Radioactivity: The First Puzzles

Abstract

Henri Becquerel, while searching for X-rays, discovers a radiation emitted by uranium. The scientific community shows no interest in such a weak and incomprehensible phenomenon with no practical applications.

Leurs métamorphoses sont soumises à des lois stables, que vous ne sauriez comprendre.

A. France, La Révolte des anges.

Their transformations are subject to stable laws which you could not comprehend.

The “Uranic Rays” of Henri Becquerel

Henri Becquerel, while searching for X-rays, discovers a radiation emitted by uranium. The scientific community shows no interest in such a weak and incomprehensible phenomenon with no practical applications.

On this Sunday morning, March 1, 1896, Henri Becquerel is working in his laboratory at the Muséum d’Histoire Naturelle in Paris. He is waiting in vain for the sun to come out [1–3] because he needs the intensity of sunlight in order to confirm some interesting observations made a week earlier and communicated to the Académie des Sciences on February 24. But in this never ending winter, the sky remains obstinately covered, day after day.

Becquerel is a distinguished physicist, born in a family with several generations of scientists [4, 5]. His grandfather, Antoine César, born in 1788, was admitted to the École Polytechnique in 1806. He distinguished himself as an officer in the Napoleonic armies. After the final fall of Napoleon in 1815, he left the army and began a successful scientific career, working on electricity, optics, phosphorescence, and electrochemistry. In 1829, he constructed the first constant current electric cell. He was awarded the prestigious Copley Medal of the Royal Society in London in 1837, and in 1838, he became member of the Académie des Sciences. In 1838, he held the first physics chair in the Muséum d’Histoire Naturelle in Paris. When he died in 1878, Henri Becquerel, his grandson, was 26 years old.

Becquerel’s father was the second son of Antoine César, Alexandre Edmond Becquerel, born in 1820. Although he passed successfully the admittance examinations to both the École Polytechnique and the École Normale Supérieure, he chose to work as an assistant to his father in the Muséum d’Histoire Naturelle. In 1852, he became Professor at the Conservatoire National des Arts et Métiers and he was elected member of the Académie des Sciences in 1863. Upon the death of his father, he succeeded him as professor in the Muséum d’Histoire Naturelle, where he specialized in electricity, magnetism, and optics. His works on phosphorescence and luminescence [6] were published in 1959 and assembled in two books [7, 8], published in 1859 and in 1867. They remained a standard reference for half a century. He invented a device, called the phosphoroscope, with which he proved that fluorescence, which had been discovered by G. G. Stokes in 1852, was nothing but phosphorescence lasting for a very short time. Alexandre Edmond Becquerel died in 1891.

Henri (Antoine Henri Becquerel, according to his birth certificate) was born on December 15, 1852, in the Muséum, the home of his parents. In 1872, he was admitted to the École Polytechnique, where he met Henri Poincaré, who was to become one of the most famous scientists of the time. They develop a long-lasting friendship. In 1876, he graduated from the Écoles des Ponts et Chaussées. First, he became an instructor at the École Polytechnique and later an assistant naturalist in the Muséum. In 1889, at the age of 37, he was elected member of the Académie des Sciences, and in 1895, he became physics professor at the École Polytechnique.

Henri Becquerel, polite and friendly, is a clever and rigorous experimentalist. Akin to many French physicists at that time, he is more inclined to observation than to theoretical speculation. His research, so far, is devoted to optics, a family tradition. In 1876, Lucie Jamin, the daughter of the Academician J. C. Jamin, becomes his wife and gives birth to a son, Jean, in 1878. She dies a few weeks later at the age of 20. On August 1890, Louise Désirée Lorieux becomes the second wife of Henri and Jean is brought up as her son. True to the family tradition, Jean will later also be admitted to the École Polytechnique and elected member of the Académie des Sciences.

The Discovery

The experiments, which Becquerel is performing in 1896, are motivated by the discovery of “X-rays,” which Wilhelm Conrad Röntgen [9–11] had made a few months earlier. Röntgen had studied the “cathode rays” produced by electrical discharges in gases. When a voltage exceeding a 1,000 V is created between two conductors placed in a container of gas maintained at low pressure, an electrical discharge occurs. The discharge consists of cathode rays emanating from the negatively charged conductor, called the cathode (We know today that cathode rays are electrons). Röntgen discovered that, when the cathode rays hit the glass wall of the container, they emit an unknown radiation which has a greater penetration power than light. He called them “X-rays.” This discovery caused quite a stir and physicists, among whom Henri Becquerel, were quite excited. In the session of January 20, 1896 of the Académie des Sciences, two medical doctors, Paul Oudin and Toussaint Barthélémy, displayed X-ray photographs. Poincaré received a reprint of the paper of Röntgen. He and Becquerel were particularly impressed by the fact that the X-rays were emitted from the luminescent spot which was produced on the glass container by the impinging cathode rays. In a paper devoted to X-rays and published on January 30, 1896 in the Revue Générale des Sciences, Poincaré wrote:

It is the glass which emits the Röntgen rays and it emits them by becoming phosphorescent. Are we not then entitled to ask whether all bodies, whose phosphorescence is sufficiently intense, emit X-rays of Röntgen, in addition to light rays, whatever the cause of the fluorescence is? [12].

This is precisely what Becquerel is investigating in his laboratory of the Muséum d’Histoire Naturelle. He is quite familiar with luminescence which he had studied at length with his father. Luminescent bodies are not spontaneously luminous but, when they are exposed to light, they radiate their own light, almost immediately¹ in the case of fluorescence, or within a variable laps of time, in which case the phenomenon is called phosphorescence.² Becquerel possesses thin strips of double uranium and potassium sulfate, and he is quite familiar with their phosphorescence which is intense but lasts only about a hundredth of a second. He then performs the following experiment, which he later described in a communication to the Académie des Sciences, dated February 24:

We wrap a Lumière photographic plate, composed of a bromide gel, between two sheets of very thick black paper, such that the photographic plate does not become veiled when exposed to sunlight during a whole day. On top of the paper sheet, we place a strip of a phosphorescent substance, and the lot is exposed to the sun during several hours. When the photographic plate is subsequently developed, the silhouette of the phosphorescent substance appears in black on the photograph […] We are led to conclude from these experiments, that the phosphorescent substance emits a radiation capable of passing through the paper which is opaque to light [13].

Becquerel exposes this assembled package to sunlight, the most intense source of light at his disposal. The following Wednesday, February 26, he attempts to make an X-ray photograph. He repeats the experiment, but this time, he slips a thin strip of copper, in the shape of a Maltese cross, between the phosphorescent uranium sulfate sheet and the photographic plate, the latter being again wrapped in thick black paper. He knows that the copper strip is opaque to X-rays, and he expects that, after a similar exposure to sunlight, a Maltese cross will appear in white on the developed photographic plate. He proceeds to expose this newly assembled package to sunlight in order to produce the phosphorescence. The sky is clear until 10 a.m. but obstinately remains clouded thereafter. The following day, the sun shines only between 3 p.m. and 7 p.m. when new clouds appear. Becquerel then puts the package into a drawer, pending better weather. The following 2 days remain grey. No sign of improvement on the following Sunday, March 1, when it even begins to rain [14].

Rather than wait, possibly several days more, Becquerel decides to develop the photographic plate in his drawer. He expects to obtain a weak picture because the plate was exposed to sunlight for a short time only, and the induced phosphorescence was expected to be weak. However, contrary to his expectations, the developed photographic plate shows that it had been intensely exposed. It also displays a somewhat blurred shape of the Maltese cross! Becquerel is surprised and, true to the clear-sighted and rigorous physicist he was, he repeats the experiment maintaining this time the assembled package in complete darkness. The photographic plate is again strongly exposed! On Monday, March 2, 1896, he presents the following note to the Académie des Sciences:

I insist on the following feature, which I consider very important and not in accord with the phenomena we might have expected to observe: the same crystalline strips, placed upon the photographic plates, under the same conditions and with the same screens, but protected from incident radiation and maintained in darkness, produce the same exposure on the photographic plate […] I immediately thought that this action had necessarily continued in darkness [15].

Henri Becquerel has just discovered what we call today radioactivity.

Is It Really Phosphorescence?

At first, Becquerel believes that the physical process which he is observing is phosphorescence produced by exposure to light and that it should therefore die out in time. In order to make sure, doubt being the physicist’s best advisor, from March 3 onwards, Becquerel maintains his strips in darkness, and, from time to time, he checks their radiative power. Month after month, it persists, showing no sign of weakening. In November 1896, Becquerel notes:

…protected from any known radiation, […] the substances continued to emit active radiation which penetrated glass and black paper, and this has been going on for 6 months for some samples and 8 months for others [16].

