vac- uum tube with a very thin aluminum window and observed that some radiation was coming out of the tube, producing photographic images

C~immen1.r At. Mol Phys. 1999, Vol. 34, No. 3-6, pp. 183-199 Repr~nts available directly from the publisher Photvcopylng penn~tted by l~cense only

0 1999 OPA (Overseas Publ~shers Assoc~ation) N.V. Published by license under the Gordon and Breach

Sc~ence Publishers ~mprint. Prlnted in Malaysla

and fluorescence, but only near the window. Lenard concluded that the

radiation consisted of cathode rays, penetrating the metal, which

appeared inconsistent with their particulate nature.

FIGURE 1 Portarait of W. C. Rdntgen. From E. N. Santini, Itt photographie d travers les

corps opdques (Paris, I 896)

R<intgen began with repeating some experiments of his predecessors,

including those of Lenard. On 8 November, 1895, while checking the

dark cover of his cathode-rays apparatus (apparently, like Lenard, he

was looking for a radiation outside the tube), Rdntgen noticed a shadow

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on a fluorescent screen laying on the table far from the tube. He beganworking non-stop on the new phenomenon under complete secrecy andmade the results public early in January 1896 (Fig. 2).

FIGURE 2 The title page of W. Rcintgen's famous paper "Ueber eine neue Art vonStrahlen" (On a new kind of rays) published as a separate pamphlet. The unusual date"Ende 1895" means that the paper was urgently published in the last three days of the year1895 by order of von Kdlliker, the President of the physikalisch-medicinischen Gesells-chaft zu Wiirzburs

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Rdntgen concluded that the tube emitted radiation with amazing fea-

tures. On the one hand, it produced sharp images on a photographicplate like light (thus Rcintgen called it the "X rays"). On the other hand,the X rays considerably differed from light: they easily penetrated

opaque bodies and showed neither reflection and refraction nor polari-

zation and interference (Fig. 3). He concluded that X rays could not betransverse waves, nor could they be the cathode rays exiting the tube,because X rays penetrated air much more easily than Lenard rays anddid not deviate in a strong magnetic field. Rtintgen decided that X raysmust by longitudinal waves in the ether used by theorists in optics andsought in vain by experimentalists since the 1830s.'

FIGURE 3 The photograph taken by W. Rdntgen of his wife's hand. From W. C. Rdntgen,"On the new kind of rays," Nature 53, 276 (23 Jamary, 1896)

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2. X RAYS: WAVES OR PARTICLES?

A few physicists first learned about the discovery from personal com_munications from Rcintgen, the rest from newspapers. The firstresponses began to come in the last decade of January 1g96. While afew prominent scientists (Lord Kelvin, Ludwig Boltzmann, JosephThomson) initially supported Rcintgen's theory a number of others wereoffered, most in a few weeks. Some of them were quite extravagant andephemeral, such as Thomas Edison,s theory of sound waves and AlbertMichelson's theory of vortices. others were more sound and had a last-ing impact; these included transverse periodic waves, cathode-ray parti-cles, and wave pulses (George Stokes and J. J. Thomson). I will brieflyreview only transverse waves and particles, as the most relevant for thispaper.

A. Transverse waves

The first suggestions of transverse waves came in late January and earlyFebruary from Arthur Schuster, George Fitzgerald, and Dmitrij Gold_hammer.2 They assumed that the wavelength Jf X .uy, is comparable tothe size of a molecule and that the properties of the ether remain con-stant within this area: in this case waves could travel in the substancewith the same speed as in vacuum, producing no refraction. The sameargument explained why X rays penetrate metals in apparent violationof Maxwell's theory. The necessary high frequency had to be producedby electrical vibrations within the molecule. The theory of dispersionalso implied that the index of refraction for extremely short waves (1, =0) must be very close to l. Similar arguments also came from the theoryof diffraction: for extremely small wavelength a diffraction image is notdistinguishable from the geometric shadow.