He makes another strange observation: similar experiments performed with other luminescent substances fail to produce the effect [17]. However:

All the uranium salts which I have studied, whether they are, or not, phosphorescent, exposed to light, crystallized, melted or dissolved, gave similar effects; I was therefore led to conclude that the effect was due to the presence of the element uranium in the salts^{3 Fn1} , and that the metal would produce a stronger effect than its compounds. The experiment was performed […] and it confirmed this prediction; the photographic effect is notably more intense than that produced by a uranium salt [18].

Becquerel insists that it does not matter whether the uranium salts are crystallized, melted, or dissolved because only the crystallized form is phosphorescent. The relation between the phenomenon he discovered and phosphorescence becomes increasingly doubtful. In other words, the “radiant” activity appears to bear no relation to the exposure of the substance to sunlight.

Although he continues to use the word “phosphorescence,” Becquerel gradually gives up the original idea which led him to the discovery. To be faced with such a phenomenon, which occurs in a similar fashion independently of the chemical compound of uranium, was quite an extraordinary experience for a physicist or a chemist at the end of the nineteenth century. One thing, which chemistry had shown since Lavoisier, was precisely the fact that properties of chemical substances did not reflect the properties of the elements from which the substances are formed. Kitchen salt, for example, is sodium chloride and its properties are quite different from those of either sodium or chlorine. The radiant activity of uranium was both strange and unique.

What Is the Nature of the Radiation?

The terms “ray” or “radiation” are used to describe something which emanates from a source and propagates in a straight line, as sun rays do. In the paper announcing his discovery of X-rays, Röntgen wrote:

The reason why I allowed myself to call “rays” the agent which emanated from the wall of the discharge vessel, is partly due to the systematic formation of shadows which were observed when more or less transparent materials were placed between the apparatus and the fluorescent body (or the sensitive plate) [9].

According to the theory of Maxwell, brilliantly confirmed experimentally in 1888 by Hertz, any sudden electric or magnetic disturbance becomes the source of an electromagnetic field ^♢ which propagates in a straight line at the speed of light. This electromagnetic field is in fact light, visible light being nothing but a particular instance. Röntgen showed that X-rays propagate in a straight line and, in spite of the fact that they could neither be reflected nor refracted, he believed that they were electromagnetic waves, that is, a kind of light which is invisible to our eyes but which can be detected on a photographic plate (or on a luminescent screen).

In his second communication on the discovery of X-rays, Röntgen noted that they had the power of discharging electrified bodies [10], that is, that they allowed an electric current to pass through air, a feature which was confirmed by numerous other works [19–22]. Becquerel subjects his “uranium rays” to similar tests. For this purpose, he uses a gold leaf electroscope ^♢. When they are electrically charged, the gold leaves repel each other. But when Becquerel places a piece of uranium in their vicinity, they gradually coalesce: the electroscope discharges itself, indicating that some electricity has escaped through the air:

I have recently observed that the invisible radiation emitted under these conditions has the property of being able to discharge electrified bodies which are subject to their radiation [23].

This property will play a major role, as we shall see. Since it manifests itself by a measurable electric process, the radiation becomes detectable. This became the first detector other than the photographic plate.

A Limited Impact on Scientists and the Public

Whereas the discovery of X-rays aroused considerable interest among both physicists and the public, the “radiant activity of uranium” made a very limited impact on physicists and none on the general public. In the year 1896, more than 1,000 publications were devoted to X-rays, but barely a dozen to the radiation of uranium [24]. Indeed, X-rays provided the possibility to see the interior of the human body, the dream of medical doctors, who would not even have imagined such a possibility a year earlier. Furthermore, X-rays are easy to produce. They required a Crookes tube and a Rühmkorff coil which could be found in practically any lab. The 1897 issue of the Almanach Hachette, subtitled Petite Encyclopédie populaire de la vie pratique ⁴ noted:

It is truly the invisible which is displayed by the mysterious X-rays, which we all have heard about. To show the bone hidden under the flesh, the weapon or projectile buried in a wound; to read all the inside of the human body—perhaps even thoughts!—to count the coins through a carefully closed purse; to seek the most intimate confessions hidden in a sealed envelope; it all becomes child’s play for any amateur. And what is required to perform such miracles? Precious little: an induction coil, a glass bulb and a simple photographic plate [25].

The radiation of uranium was far less interesting. For one thing, it was very weak: exposures lasting hours were required whereas, in 1897, 10 min were sufficient to produce an X-ray photograph (the first X-ray photograph, which showed the hand of Bertha, the wife of Röntgen, was obtained in 1 h). But most of all, nobody could see what the uranium rays could be used for. The case of the English physicist Sylvanus P. Thomson is quite instructive in this respect. He was also interested in X-rays, and, like Becquerel, he thought that they were linked to phosphorescence. He even observed, at about the same time as Becquerel, that phosphorescent uranium salts emitted a radiation, which he proposed to call “hyperphosphorescence.” But Becquerel was the first to publish his observations. Thomson published his a few months later [26], in June 1896, and then he abandoned their study in order to devote his research to the study of X-rays. After November 1896, even Becquerel abandoned the study of uranium radiation for several years. With the experimental means available to him at the time, he could not see how to progress further.

Why 1896?

Becquerel used to say that radioactivity was bound to be discovered at the Muséum. He considered that his discoveries were “daughters of his father and grandfather; they would have been impossible without them.” [27] However, in a lecture delivered at the University of Yale in March 1905, Ernest Rutherford claimed that the discovery could well have been made a century earlier:

In this connection it is of interest to note that the discovery of the radioactive property of uranium might accidentally have been made a century ago, for all that was required was the exposure of a uranium compound on the charged plate of a gold-leaf electroscope. Indications of the existence of the element uranium were given by Klaproth in 1789, and the discharging property of this substance could not fail to have been noted if it had been placed near a charged electroscope. It would not have been difficult to deduce that the uranium gave out a type of radiation capable of passing through metals opaque to ordinary light. The advance would probably have ended there, for the knowledge at that time of the connection between electricity and matter was far too meagre for an isolated property of this kind to have attracted much attention [28].

Was Radioactivity Discovered by Chance?

When he developed his photographic plate on March 1, 1896, Becquerel certainly did not expect to see what he saw. Can we say that he discovered radioactivity by chance? Becquerel had designed an experiment with a well defined goal, namely, to observe a radiation, if it exists, similar to X-rays and emitted by phosphorescent substances. The lack of sunlight as well as his decision to develop the photographic plate admittedly played an important role. But his experiments would have led him, sooner or later, to the same discovery. The nature of a true physicist consists in being surprised by the right thing. In this respect, Becquerel left nothing to chance [29]. Better still, by mounting successive and rigorous experiments, he gradually showed that his initial idea was wrong, that the radiation was not linked to phosphorescence, but that instead, it was a truly new phenomenon linked to the presence of uranium. It is in this respect that he truly discovered radioactivity. Sylvanus Thomson had made the same observation in a similar fashion, but without persevering. Similarly, Abel Niepce de Saint-Victor, a French officer and amateur chemist, had observed that uranium salts could leave a trace on a photographic plate long after it had been exposed to sunlight, and he observed the same effect with tartaric acid. He published a number of papers between 1857 and 1867 on what he called “A new action of light.” [30] But he always linked the observed effects to exposure to light: he did not discover radioactivity.

The discovery made by Becquerel was truly unexpected. But is that not the nature of every true discovery?

Polonium and Radium

A young Polish student and her French husband, working outside the French university establishment, discover two new elements, polonium and radium, which are considerably more radioactive than uranium. Their discovery rekindles research on radioactivity. Pierre and Marie Curie ask the crucial question: where do radioactive elements find the energy required for them to radiate?

Two years after the discovery of radioactivity by Henri Becquerel, the study of the “radiating activity” of uranium had ceased. But on the April 12, 1898, a young Polish woman, married to a French Physicist, delivers a communication to the Académie des Sciences which ignites a fire of interest which, this time, is likely to last.

Marya Skłodowska

Marya Skłodowska [31–34] was born in Warsaw in 1868 into a family with already three daughters, Sofia, Bronisława and Helena, and a son, Joseph. Her father, Władysław Skłodowski, teaches physics at the Gymnasium in Nowolipki street. Marya was born at a particularly dark time of Polish history. The defeat of the January 1864 uprising against Russian rule is followed by a ferocious repression. The Tsar decides to Russianize the country. Russian becomes the official language and the use of Polish is forbidden, even in schools. Władysław loses his job. After considerable difficulties, he succeeds in becoming a monitor in a boarding school with a small teaching duty. The family lives in poverty. Sofia dies from typhus in 1876 and Mrs. Skłodowska catches tuberculosis. She dies May 9, 1878, when Marya is barely 11 years old.

On June 12, 1883, at the age of 15, Marya graduates brilliantly from secondary school, earning a gold medal. But universities are closed to women. Her elder sister Bronia would also like to attend university and so the two sisters decide to make a deal: Marya will help Bronia financially to go to Paris by becoming a primary school teacher. Once Bronia gets the required diploma, she will in turn help Marya to join her in Paris. Seven years pass before Bronia, who has almost finished her medical studies and is married, can welcome her sister.

In the fall of 1891, in Paris, Marya attends the lectures of Gabriel Lippmann, Edmond Bouty, and Paul Appell at the Sorbonne. In July 1893, after living in considerable poverty for 2 years, she obtains a bachelor’s degree in physics; she is the best student in her class. She goes back home to Poland for a vacation, fearing that she might not find the money to return to Paris. But, thanks to a heaven-sent subsidy (an Aleksandrovič grant of 600 rubbles), she returns to Paris and, in July 1894, she obtains a bachelor’s degree in mathematics, graduating as second best in her class.

While preparing her bachelor’s degree in mathematics, Marya begins to work in the laboratory of Gabriel Lippmann where she receives an assignment which pleases her: the Société d’Encouragement de l’Industrie Nationale ⁵ asks her to study magnetic properties of various steels. However, she lacks both the necessary funds and know-how. Then 1 day she mentions this to a Polish friend, Jósef Kowalski, physics professor in Freiburg, who was passing through Paris. He proposes to present her to Pierre Curie, a physicist who had done important work on magnetization.

Pierre Curie

Born on May 15, 1859, Pierre Curie is then 35 years old [35–38]. His brother Jacques is 4 years older. His father, Eugne Curie, was a medical doctor. Pierre never went to school: he was educated by his parents, some friends, and private tutors. He was described as a dreamy person who loved to walk in the country, where, thanks to his father, he could name every plant and animal he would come across. At the age of 14, his father entrusted him to a mathematics teacher, Albert Bazille. He passed the baccalauréat ⁶ at the early age of 16. The following year, he became an assistant to Paul Desains, a specialist of infrared radiation, after which he began to work in the laboratory of Charles Friedel, where he joined his brother Jacques. The two brothers discovered that some crystals, when compressed or elongated, emit electricity. Ten years, later the phenomenon was called piezoelectricity [39]. Pierre used this property to construct an extremely sensitive and precise electrometer.

In 1882, Pierre becomes an assistant at the newly founded École Municipale de Physique et de Chimie Industrielle.⁷ Strictly, he does not have a lab at his disposal because the school’s lab is reserved for the students. Fortunately, however, the director, Léon Schützenberger, a chemist who is also professor at the Collge de France, is an intelligent and liberal minded man who permits Pierre to pursue his personal research there. Pierre continues to work on crystallography. He believes that the symmetries displayed in the beautiful geometrical figures of crystals reflect deeper symmetries of the constituent atoms [40]. The importance which Pierre Curie attached to symmetry makes him appear today as a precursor [40, 41].

In 1891, he begins to study magnetization. He discovers and formulates what we call today the “Curie law” ^♢ which exhibits a critical temperature (the Curie temperature ^♢) above which ferromagnetic substances lose their magnetization [42]. In spite of the fact that he holds no university position and has no official laboratory to work in, he becomes a well known scientist, especially abroad. It is therefore quite logical for Jósef Kowalski to suggest that Marya Skłodowska should consult him for her work on magnetization. They meet 1 day in the spring of 1894. The meeting becomes a mutual discovery and they are married a year later, on July 25, 1895, after some hesitation of Marya, to whom marriage means that she must give up the idea of returning to her father in her home country. She has the feeling of somehow betraying her country by getting married to a Frenchman and settling in France. But Pierre insists on the fact that she can continue her scientific work in France. And, after all, they are in love…

Polonium and Radium: Pierre and Marie Curie Invent Radiochemistry

Following the advice of Pierre, Marya, who now bears the name of Marie, completes her work on magnetization [43, 44] and searches for a subject for her PhD. This by itself is exceptional: so far, no woman in France had defended a PhD thesis in physics. Pierre suggests studying the “Becquerel rays” a subject that had been neglected for about 2 years. He even offers her a quartz piezoelectric electrometer with which she can measure the extremely weak electric current produced by the radiation of uranium. Although quadrant electrometers were available, his electrometers made it possible to measure the absolute value of the current in units of amperes (in fact tiny fractions of amperes). As Marie later stated:

We obtain thus not only an indication but a number which accounts for the amount of active substance [45].

Where should she begin? Together with Pierre, Marie decides to find out whether substances other than uranium emit similar radiations. She soon discovers that thorium also radiates [46]. By coincidence, the German physicist Gerhard Schmidt published only a week earlier his observation that thorium was “active,” that is, it emitted radiations [47]. However, the attention of Marie is attracted to a small detail. In practically all the cases she had studied, the activity of the uranium compound was precisely that which she could calculate, knowing the amount of uranium in the sample. She finds, however, one exception: two uranium minerals, namely, pitchblende (uranium oxide) and chalcolite (a copper and uranyl phosphate), are more active than what their uranium content would grant. She sees in this remarkable feature a hint that these minerals contain an element which is far more active than uranium. This is where the electrometer of Pierre turns out to be useful because it makes it possible to measure precisely weak currents of the order of 10^− 11 amps,⁸ in order to detect such anomalies. Marie Curie is surprised by the right thing. The mineral certainly contains another active substance, but the amount is far too small to be measured by a chemical analysis. Marie Curie has a brilliant idea. Since this substance can only be detected by its radiation, why not use its radiation to follow its trace? With the help of Pierre who discontinues (for a while only, he believes) his work on crystals, she begins with a chemical separation, or at least a concentration of a special kind. She proceeds with several successive chemical reactions with the aim of progressively separating the elements while retaining the most radioactive ones. The radioactivity increases each time she makes a chemical reaction which concentrates bismuth, as if she was extracting bismuth from the mineral:

We obtain more and more active products. We finally obtained a substance which is about 400 times more active than uranium. We therefore believe that the substance which we have extracted from the pitchblende contains a metal which has not yet been reported, similar to bismuth in its analytic properties. If the existence of this new metal is confirmed, we propose to call it polonium, from the name of the country one of us originates from [45].

Pierre and Marie Curie have just invented what we call today radiochemistry. It is in the title of their publication, “About a new radioactive substance contained in pitchblende,” that the term radioactive appears for the first time. The terms radioactive and radioactivity will soon be adopted worldwide.

They soon make a new discovery, which is communicated to the Académie des Sciences on December 26, 1898:

We have discovered a second substance which is strongly radioactive and which differs from the first [polonium] by its chemical properties [48].

As in the case of polonium, they perform chemical separations guided by measuring the radioactivity. This time, they find that:

The new chemical substance which we found has all the chemical appearances of barium.

In fact, they cannot separate the new substance from barium, but:

Barium and its compounds are usually non radioactive; however, one of us has shown that radioactivity appears to be an atomic property which persists in all the chemical and physical states of the matter. If we adopt this view, the radioactivity of our substance is not due to barium and has to be attributed to a new element.

This is the first time that radioactivity is considered to be an atomic phenomenon. Oddly reference is made to the first publication of Marie Curie on the subject [46], where the word “atomic” is only used in the term “atomic weight.”

To make sure, they ask the expert Eugène Demarçais to make a spectroscopic analysis of their substance. His results confirm their hypothesis: Demarçais observes an unknown optical spectral line which becomes more intense when the sample is more radioactive [49]. They conclude:

The various reasons mentioned above make us believe that the new radioactive substance contains a new element which we propose to call radium .

This is the discovery of radium which was soon to become famous. They also note:

The new radioactive substance certainly contains a strong fraction of barium; in spite of this, its radioactivity is considerable.

There is more: a platinum-cyanide screen, known to become luminous when exposed to X-rays, becomes also luminous when it is placed in the vicinity of the substance. But they believe that this raises a problem:

We obtain this way a source of light, an admittedly very weak source, but which works without a source of energy. This at least appears to contradict the law of Carnot.

They refer to the second law of thermodynamics. They would not have questioned the first law which states that energy is conserved and that energy cannot be created from nothing. But the second law states that the energy of the luminous source cannot be extracted from the surroundings by cooling it, for example. So where does the energy come from? This is the first time that the question is clearly raised.