Attempts to test this hypothesis by direct measurement of the wave-lengths of X rays in interference or diffraction experiments failed. Dueto technical difficulties, the results varied from I nm to g30 nm, whichofcourse provided no proofeven ofthe wave nature ofx rays. The firstwavelengths of X rays that were in the correct range (about 0.1 nm)were obtained by Haga and wind (in Holland) in lg99 using diffractionon a narrow slit.

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Experimenting with polarization did not help either. The early positive

results of Boris Galitzin and A. Kamojitsky were never confirmed.3 The

absorption of X rays by two overlapping crystals did not depend on the

mutual orientation of their optical axes, nor did a single crystal change

its absorption when its axis made different angles with the beam of X

rays. An attempt to polarize X rays by transmission through a diffrac-

tion grating also failed. The first positive (albeit indirect) signs of polar-

ization of X rays were obtained by Charles Barkla only in 1905.

B. Particles

With no real direct experimental support for waves the corpuscular the-

ory enjoyed an equal standing. A number of physicists believed that X

rays were a part of cathode rays exiting the tube, more penetrating and

less deviated by a magnet than those inside the tube. In 1897, Rdntgen

and Lenard switched to this hypothesis. The supporters ofthe corpuscu-

lar theory of the cathode rays extended it to X rays. One of the argu-

ments was that a wave theory cannot explain that absorption of X rays

depended primarily on the density of substances. In particular, Oliver

Lodge supposed that X rays consisted of particles in the cathode-ray

beam that lost their charge when colliding with the glass wall or the

anti-cathode.4 The neutrality of these particles explained the absence of

deviation of X rays in a magnetic field and their high penetrability.

Some physicists claimed on the basis of experiments that X rays contain

charged panicles together with the neutral ones.

Thus, for several years the nature of X rays was not at all demon-

strated. Physicists' views on the subject were based on negative evi-

dence and shaped by their personal predilections. Frustrated with their

efforts to prove the wave properties of X rays, physicists switched to

studying their absorption which apparently did not require any knowl-

edge oftheir nature. It is there where they found additional evidence for

the nature of X rays, and it came from an unexpected direction.

3. BECQUEREL'S DISCOVERY

The absence of a theory of X rays prompted a number of scientists to

depart from a "normal" way of following up their discovery, by repeat-

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ing and modifying Rcintgen's experiments, in favor of searching altema-tive ways of producing X rays, that is, invisible and penetrating rays,but without a cathode-ray tube. For instance, a week after Henri poin-car6 briefly informed the members of the paris Academy of Sciences ofRcintgen's discovery on 20 January, 1896 (meetings were herd eacnMonday), Gustave Le Bon demonstrated a new sort of invisible andpenetrating radiation. He placed a sensitive photographic plate in awooden holder, on top of the holder an object, and on top of this a metalplate completely covering the front. He exposed the apparatus for sev_eral hours to the light of a lamp or the sun and found, after developingthe plate, an image of the object. He stated that light p"n"t ut"d

"-d--board, thin iron, and copper, and claimed the existence in ordinary lightof a penetrating component, which he called brack right.It is worth noring that he had conducted such experiments for two years, but decidedto publish them only after Rcintgen's discovery.s

The following week, on 3 February, G. H. Niewenglowski reported tothe Academy that he repeated Le Bon's experiment without illuminatingthe metal plate and sti l l found images. He supposed thar right producin!images was stored in the object during prior exposures. Le Bon repliedthat he used only objects kept in the dark for a day. Another researcherreported finding no X rays in the light produced by a voltaic arc. Stillanother demonstrated photographs which proved, in his view, that apowerful induction coil created X rays without any vacuum tubes.6

At about this time Poincar6 published a paper on Rdntgen rays where,while discussing various hypotheses oftheir nature, gave special promi-nence to the fact discovered by Rdntgen that X rays originate from thefluorescent spot on the glass wall of a vacuum tube. He supposed that allbodies producing intensive fluorescence, whatever its cause, emit. inaddition to l ight rays, X-rays.7 This hypothesis had considerable reso-nance among scientists.