For months, even years, Pierre and Marie Curie extract and purify radium. Finally, in 1900, after painstaking labor, they succeed in extracting a few decigrams of pure radium from 2 T of mineral [50]! The task was made more difficult by the fact that the room, which the good Schützenberger allowed them to use, was no longer suitable. Charles-Marie Gariel, the new director of the École, allowed them to use an abandoned shed in the courtyard. The shed was hot in the summer and dead cold in the winter, but most of the chemical treatments had to be performed outside in the open. At each step of the purification process, Marie Curie made a chemical measurement of the atomic weight ^♢ of the radium. In 1902, she obtained a value of 225 with an uncertainty of one unit [51], a value confirmed by a later measurement [52], in 1907, which gave the value of 226.18 (the value measured today is 226.097 in units used at that time, namely, 1/16 of the mass of oxygen). Radium is indeed a new element, several million times more radioactive than uranium.

Enigmas

What is the nature of the Becquerel rays and where does their energy come from? The problem is reviewed in a paper published in 1899 by Marie Curie in the Revue Générale des Sciences [53]:

Becquerel radiation is spontaneous; it is not sustained by any known agent. […]What is more remarkable is the constancy of the radioactivity of uranium in its various physical and chemical states. […] The uranic radiation appears therefore to be a molecular property inherent in the uranium substance itself.

In a new paper published in 1900 in the Revue Scientifique, known as the Revue Rose, she is more specific:

Radioactivity is therefore a property which is tagged to uranium and thorium in all their states—it is an atomic property of these elements [54].

But the deep enigma of radioactivity is the origin of its energy:

The emission of uranic rays is spontaneous, meaning that it is not produced by any known cause. For a long time, Mr Becquerel believed that it was caused by light, that uranium somehow absorbed light and that the energy thus absorbed was re-emitted in the form of uranic rays. […] But experiment does not confirm this interpretation […]. The emission of uranic rays is remarkably constant and does not change either with time, nor with its illumination nor with its temperature. This is its most troubling feature. When we observe Röntgen rays, we furnish electrical energy to the tube; this energy is provided by batteries, which have to be renewed, or by machines which are set into motion by work which we supply. But the matter is not modified when it emits, admittedly weakly, uranic rays continuously.

Marie Curie finally raises the question of the nature of the radioactivity. Is it “materialistic” like cathode rays, which J. J. Thomson had shown to be material particles, with measurable mass, charge, and velocity? If such is the case, she claims, we must face the consequences of upsetting several laws of chemistry:

The materialistic theory of radioactivity is very tempting. It explains many features of radioactivity. However, if we adopt this theory, we are forced to admit that a radioactive material is not in a usual chemical state; its atoms are not in a stable state, since particles, smaller than the atoms are radiated. The atom, which is an indivisible unit in chemistry, is divisible in this case and sub-atoms are in motion. The radioactive substance therefore undergoes a chemical transformation which is the source of the radiated energy; but it is not an ordinary chemical transformation, because usual chemical transformations leave the atom unchanged. In a radioactive material, if anything changes, it is necessarily the atom, because the radioactivity is attached to the atom. The materialistic theory of radioactivity leads us therefore quite far.

Marie Curie concludes thus that the materialistic theory leads inevitably to the transformation of atoms, therefore to transmutations, which she is not ready yet to admit. She continues:

Even if we refuse to admit its consequences, we cannot avoid being embarrassed. If the radioactive material is not modified, where does the energy of radioactivity come from? If we are unable to find the source of energy, we contradict the law of Carnot, which is a fundamental law of thermodynamics, according to which a substance, at a given temperature cannot furnish energy if it does not receive some from the outside. We are then forced to conclude that the law of Carnot is not a general law, that is does not apply to certain molecular phenomena, and that radioactive substances possess means of producing work from the heat of its surroundings.

And Marie Curie concludes… that it is difficult to conclude:

Such a hypothesis bears a blow which is as serious to the ideas admitted in physics as to the hypothesis of the transformation of elements in chemistry, and we see that the problem is not easy to solve.

Emanation from Thorium

A young physicist from New Zealand, Ernest Rutherford, begins to study radioactivity. He shows that radioactivity consists of two distinct radiations which he calls α and β rays and the latter are identified to electrons. He discovers that a radioactive gas, belonging to the family of argon, is continuously produced by radioactive thorium. This leads him to the discovery of exponential decay, the fundamental law of radioactivity. But the energy of radioactivity remains an enigma.

In those years, the Cavendish Laboratory in Cambridge is the stage of intense activity: J. J. Thomson [55] and his team are studying cathode rays and the flow of electricity through gases. In November 1895, X-rays are discovered and it is observed that air conducts electricity when it is exposed to X-rays. J. J. Thomson immediately suggests an explanation: the X-rays ionize the molecules of air, splitting them into two “ions,” one positively charged the other negatively. The explanation needs to be confirmed. A couple of months earlier, a young physicist arrives from New Zealand.

Ernest Rutherford

Ernest Rutherford [56–58] was born on September 30, 1871, in a family of New Zealand farmers. His father arrived there at the age of 3. The country was occupied by the British. The home in which the young Ernest was raised was governed by his mother, a woman with a strong character who remained active until she died in 1935 at the age of 92. A former teacher, she loved reading and playing the piano. During all his life, Ernest Rutherford was an eager reader, particularly of detective novels. He had just reached the age of 6 when New Zealand made education compulsory for children between the ages of 6 and 13. He was a brilliant pupil. At the age of 15, he was granted a scholarship which enabled him to study at the Nelson College, which today, noblesse oblige, is called the Rutherford College. He excelled in English and French literature, history, Latin, mathematics, and rugby. Two years later, he obtained a scholarship from the University of New Zealand, which enabled him to enter the Canterbury College, in Christchurch. He graduated M.A. in 1893 and received the B.Sc. degree in 1894.

That same year, he began to study magnetism and the detection of the recently discovered Hertzian waves. In 1894, he published his first papers in the local scientific journal, the Transactions of the New Zealand Institute [59, 60]. He was endowed with a strong personality and was always ready to help his colleagues. Thanks to his charm and power of persuasion, he was always able to obtain help for his projects. In 1894, he ranked second in a competition, which had been initiated during the 1851 “Great Exhibition” in London, and which granted the winner a scholarship allowing him to study in England for 2 or 3 years. It so happened that personal reasons prevented the winner from going, so that the scholarship was given to Rutherford. He chose the Cavendish Laboratory where he arrived in October 1895. He wrote letters to his mother, roughly every 2 weeks, until her death. The letters are a precious testimony in spite of several being lost.

At the Cavendish, he first pursues his research on the detection of electromagnetic waves. However, in February 1896, J. J. Thomson suggests that he should join him in the study of the mechanism which makes air an electric conductor when it is exposed to X-rays. Rutherford rapidly confirms the ideas put forth by J. J. Thomson: the X-rays decompose the molecules of the gas into pairs of “ions” with opposite electric charges, in the same way as dissolved salts do during electrolysis ^♢. An electrically charged neighboring body attracts electric charges of opposite sign, thereby producing an electric current in the gas [61]. Rutherford studies this “ionization” of the gas by X-rays with the meticulous care which characterizes his work throughout his life: he measures the rate of production and recombination of the ions, as well as their velocity in the gas [62, 63].

Rutherford Studies Radioactivity: α-and β-Rays

In 1898, Rutherford turns his attention to the “rays of Becquerel.” He wants to find out if they produce the same “electrification of air” as X-rays do and he quickly confirms that this is indeed the case. This leads him to make a detailed study of the penetration of the rays in different substances. He discovers that they are in fact composed of two very different kinds of rays: some ionize strongly the gas which they pass through (meaning that they produce a large number of ions) and they can be stopped by a piece of cardboard; the others have a much stronger penetrating power while ionizing less:

These experiments show that the uranium radiation is complex, and that there are present at least two distinct types of radiation—one that is very readily absorbed, which will be termed for convenience the α radiation, and the other of a more penetrative character, which will be termed the β radiation […] [64].

α- and β-rays are born. The former are often referred to as “weakly penetrating radiation” and the latter as “strongly penetrating radiation.”

β-Rays Are Electrons

The discovery of radium marks a new stage in the study of radioactivity: indeed, a tiny sample of radium is an intense and almost point like radioactive source, thereby making it possible to perform a much finer study of the radiation than with the weakly radiating uranium.

As early as 1899, Friedrich Giesel [65], in Braunschweig, Germany, as well as Stefan Meyer and Egon von Schweidler [66] showed that some of the rays emitted by radium could be deflected by a magnetic field, while others could not. Independently, Becquerel observed the same thing and he noted that the rays which could be deflected had properties similar to cathode rays, that is, to electrons. Pierre Curie undertook a more quantitative study. He noticed that the rays which can be deflected have a greater penetrating power than those which cannot [67]. Working with Marie Curie, he showed that the transported electric charge was negative [68]. The rays which can be deflected appear to be the same as those which Rutherford had coined as β-rays. By measuring the deviation produced by magnetic and electric fields, Becquerel was able to measure the ratio of the mass and of the electric charge of the deflectable rays: it turned out to be the same as that of cathode rays [69, 70]. Finally, in 1902, the German physicist Walter Kaufmann made a careful measurement of this ratio for the rays emitted by radium, and he confirmed that it was identical to that of cathode rays, that is, of electrons [71]. The conclusion was that β-rays were very fast electrons with a velocity certainly higher than that of cathode rays. But the value of the velocity of the β-rays was badly known.

Rutherford in Montreal: The Radiation of Thorium, the Exponential Decrease

In 1899, the scholarship of Rutherford expires. A research professor position becomes available at the McGill University in Montreal. This position as well as the attached laboratory are funded by a tobacco millionaire, named MacDonald. The salary is modest but the laboratory has the best equipment in the world. J. J. Thomson is consulted and he strongly recommends Rutherford, who thus becomes MacDonald Professor of Physics in the University of McGill at the age of 28.

Rutherford immediately resumes his research on radioactivity. As he later explains in a letter to his mother:

I have to keep going, as there are always people on my track. I have to publish my present work as rapidly as possible in order to keep in the race. The best sprinters in this road of investigation are Becquerel and the Curies in Paris, who have done a great deal of very important work in the subject of radioactive bodies during the last few years [72].

He begins a collaboration with R. B. Owens, another professor at the university who studied the ionization of air by thorium. Then, 1 day, they discover a phenomenon, surprising at first:

The sensitiveness of thorium oxide to slight currents of air is very remarkable. The movement of the air caused by the opening or closing of a door at the end of the room opposite to where the apparatus is placed, is often sufficient to considerably diminish the rate of discharge [73].

Rutherford can think of only one explanation:

Thorium compounds continuously emit radio-active particles of some kind, which retain their radioactive power for several minutes. This “emanation,” as it will be termed for shortness, has the power of ionizing the gas in its neighborhood….

Always very careful, Rutherford does not claim the agent to be a gas, but he proceeds with careful experiments to show that the emanation is neither due to a fine dust of thorium particles nor to thorium vapor. Furthermore, he notes an essential feature: the activity of the emanation decreases geometrically, that is, exponentially as we would say today. This means that if the activity of the sample diminishes by a factor of 2 after a certain time, which we call today the radioactive half-life ^♢, the activity again diminishes by a factor of 2 in the following same interval of time and continues to do so. The measurements of Rutherford showed that the radioactive half-life of the “emanation from thorium” was 60 s. This means that it is reduced to 1/2 in 1 min, to 1/4 in 2 min, to 1/8 in 3 min, and so on. It becomes a 1,000 times weaker after 10 min and a million times weaker after 20 min.

This law of radioactive decay has a great importance: it will soon be observed that the radioactive half-life is a property of each radioactive substance and that it can be used to characterize a radioactive substance, to detect its presence, even in very small quantities.

“Induced” and “Excited” Radioactivity

Pierre and Marie Curie make an observation which they communicate to the Académie des Sciences on November 6, 1899:

While studying the strongly radioactive samples which we prepared (polonium and radium), we noticed that the rays emitted by these substances were able to transfer the radioactivity to otherwise inactive substances and that this radioactivity lasts for quite a long time [74].

Like Rutherford, they observe that this “induced radioactivity” decreases with time:

If one isolates the activated sample from the influence of the radioactive substance, it remains radioactive for several days. However, its radioactivity decreases, fast at first and then progressively more slowly. It appears to disappear asymptotically.

This is a qualitative description of the exponential law of Rutherford. As good experimentalists, Pierre and Marie Curie make sure that this is not due to a trivial cause, or to an illusion:

The aim of this work was mainly to find out whether this induced radioactivity was not due to traces of radioactive material which could have been transported in the form of vapor or of dust onto the exposed strip. […] we believe that we can claim that it is not so and that there exists an induced radioactivity .

This radioactivity has a surprising feature:

We examined the effect of the Becquerel rays on various substances: zinc, aluminum, brass, lead, platinum, bismuth, nickel, paper, barium carbonate, sulphuric bismuth. We were very surprised not to discover order of magnitude differences in the radioactivity induced in these various substances, which all appeared to behave in a similar fashion.

How could one explain this phenomenon? Can it be compared to the emission of electrons when the X-rays of Röntgen impinge on a substance?

The induced radioactivity is a kind of secondary radiation, caused by the Becquerel rays. However it differs from that which is known to occur with the rays of Röntgen. Indeed the secondary rays of Röntgen, which have been studied so far, are created at the instant when the rays of Röntgen impinge on the substance, and they cease as soon as the they are suppressed. In view of the facts reported above, we may ask whether radioactivity, which is apparently spontaneous, is not an induced effect for certain substances.

On November 22, 1899, Rutherford, who had not yet read the communication of Pierre and Marie Curie on induced radioactivity, sends a second paper to the Philosophical Magazine, describing his observations of induced radioactivity:

Thorium compounds under certain conditions possess the property of producing temporary radioactivity in all solid substances in their neighborhood. The substance made radioactive behaves, with regard to its photographic and electric actions, as if it were covered with a layer of radio-active substance like uranium or thorium [75].

Rutherford shows that this “excited” radioactivity is always associated to an “emanation.” He measures its rate of decay and finds that the half-life is about 11 h. It is the same for all the substances which are exposed to the radioactivity, and the half-life is considerably longer than that of the emanation itself (1 min). After a thorough discussion, a prime example of rigor and imagination, he proposes the only plausible explanation he can think of: the “excited” radioactivity⁹ must be caused by radioactive particles originating in the thorium and most likely transported by the “emanation”:

The power of producing radioactivity is closely connected with the presence of the “emanation” from thorium compounds, and is in some way dependent upon it.

Rutherford observes one more thing: in the absence of an electric field, the induced radioactivity is uniformly distributed on the surface of the surrounding material. But if a body carries a negative electric charge, the radioactivity becomes concentrated on this body, suggesting that the induced radioactivity is associated to positive electric charges:

All thorium compounds examined produce radioactivity in substances in their neighborhood, if the bodies are uncharged. With charged conductors the radioactivity is produced on the [negatively] charged body.

Elster and Geitel: The Radioactivity of the Air and of the Earth

Two German physicists, Julius Elster and Hans Geitel, provide further data. Elster was born on December 24, 1854, in Bad Blankenburg, Germany. In school, he becomes a friend of Geitel, who is a few months younger (born on July 26, 1855, in Brunswick). After studying in Heidelberg from 1875 to 1877, and in Berlin in 1878, Elster returns to Heidelberg where he obtains his Ph.D. He passes successfully an examination allowing him to become a high school professor and he obtains a position in Wolfenbüttel, where Geitel, who passed the same examination in Berlin in 1879, had been teaching for a year. When Elster got married, he had a house constructed and Geitel came to live there. They installed a laboratory and soon embarked on their research. At first, they were interested in the conduction of electricity by gases, bearing particular attention to electrical phenomena in the atmosphere, well before the theory of ionization could explain it. In 1889, they studied the photoelectric effect and made important contributions to the field. As soon as the discovery of radioactivity becomes known in 1896, they begin to work on it.

In an address to the British Association on September 7, 1898, Crookes makes a daring suggestion:

It has long been to me a haunting problem how to reconcile this apparently boundless outpour of energy with accepted canons[…] It is possible to conceive a target capable of mechanically sifting from the molecules of the surrounding air the quick from the slow movers[…] Let uranium or polonium, bodies of densest atoms, have a structure that enables them to throw off the slow moving molecules of the atmosphere, while the quick moving molecules, smashing on to the surface, have their energy reduced and that of the target correspondingly increased [77].

This conjecture appears to contradict the second principle of thermodynamics because it implies that heat could flow from one body to another at the same temperature.

Three weeks later, Elster and Geitel disprove the conjecture of Crookes by showing that the radioactivity of uranium is the same in air at the atmospheric pressure, in vacuum, or in a vessel under pressure [78]. The air is therefore not responsible for the occurrence of radioactivity.