The first test of this hypothesis was reported on l0 February byCharles Henry. He informed the Academy that a thin layer of phospho_rescent zinc sulfide became a source of invisible rays (later, he identi-fied them with X rays) after being stimulated by X rays or even bydaylight. This convinced Henry that poincar6's hypothesis *u,

"or.".t.A similar result was described at the next meeting by Niewenglowskiwho was working with phosphorescent calcium sulfide

"*por"Jto ,un_

light: the radiation penetrated black paper and darkened a photographic

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plate. Then a week later, on 24 February, Henri Becquerel entered the

race.Antoine-Henri Becquerel, who was then 44, was a third generation

physicist who inherited his grandfather's and father's several physics

chairs in Paris, and was already a member of the Academy. He special-

ized in magneto-optics and phosphorescence. In his paper, he described

experiments with another phosphorescent substance: a crystalline plate

of double sulfate of uranium and potassium.

Becquerel wrapped a photographic plate in thick black paper, placed a

crystal on top of it and exposed the apparatus to sunlight for many

hours. The developed plate showed an outline of the crystal and also

images of metal objects interposed between the crystal and the plate. To

exclude the possibility of chemical action due to vapors produced by

sunlight, he placed a very thin glass between the crystal and the paper:

the result was the same. Becquerel's conclusion was very modest: thephosphorescent substance in question emits radiation which traverses

opaque paper and acts on a photographic plate.

The next week, Becquerel made a plate-holder out of opaque cloth

with a window in the front covered with an aluminum plate, the crystal

being attached to the aluminum cover. Several preparations were made

on Feb. 27 and 28, but since the sun showed up only intermittenly, the

apparatus was put in a dark drawer as it was. On March 1, seeing no

improvement in the weather and thinking of the Academy meeting next

day, Becquerel developed the plates with the expectation of finding very

light darkening due to the short exposure to sunlight. To his surprize, he

found very dark and clear shadows (Fig. 4). He repeated the experiment

for five hours, keeping the crystal attached to the plate-holder in the

dark, without any exposure to light. Again, he found images, although

not as dark. Becquerel concluded that his rays were similar to those ofLenard and Rcintgen, and perhaps they resulted from phosphorescence

whose persistence time was infinitely longer than that of visible light

radiation emitted by these bodies.6

Guided by this analogy with X rays, Becquerel tried to find someproperties in his radiation that were known or suspected for X rays.

Soon after the ionizing properties of X rays became public, heannounced on 9 March a similar result with uranium salts. He alsochecked his rays for reflection and reported a positive result: when apart of the crystal was covered with a metal mimor, its image was dif-

fuse as if it were a combination. of two images.e

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'tult' $/,&

FIGURE 4 Photograph of a copper cross taken by H. Becquerel: the cross was placedbetween the crystal of double sulphate of uranium and potassium and an aluminum coverof a photographic plate. This is the famous plate that sat in the dark drawer for two daysbefore being developed. From H. Becquerel, "Recherches sur une propri6t6 nouvelle de lamatidre," M6m. Acad. Sci. Paris 46, 5, pl. I (1903)

At the same meeting, Louis Troost demonstrated clear images pro-duced by crystals of hexagonal blende placed together with a photo-graphic plate and other opaque objects in the dark and illuminated bythe combustion of a magnesium band. These results, he said, confirmedPoincar6's hypothesis and provided us with a simple and easier way toreplace the vacuum tube as a source of X rays.

On 23 March, Becquerel described measuring absorption using anelectroscope. He found no significant increase in the radiation afterexposing the crystal to strong light. It meant that if the effect was due toan invisible phosphorescence, it had no connection with visible phos-phorescence or fluorescence. For example, uranium nitrate loses thephosphorescing property in solution but this is not true for the invisibleradiation.

l 9 l

Having found qualitative evidence for refraction, Becquerel checked

the radiation for double refraction and polarization. On 30 March, he

reported finding the first important difference between X rays and ura-

nium rays; thin plates of tourmaline showed polarization with uranium

rays but not with X rays.lo

Two months later Becquerel announced that the radiation was due

solely to the element uranium rather than a variety of substances. That

ended the first stage of his research into radioactivity, when he used

extensively the techniques developed for X rays and set the problems

similar to those for X rays. It turned out that developing a closer analogy

between X rays and uranium rays benefited research on X rays as well.