They then investigate whether the radioactivity of a sample varies under different conditions: when it is subject to cathode rays, when it is heated to different temperatures, and when it is taken to a high altitude or even to the bottom of a mine (852 m deep). Since the radioactivity seems to remain obstinately insensitive to these conditions, they conclude that:

Since the property of emitting Becquerel rays belongs, as it seems, to all chemical compounds of an active element, it is difficult to interpret it as the sign of a chemical process in the true sense; indeed one should rather seek the source of energy in the atom of the element concerned. One is not far from the idea that a radioactive element, like the molecule of an unstable compound, turns into a stable state. In fact this idea leads to assume a gradual transition of the active substance toward an inactive substance, and therefore, logically, an alteration of its elementary properties [79].

In 1901, they discover that when an electric conductor, in the air, is connected to the negative pole of a battery thereby becoming negatively charged, it becomes radioactive. It has attracted positively charged radioactive particles present in the air, which is therefore weakly radioactive, and so is the earth, as they soon discover [80, 81].

Within a few years, Elster and Geitel acquired a great scientific reputation. Known as the “Castor and Pollux” of physics, they did all their research together. In 1899, the University of Breslau offered a professorship to both of them. However, fearing for their independence, they preferred to remain professors at the Gymnasium of Wolfenbüttel. The respect and esteem which they enjoyed in the scientific community was expressed in 1915 by the edition, for their 60th birthday, of a voluminous commemorative edition with contributions from the greatest German physicists of the time, namely, Max Born, Max von Laue, Philip Lenard, Max Planck, and Arnold Sommerfeld. Elster died on April 6, 1920, in Bad Harzburg, and Geitel died on April 15, 1923, in Wolfenbüttel [82, 83].

A Third Type of Ray: γ-Rays

On April 9, 1900, at a session of the Académie des Sciences, Paul Villard presented a communication on “The reflection and refraction of cathode rays and of deflectable uranium rays.” Under this somewhat trivial title, he included a “remark on the radiation of uranium”:

I almost always observed that, in addition to the refracted beam, a beam propagating in a straight line was superposed. […] These observations lead us to admit that the emission of radium contains a very penetrating radiation, which can pass through metallic strips and which the photographic method is able to detect [84].

A few months later, pursuing his study, Villard showed that these “non deflectable” rays have a penetrating power about 160 times larger than that of β-rays [85]. He believed that they are similar to X-rays and he called them “radium X-rays.” They will soon be called γ-rays, the third Greek letter after α and β.

The Emanation of Thorium Is a Gas Belonging to the Argon Family

When Rutherford returns to New Zealand for a vacation, an important event takes place. Some 6 years earlier, still a student in Christchurch, he rented a room in the house of Mrs. Arthur de Renzy Newton, a widow with four children. The young Ernest fell in love with the eldest daughter, Mary. They became engaged, but there was no question of marriage as long as he was unable to provide for the needs of the family. Now, in 1900, the time is ripe: they are married and they return to Montreal.

Rutherford continues his detailed observations of the “emanation” of thorium. He proves not only to be an outstanding physicist but also a leader. He builds a small research team which includes Frederick Soddy, a young chemist from the University of Oxford. This is the beginning of a fruitful collaboration. In a paper, published in 1901, they study the chemical properties of the “emanation of thorium […], [which] behaves in every way as a temporarily radioactive gas.” This leads them to an important conclusion:

It will be noticed that the only known gases capable of passing in an unchanged amount through all the reagents employed are the recently discovered gases of the argon family [86].

Note the caution and the art with which Rutherford expresses himself: he incites the reader to note and to conclude for himself that the emanation is a gas belonging to the argon family.

A Proliferation of ”X” Radiations

Rutherford and Soddy have hardly finished writing their paper on the radioactivity and the emanation of thorium compounds, when they discover a new phenomenon which, yet again, leaves them puzzled. They do not rewrite their paper. Instead, they write:

…developments have been made in the subject which completely alter the aspect of the whole question of emanation power and radioactivity.

They observe that the amount of emanation of a given quantity of thorium varies from one chemical compound to another! Rutherford and Soddy make a fractional chemical analysis, similar to the one used by Marie Curie, and they reach the only possible conclusion (because they cannot question the atomic nature of radioactivity):

There seems little doubt of the actual existence of a constituent ThX to which the properties of radioactivity and emanating power of thorium must be ascribed [86].

Further down, they add:

The manner in which it makes its appearance, associated with each precipitate formed in its concentrated solution, resembles the behavior of Crookes’ UrX.

They refer to a paper written by the English chemist William Crookes who, at the age of 68, embarked on a study of the radioactivity of uranium and showed that it was possibly not due to uranium proper but to another constituent:

…the radioactive property ascribed to uranium and its compounds is not an inherent property of the element, but resides on some outside body which can be separated from it [87].

Crookes calls this substance “uranium X,” UrX in short, in order to specify that it is associated to uranium (although it is not uranium). Eighteen months earlier, Becquerel had performed similar experiments: he succeeded in separating a uranium salt from a substantially more radioactive substance mixed with barite sulfate [88, 89]. However, he kept the inactive uranium preparation and he discovered that the uranium had recovered its original radioactivity:

I studied again the progressively weakened products which I had prepared 18 months ago and, as I expected, I found that all the products were identical […] Thus the lost radioactivity was spontaneously recovered. On the other hand, the precipitated barite sulfate, which before was more radioactive than uranium, is completely inactive today. The loss of radioactivity, which is a property of activated or induced substances, shows that barium did not decrease the essentially active and permanent part of uranium [90].

The mystery grows.

“An Enigma, a Deeply Astonishing Subject”

1900 is the year of the universal exposition in Paris, and on this occasion, the Société Française de Physique (the French Physical Society) decides for the first time to organize an international physics meeting. The first circular is mailed to physicists throughout the world in June 1899. There is a large response and over 800 physicists attend the opening session in the Grand Palais on Monday, August 6, 1900. It is presided by Alfred Cornu and Lord Kelvin is named honorary president, by acclamation. The meeting lasts 6 days. It is divided into seven sessions, each one devoted to an important problem at the time. One session is devoted to cathode and uranic rays. The physicists are invited to the Élysée Palace by the president of the Republic Émile Loubet and the prince Roland Bonaparte offers a reception in his private mansion in avenue d’Iéna. They are also guided to the top of the new Eiffel Tower. Visits to laboratories are organized as well as some general talks, among which two, devoted to radioactivity, are delivered by Becquerel and Pierre Curie. Becquerel speaks about the radioactivity of uranium [91], and the talk of Pierre Curie, signed also by Marie Curie, is devoted to “new radioactive substances” and to general problems of radioactivity [92]. Their conclusion bears yet another question mark:

But the spontaneous nature of the radiation is an enigma, a deeply astonishing subject. What is the source of the energy of the Becquerel rays? Should one search for it within the radioactive substance itself or outside? […] In the first case, the energy could be drawn from the heat of the surrounding matter, but such a hypothesis would contradict the Carnot principle. In the second case, […] radium would continuously emit very small particles carrying negative electric charge. The available energy, stored as potential energy would progressively dissipate, and this view would necessarily lead us to abandon the idea that atoms are invariable.

The Puzzle Is Disentangled

Rutherford shows that radioactivity is the transformation of an atom into another: the atom explodes while violently ejecting microscopic particles. Radioactivity draws its energy from within the atom, in an enormous quantity.

In this new year of 1902, Rutherford and Soddy pursue their patient and obstinate study in their laboratory at McGill University. The situation is really confusing. The simple and constant radioactivity of uranium or thorium (or still radium) is presently replaced by a multitude of further phenomena: the induced radioactivity, as Pierre Curie calls it, the emanation, loss and recovery of the activity of thorium and of uranium. Furthermore Rutherford and Soddy recently discovered “thorium X,” coined ThX. It is not thorium since it can be separated by chemical methods. Nonetheless, Rutherford calls it “thorium X” as a reminder that it is associated with thorium. In a similar fashion “uranium X” is not the same element as uranium. They make a hypothesis:

It therefore follows that […] the experimental curve will be explained if two processes are supposed to be taking place:

• 1. That the active constituent ThX is being produced at a constant rate.

• 2. That the activity of ThX decays geometrically with time [93].

Several observations corroborate this idea. Paying attention to the question recurrently raised by Pierre and Marie Curie, they carefully discuss the origin of the energy involved:

Energy considerations require that the intensity of radiation from any source should die down with time unless there is a constant supply of energy to replace that dissipated. This has been found to hold true in the case of all known types of radioactivity with the exception of the “naturally” radioactive elements […] In the case of the three naturally occurring radioactive elements, however, it is obvious that there must be a continuous replacement of the dissipated energy, and no satisfactory explanation has yet been put forward to account for this.