4. WHAT ARE X RAYS FROM THE PERSPECTIVEOF URANIUM RAYS?

A number of researchers experimenting with both X rays and "other"

rays (which they frequently lumped together) developed a new view of

X rays. Henry and Sylvanus Thompson summarized the experimentalresults as proving that Becquerel rays are transverse waves with wave-

length much shorter than those of ultra-violet light. Using the analogy

with X rays, they concluded that X rays must also be transverse waves

with even shorter wavelengths.ll Lodge concurred, proclaiming that

transverse waves came to dominate after Becquerel's discovery.12

That was very important for X-ray research. Before 1899, having no

direct experimental proof, the wave theory of X rays hung in the air, and

the whole field was on the verge of abandonment. To the end of thisperiod, physicists found enough interest in the problem of absorption to

keep them going without solving the problem of the nature of X rays.

But in the meantime they needed all the arguments they could find to

support their faith in X rays being a kind of light. And there Becquerel's

rays came in very handy.

5. "DISCOVERY OF THE ELECTRON''

If this term is of any use, its meaning must be very different from the"discovery of X rays" or the "discovery of radioactivity." The latter two

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are experimental discoveries and, thus, can be associated with a certainperson and a certain date. If the "discovery of the electron" refers to auniversal particle, which is both a part of any atom and a carrier oJ'electricity in certain media, then the "electron'i is a theoretical concept,and its introduction into science must be a process. The traditional corr-nection of the electron with 1g97 and J. J. Thomson reflects only a smallpart of this process.

It is known that the idea of discrete electricity originated from Fara-day's work on electrolysis, and that by the end of the century a numberof scientists used it for electrolysis. The charge e of that discrete ele-ment of electrolytic electricity was calculated from experiments, and itbecame known as the "electron" (Johnstone Stoney). The ratio e/m forthe hydrogen ion was found to be about tO8 Cltg.

In the 1890s, some theoreticians tried to apply the idea of particulateelectricity to conductivity in other media (Hendrik Lorentz), but theydid not offer any magnitude for either the charge e or mass _ of rfr"r"particles. Apparently, the first hint for a possible range of magnitudescame from the Zneman effect discovered late in lg96: pieterZeemanand H . Lorentz fowd elm fo;the particles of a gas heated by a flame tobe of the order of 1011 C/k*.t:

In January 1897, Emil Wiechert found, on the basis of his experimentswith the bending of cathode rays in a magnetic field, that particles incathode rays have elm of the same order l0rr ck .t+ Four mtnths later,from similar experiments, J. J. Thomson independently arrived at thesame ratio for cathode rays (Fig. 5;.15 wiechert assumed that all parti-cles have the same charge, equal to that of the hydrogen ion, whileThomson first accepted charge multiples of e. Both concluded that thehigh elm ratio for cathode rays, compared to hydrogen, should beexplained by the fact that their mass was smaller ttran itre mass of thehydrogen atom by a factor of r000. Both asserted that only a particulatenature of cathode rays with such minute particles could reconcileLenard's law that the absorption of cathode rays depended only on thedensity of a gas. Both suggested that these ..electric

atoms,, (Wechert)or "corpuscles" (Thomson) are building blocks of chemical atoms. In1899' J' J' Thomson found the charge ofparticles in photoelectric phe-nomena to be close to that of the hydrogen ion, which, assuming thesame charge for cathode.-ray particles, confirmed the magnitude of-theirmass suggested earlier. 16

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FIGURE 5 Photograph taken by J. J. Thomson of the trajectory of cathode rays in a mag-

netic field in a tube filled with hydrogen at a low pressure. From J. J. Thomson, "Cathode

rays," Notices of the Proceedings of the Meetings of the Royal Institution of Great Britain

15,419 (30 Apr i l , 1896)

In May 1897, Walter Kaufmann began his measurements of elm for

cathode rays and eventually considerably improved their precision. One

of his students, S. Simon, claimed to reduce the error to 0.l57o.In fact,

that was the precision of a specific apparatus under specific conditions,

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while the results of different researchers differed 2-3 times, and by 1900elm for cathode ray particles was known only within an order of magni-tude.

From the beginning of this research on particles as carriers of electric-ity in different media, physicists displayed considerable propensity forspeculation. For instance, after his first measurements, J. J. Thomsontook note that his result for elm was the same as the one from the Zee-man effect. Physicists measuring elm for cathode rays presumed thatthey all dealt with the same particles, however different the conditionsin their tubes and however widely varied their results. J. J. Thomson hadno doubt that the charge of photoelectric particles was the same as thosein cathode rays. Soon these generalizations penetrated a newly discov-ered field of electricity produced by radioactive substances.

6. RADIOACTIVE RAYS: WAVES OR PARTICLES?

While in 1896 everyone believed that uranium rays were waves, in thefollowing two years the wave theory began to yield to the corpuscularone, primarily because their absorption was much more similar to thatof cathode rays or X rays than of light (at that time many believed Xrays were particles). The final proof came, however, only when it wasshown that radioactive radiation contains charged particles. This hap-pened after Pierre and Marie Curie discovered, in 1898, much morepowerful sources of radiation, polonium and radium.

Ifradioactive rays are similar to cathode rays, they should deviate in amagnetic field. The first qualitative experiments were conducted byFriedrich Giesel, Stefan Meyer and Egon von Schweidler betweenOctober and November 1898. The deviation of rays by a magnetic fieldwas observed on a fluorescent screen. The researchers noted that therays deviated in the same direction as cathode rays and concluded that"the rays behave fully analogously with cathode rays.""

Unaware of this research, Henri Becquerel began his own experimentsearly in December 1899. His need to use a magnetic field came from arevision of some of his early experiments of 1896, from which he thenconcluded that uranium rays were a sort of light The problem was thateven later in 1896 Becquerel could not repeat his initial success, demon-strating that the new rays possess regular reflection, refraction, andpolarization. Two years later, negative results on polarization werereported by Schmidt and Rutherford.

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Becquerel decided to repeat his early experiments with polonium and

radium: the results were negative. Note that he assumed that spontane-

ous radiation from these different substances must have the same prop-

erlies. ln March 1899, Becquerel announced that he had misinterpreted

some of his early experiments, in particular, because of the presence of

secondary ra1s, similar to those discovered in 1897 by G. Sagnac for Xrays. Becquerel concluded that radioactive rays resemble X rays rather

than light, which were mostly viewed at the time as particles.

Becquerel decided to use a magnetic field as a new means of analyz-ing radioactive rays that could reveal heterogeneity unnoticeable in

absorption. From his first photographs of magnetic deviation of radium

rays, presented on 11 December, 1899, Becquerel concluded that therewas a "close similarity" between the radium rays and the cathode rays.He used this analogy to suggest how to calculate the velocity of radiumrays from measuring their deviation in both magnetic and electrostaticfields.

It was found that not all rays were deviated by a magnet, as the Curiesfound in January 1899: only a part ofthe radium rays were. Three weekslater, on 29 January,1900 Becquerel presented the first quantitative dataon magnetic deviation, from which he calculated the ratio mvle = 1500,in mixed Gaussian units (e= 1.6 x 10-20 emu) (where m is mass, e ischarge, and v is velocity). Becquerel thought this to be quite close to thenumbers obtained for cathode rays by J. J. Thomson, P. Lenard and W.Wien (from 1030 to 1213, with velocities between 0.67 x 1010 and 0.81x 1010 cm/s). To determine elm he needed an independent method ofmeasuring the velocity.