Rutherford and Soddy then propose the following explanation of all the observations: by its radioactivity, thorium constantly produces “thorium X,” which in turn progressively disappears by radioactivity. But since it remains mixed to the thorium, an equilibrium is reached between the production and disappearance of “thorium X.”

The material constituent responsible for the radioactivity, when separated from the thorium which produces it, behaves in the same way as the other typically radioactive substances. Its activity decays geometrically with time, […]. The normal radioactivity is, however, maintained at a constant value by a chemical change which produces fresh radioactive material…

They consider that this explanation applies equally well to uranium and radium, which allows them to conclude:

All known types of radioactivity can thus be brought into the same category.

This is not the end yet, but they hold the thread which will lead them out of the labyrinth. Radioactivity is not a simple phenomenon but a cascade of superimposed events. Radioactive substances are subject to continuous transformations, while emitting radiation and producing new substances which are themselves transformed. And to complicate things further, each transformation occurs at a different rate. What is observed is a mixture of the radiations of these substances. A well-entangled process! Rutherford insists that it is an atomic phenomenon:

All the most prominent workers in this subject are agreed in considering radioactivity an atomic phenomenon. M. and Mme Curie, the pioneers in the chemistry of the subject, have stated ( Comptes Rendus 1902, 134, 85) that this idea underlies their whole work from the beginning and created their method of research.

Furthermore, the radiation consists of material particles and not of waves similar to electromagnetic waves or X-rays:

M. Becquerel, the original discoverer of the property for uranium, […] points out the significance of the fact that uranium is giving out cathode rays. These, according to the hypothesis of Sir William Crookes and Professor J. J. Thomson, are material particles of mass one-thousandth that of the hydrogen atom.

Induced radioactivity behaves as a deposit of a certain kind of radioactive material:

The present researches had their starting point in the fact that had come to light with regard to the emanation produced by thorium compounds and the property it possesses of exciting radioactivity on surrounding objects. In each case, the radioactivity appeared as the manifestation of a special kind of matter in minute amount. The emanation behaved in all respects like a gas, and the excited radioactivity it produces as an invisible deposit of intensely active material independent of the nature of the substance on which it was deposited, and capable of being removed by rubbing or by the action of acids.

After having carefully assessed these observations, Rutherford and Soddy inevitably conclude:

The position is thus reached that radioactivity is at once an atomic phenomenon and the accompaniment of a chemical change in which new kinds of matter are produced. The two considerations force us to the conclusion that radioactivity is a manifestation of a subatomic chemical change.

This is quite an extraordinary conclusion! New forms of matter and possibly new elements can be created. At this point, Rutherford avoids being explicit and he avoids the word “transmutation” although that is what it is all about: he does not want to be coined an alchemist. He restricts himself to a minimal formulation which is, however, strongly stated as “the two considerations force us to the conclusion….” Such a personal formulation is extremely rarely used by him. Finally, Rutherford and Soddy observe that “disactivated” thorium (meaning distinct from ThX) is not really inactive: it remains endowed with a residual activity of its own. Soddy makes a similar observation for uranium [94].

α-Rays Revisited

At the same time, Rutherford studies the weakly penetrating α-rays. They had not been the subject of as much attention as the penetrating β-rays. Becquerel had shown that the radiation of radioactive substances was similar to cathode rays, that is, to electrons but with a velocity which is much higher than that of cathode rays [69, 95–98]. The radioactivity discovered in 1896 by Becquerel was in fact composed of β-rays because the α-rays emitted by uranium were stopped by the cardboard which he placed between the uranium and the photographic plate in order to protect it from sunlight. It is not easy for Rutherford to study α-rays for this same reason: they have a weak penetrating power. A thin layer of matter is enough to stop them so that an extremely thin sample of radioactive material needs to be used if the α-rays are to be emitted into the air before becoming absorbed. By using a magnet, more powerful than the one of Stefan Meyer and Egon R. von Schweidler,¹⁰ he notices that α-rays are indeed also deflected by a magnetic field. They therefore carry an electric charge. Rutherford succeeds in deflecting them by an electric field and this enables him to estimate their mass [99, 100] which he discovers to be much larger than the mass of the electron. However, they have a considerably smaller velocity of about 25,000 km/s. The α-rays are thus more akin to ionized atoms. They could be hydrogen or helium atoms. Their higher mass allows them to transport a higher energy in spite of their slower velocity.

Radioactivity Is an Atomic Decay

In the spring of 1903, the University College of the University of London offers a position to Soddy in the laboratory of Sir William Ramsay, the famous chemist, who had isolated, purified and identified several new elements, namely, argon in 1894 with Rayleigh as well as helium, neon, krypton, and xenon [101]. These elements formed a new column which was added to the Mendeleev table ♢. They are “rare” or “noble” gases which neither combine with themselves nor with any other elements. Ramsay is particularly interested in the “emanations” which bear a striking resemblance to “his” rare gases.

Before Soddy embarks to England, he and Rutherford publish several papers in which they summarize the work done at McGill University. They first rewrite two papers, originally published in the Journal of the Chemical Society, and send them to the Philosophical Magazine [102, 103], which enjoys a much wider audience, among physicists in particular. In their second paper, they add a very important remark:

So far it has been assumed, as the simplest explanation, that the radioactivity is preceded by chemical change, the products of the latter possessing a certain amount of available energy dissipated in the course of time. A slightly different view is at least open to consideration, and in some ways preferable. Radioactivity may be an accompaniment of the change, the amount of the former at any instant being proportional to the amount of the latter. On this view the non-separable radioactivity of thorium and uranium would be caused by the primary change in which ThX and UrX are produced.

Thus, the radiation would be emitted during the transformation of the element thorium (or uranium) into the mysterious ThX (UrX). The radioactive transformation would be a kind of an explosion during which α-particles would be expelled: a real decay!

The Puzzle Is Unravelled: Radioactive Families

In the fall of 1903, Rutherford and Soddy publish the last paper belonging to their collaboration in the laboratory of McGill. They first make a systematic review of their results. Their view of the nature of radioactivity has matured and they take another step forward:

There is every reason to suppose, not merely that the expulsion of a charged particle accompanies the change, but that this expulsion actually is the change [104].

In other words, radioactivity is the way in which the transformation of an atom is visible to us. They give a simple formulation of the law of radioactive decay:

The proportional amount of radioactive matter that changes in unit time is constant.

This is an alternative way to express the law of exponential decay. They add:

The complexity of the phenomena of radioactivity is due to the existence as a general rule of several different types of matter changing at the same time into one another, each type possessing a different radioactive constant.

The nature of radioactivity can only be a decay:

Since radioactivity is a specific property of the element, the changing system must be the chemical atom, and since only one system is involved in the production of a new system and, in addition, of heavy charged particles, in radioactive change the chemical atom must suffer disintegration.

The situation is clear: radioactivity is the decay of an atom which expels a particle. The residual atom is different, a chemical transformation has taken place.

However, Rutherford and Soddy are not content to simply state that the observed radioactivity is the mixture of several radioactive decays. They also classify radioactive substances into three groups, corresponding to the decay of uranium, thorium, and radium (see Fig. 1). This is the first example of what will soon be called “radioactive families.” True, there still remain substances and emanations of unknown nature, but the direction is defined. Physicists must now complete this table, identify the various elements, measure their decay rate, etc. The general framework will not change. Rutherford and Soddy ask what name should be given to the intermediate atomic fragments:

Fig. 1

The three radioactive families proposed by Rutherford and Soddy in 1903. Each known element (uranium, thorium, and radium) is the starting point of a succession of radioactive transformations: uranium transforms into uranium X, what follows is not known; thorium transforms into thorium X, which transforms into the “thorium emanation,” etc. [104]

… which remain in existence only a limited time, continually undergoing further change. Their instability is their chief characteristic. On one hand, it prevents the quantity from accumulating, and in consequence it is hardly likely that they can ever be investigated by ordinary methods. On the other hand, the instability and consequent ray-expulsion furnishes the means by which they can be investigated. We would therefore suggest the term metabolon for this purpose [104].

The term metabolon did not survive. Clearly, the prudent Rutherford did not wish to expand the list of elements too hastily, even with question marks. After all, there existed no experimental evidence, neither chemical nor spectroscopic, which would point to new elements. He said that they were fragments of atoms with specific features; their short lifetime implies that they appear in too small quantities to be identified by usual chemical methods or even by spectroscopic methods. Each one, has however, a unique feature: its radioactive half-life. But to conclude from this that they should be new chemical elements is a step which Rutherford carefully avoids.