On 26 March, 1900, Becquerel gave the first data for electrostaticdeviation of radium rays, and from these data, combined with those onmagnetic deviation, he determined the velocity of particles to be aboutone-half the speed of light, and the ratio elm about 10" C&g. Bothnumbers, he said, were within the range of the numbers obtained forcathode rays. From then on, he talked of the deviable rays as identical tocathode ,uyt.18

In 1901, Kaufmann entered the field, calling for improving the preci-

sion of measuring elm for B-rays. His purpose, however, had nothing todo with identifying different particles: he wanted to check the theoreti-cal prediction that the mass of a very fast moving particle may dependon its velocity. To accomplish this task he needed faster particles thancathode rays, and B-rays appeared to provide this. By 1903, he claimed

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to have reduced the experimental error sufficiently to confirm MaxAbraham's theory that the mass of the electron is fully electromagnetic,within l.4o/o.

7. SAME PARTICLES OR DIFFERENT?

However, what did this precision actually mean? It was a precisionachieved fbr a specific apparatus and a specific sample of radium; itcould not prove that B-rays emitted by different substances have thesame e/m. That would require a greater variety of experiments. Withonly a few observations done by 1903, the conclusion that negative radi-oactive particles observed in different experiments were the same wastrue only within an order of magnitude.

If so, identifying B-rays with cathode rays could not be more precisethan that, even if elm for the latter were positively known. This conclu-sion appears to contradict Kaufmann, who stated that elm forslower-moving B-particles was the same as the one for cathode ruys.19To supporl his statement he compares his result for B_-rays, 1.84 x 10/,with S. Simon's result for cathode rays, 1.865 x l0/, again in mixedGaussian units. The implication is that elm is the same for negativelycharged particles in the two phenomena within l.4Vo. However, evenassuming that either result was the best in its field, the comparisonmakes sense only if the ratio had already been proven constant in thatfield, which, was, of course, not the case.

We see that impoftant conclusions about the identity of negativelycharged particles emitted by different radioactive substances, as well astheir identity to cathode rays, were based on their ratio elm being equalonly within an order of magnitude./ The whole business of "identifica-tion" of particles at the time appears to be too simplified from themodem perspective. Becquerel, for instance, said that, to complete theidentification of deviable radiation of radium with cathode rays it is suf-ficient to show either that this radiation transpol'ts negative electriccharges or that it deviates in an electrostatic field.20

Kaufmann had a similar attitude, for after quoting elm for cathoderays obtained by different researchers which differed by factors 2-3among themselves, he stated: "in any case, the numbers so closell-approached those determined from the Zeeman effect that the hypothe-sis probably first put forward by Wiechert may be unhesitatingly

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adopted: that we have to do in both cases with the same particles - viz.,

the electrons."2l (By that time "electron" began to be used to denote

presumably the same negative particles in different phenomena.) Thus,

like Becquerel with B-rays, Kaufmann did not expect particles in cath-

ode rays to differ in mass and charge. Thomson described Lenard's and

Kaufmann's measurements of e/m for cathode rays as "confirming"

rather than "improving" those of his own. He also found Becquerel's

result for p-rays "the same" as those for cathode rays. This means that

Thomson was completely happy with an agreement within an order of

magnitude.

If so, how do we reconcile such a low demand for precision in thecases just described with striving for l.4Vo of error in Kaufmann'sexperiments with p-rays? I would say that the two do not contradict oneanother because they refer to different situations. Neither Becquerel, norKaufmann, nor anyone else expected more than one new particle withelm mlch greater than that for the hydrogen ion. For this reason, it islikely that even if this magnitude varied for different researchers 30times instead of 3, it would still be acceptable. There was no theoreticalreason for or against a single small particle versus many, which woulddiffer in mass (or even charge), say,2-3 times. Consequently, physicistsopted for the simplest solution. It worked well at the time, but such anapproach would not have worked for mesons 50 years later.

Now, having known that particles observed in different phenomenaare the same, one could select such a phenomenon and procedure formeasuring elm that would make it simpler and more precise. That waswhat Kaufmann did. However, he could not know of such an identity;he could only assume it. The hypothesis of a single universal particlewas first formulated by Wiechert in January 1897. By the time it wasrestated by J. J. Thomson at the First Physics Congress in Paris in 1900,it had much more evidence behind it, and it had already been adopted bya number of physicists as a working hypothesis." Still, it remained ahypothesis for quite a while.

8. CONCLUSION

The discovery of X rays lead directly to the discovery of radioactivityand revived cathode-ray research, which made an important contribu-tion to establishing the concept of the electron. Radioactivity alsoplayed an essential role in this.

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There is more in this than a chain reaction of discoveries. Using anal-ogies between X rays, cathode rays, and radioactive rays in alllirec_tions not only opened new fields but reinforced research in the"progenitor" field ofX rays. A close interdependence between researchon X rays, radioactivity, and the electron between lg96 and r90l andthe brevity ofthe period suggest that their discoveries can be consideredcomponents of one meta-discovery that opened a window into the atom.

NAHUM KIPHIS-

,,,:: : : "tr : : ;;:: l:: i; # i I ;References

l. W. K. Rdntgen, Eine neue Art von Strahlen (Wiirzburg, Ende 1g95).2. A. Schuster, Nature 53, 26g (23 Jan.,lg96); D. Goldhammer, Ann. phys. (Leipzig)^

57, 635 (t 896); o. Lodge, Etecrrici an 31, il z ( u ruiv, r sqo).3. B. Galitzine and A. Kamojitsky, C. R. Acad, Sci. 122,711(23 Mar., 1896).4. O. Lodge, Electrician 36, 4?t (Feb. 7, lg96); p. Lenard, Ann. phys. (Leipzig) 63,253 (1891); A. Vosmaer and F. L. Ortt, Nature 56, 3l; (1897).5. G. Le Bon, C. R. Acad. Sci. l22, t8g (27 Jan., 1896).6. G. Niewenglowski, rDld., 232 (3 Feb., lg96); G. Le Bon, ibid.,233 Nodon, ibld.,237; Moreau, ibid., 238.7. H. Poincar6, Rev. G6n. 1ci.7,52 (30 Jan.. lg96).8. H. Becquerel , C. R. Acad. Sci . 122.501 r2 March, lg96).9. H. Becquerel, C. R. Acad. Sci. 122,559 (9 March, 1896).

10. H. Becquerel, C. R. Acad. Sci. 122,762 (30 March, 1g96).l l . S. Thompson, phi los. Mag.43, 103 (1896); Ch. Henry, C. R. Acad. Sci .122,78j( | 896).12. O. Lodge, Electrician 37 , 3t0 (189ft.I 3. P Zeeman. phi los. Mag.. 43.226 ( lggT ) .14. E. Wiechert, Schr. phys. Ges. Krinigsbeng 3g, I (1g97).15. J. J. Thomson, Not. proc. Roy. Inst. 1S,419 (1g97),16. J. J. Thomson, philos. Mag.48, 547 (t8gg).17. S. Meyer and E. Schweidler. phys. Z. l. I 13, cit. p I 14 (1899).18. H. Becquerel , C. R. Acad. Sci .809 (1900).19. W. Kaufmann. phys. Z. 4.54 \ lg02t .20. H. Becquerel, in Rapports prisentds au Congr?s International de physique, editedby C. Guillaume and L. poincar6 (GauthierlMllarr, p_ir, 1900),

".,3,; It i;;;p. 69, italics added).

21. W. Kaufmann, Electrician 97,(1901),italics added.22. J. J. Thomson, in Rapports prdsentds au Congris International d.e ph,tt,\syyu,tr pter€rLres au Longres International de physique, editedby C. Guil laume and L. poincar6 (Gauthier-Vil larr, p".r, ' iS00l, v.3, p. l3g.

E Correspondence Author.

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