Where Does the Energy of Radioactivity Come from? The Conjecture of Rutherford

Rutherford paid much attention to the problem of the energy involved in radioactivity. It appeared to stem from nowhere. This question was also a major concern for Pierre and Marie Curie. Rutherford conjectured that atoms possess and internal “latent” energy which is released during the radioactive decay. In a similar fashion, heat is released during the combustion of hydrogen and oxygen, the process during which two atoms of hydrogen stick to an atom of oxygen so as to form a water molecule. However, this process is not a decay but rather the coalescence of three atoms which form a molecule. In the fall of 1902, Rutherford and Soddy attempt to measure the energy released during the radioactive transformation of radium. They know roughly the mass and the velocity of the emitted “rays” which they assume to be material particles. They estimate their number which in turn allows them to estimate the amount of energy which is released. They find an enormous number: a hundred million (10⁸) calories are released by 1 g of radium (assuming it decays completely) and that is a minimal estimate:

10 ⁸ gramme-calories per gramme may […] be accepted as the least possible estimate of the energy of radioactive change in radium. The union of hydrogen and oxygen liberates approximately 4 × 10 ³ gramme-calories per gramme of water produced, and this reaction sets free more energy for a given weight than any other chemical change known. The energy of radioactive change must therefore be at least 20 000 times, and may be a million times, as great as the energy of any molecular change [104].

They calculate that, in the span of 1 year, a single gram of radium releases at least 15 000 g-cal. This leads them to explain the apparent constancy of the radioactivity of radium and thorium:

Since the α radiation of all the radio-elements is extremely similar in character, it appears reasonable to assume that the feebler radiations of thorium and uranium are due to these elements disintegrating less rapidly than radium. […] We obtain the number of 6 × 10 ⁻¹⁰ as a maximum estimate for the proportionate amount of uranium or thorium undergoing change per year. Hence in 1 “gramme” of these elements less than a “milligramme” would change in a million years. In the case of radium, however, the same amount must be changing per year. The “life” of the radium cannot be in consequence more than a few thousand years.

In other terms, uranium and thorium do not last forever, but their decay rate is very weak: only one part of a sample in a thousand will disintegrate in the span of one million years. About a billion years (10⁹ years) will pass before half of the sample disappears. This is why it appears to be permanent. But radium decays a million times faster, so that its half-life is of the order of a 1000 years. The order of magnitude of these estimates is correct. The present-day measured half-lives are 4.47 billion years (4. 47 ×10⁹) for uranium, 14 billion years (14 ×10⁹) for thorium, and 1600 years for radium.

There is more. These results suggest an explanation of a completely different phenomenon, namely, the source of energy of the sun:

The energy latent in the atom must be enormous compared with that rendered free in ordinary chemical change[ …] It must be taken into account in cosmic physics. The maintenance of solar energy, for example, no longer presents any fundamental difficulty if the internal energy of the component elements is considered to be available, i.e. if processes of sub-atomic change are going on.

This is a bold conjecture. Rutherford had little inclination towards abstract or adventurous speculation. He simply acknowledged the fact that the internal energy of atoms was probably of the right order of magnitude to provide for the energy of the sun, a controversial subject at the time.

At the same time, Pierre Curie, together with his young assistant Albert Laborde, measured the increase in temperature of a water bath in which they immersed a vessel containing a small amount of radium (in the form of a chloride mixed with some barium chloride). They obtained a huge number:

1 ^g of radium releases a quantity of heat which is of the order of 100 small calories per hour.

1 gramme-atom of radium (225^g ) would release, during each hour, 22, 500 ^cal , a number comparable to the heat released by the combustion of 1 gramme-atom of hydrogen in oxygen.

The release of such an energy cannot be explained by an ordinary chemical transformation. If we seek the origin of this production in an internal transformation, this transformation must be of a deeper nature and it must be a modification of the radium atom itself […] Thus if the preceding hypothesis were exact, the energy involved in the transformation of atoms would be extraordinarily large.

The hypothesis of a continual transformation of the atom is not the only one which is compatible with the heat released by radium. The heat release can also be explained by assuming that radium uses an external energy, the nature of which is unknown [105].

Let us ignore units such as the gramme-atom, the small calorie ♢, and gramme-calories ♢, which are no longer used today, in order to retain this essential fact: 225 g of radium release as much energy per hour, and at a seemingly constant rate, as the total combustion of 22 l of hydrogen. Just like Rutherford, Pierre Curie notes that this energy is much larger than energies involved in chemical reactions and that radioactivity must correspond to a deep transformation of the radium atom. But he does not yet abandon the hypothesis of a cosmic energy flux which radium (as well as other radioactive substances) would be able to pick up. Pierre Curie maintains this hypothesis because radium releases energy permanently and that one cannot detect a corresponding decrease of its weight. He is not yet aware of Rutherford’s explanation which is the key to this enigma: it is precisely because the energy released by the decay of each atom is so large that this energy can manifest itself at our scale while only a tiny fraction of atoms actually decay. Indeed barely 0.04 % of the radium atoms and only one out of ten billion (10¹⁰) uranium or thorium atoms decay in 1 year. The constancy of these elements is an illusion.

How do the measured values of Pierre Curie compare with those of Rutherford? It takes 1,600 years for half a gram of a radium sample to decay so that, according to the estimate of Rutherford, it would release at least a dozen gramme-calories per hour, possibly 10 or a 100 times more. This estimate is quite compatible with the measurement of Pierre Curie, who quotes 100 cal/h.

The theory of Rutherford and Soddy was not immediately approved by everyone. J. J. Thomson was readily convinced, but the idea of a transmutation, even if the word wasn’t used, was difficult to swallow, especially by chemists and also by Lord Kelvin, who found it hard to believe that so much energy could be stored in an atom. He preferred to think, like Pierre Curie, that some atoms could absorb some radiation propagating in space. Pierre Curie was somewhat reluctant to believe in the existence of such substances whose existence is only revealed by a radioactivity of a given half-life.

But the theory of Rutherford was effective and it explained the great complexity of radioactive phenomena. By 1904 it was accepted. In that same year, Rutherford published his first book, Radioactivity [106], which he dedicated to J. J. Thomson “to acknowledge his respect and admiration.” In the preface, he states his philosophy:

The phenomena exhibited by the radioactive bodies are extremely complicated, and some form of theory is essential to connect in an intelligible manner the mass of experimental facts that have now been accumulated. I have found the theory that the atoms of the radioactive bodies are undergoing spontaneous disintegration extremely serviceable, not only in correlating the known phenomena, but also in suggesting new lines of research.

It must be admitted that his theory does explain the observations and succeeds in unravelling what appeared to be an inextricable puzzle.

Experimental Evidence of Transmutation

Rutherford and Soddy noticed that uranium ore always contained a certain amount of helium. The element helium was first observed in 1868 by Lockyer [107], the founder of Nature, in the form of a bright yellow line in the spectrum ♢of solar light in a prominence which he observed during the total eclipse of August 18, 1868. He attributed this spectral line to a new element which he called helium, in order to stress its solar origin. Twenty seven years later, William Ramsay detected its presence in cleveite, a rare variety of pitchblende which is found in Sweden. He showed that it was an inert gas with atomic weight ♢4, four times heavier than hydrogen. Crookes showed that it gave rise to the spectrum observed by Lockyer.

In 1903, Frederick Soddy joins William Ramsay in the laboratory of University College in London. Soon, they begin to study the emanation of radium. They enclose a 20 mg sample of radium in a closed vessel and, after a few months, they discover that the vessel contains helium [108]. This is a proof that the radium atom splits while emitting a helium atom. What remains of the radium atom is then necessarily another element. This observation adds considerable credit to Rutherford’s general theory of radioactive transformations which becomes generally accepted.

Radioactivity is Understood. Radioactive Families

The physical phenomenon is now understood. It is still necessary to identify each one of the successive transformations of naturally radioactive elements. On May 19, 1904, Rutherford is invited to deliver the prestigious annual Bakerian Lecture at the Royal Society in London [109]. In 1775, Henry Baker donated £100 with the instructions that a Fellow of the Royal Society should deliver a talk on “some part of natural history or of experimental philosophy at a date and in a fashion found suitable to the President and the Council of the Royal Society.” Previous speakers included William Thomson (Lord Kelvin), James Dewar, Norman Lockyer, William Crookes, James Clerk Maxwell, Lord Rayleigh and Michael Faraday. Rutherford reports on progress in the theory of radioactive transformations. The list of radioactive families has grown and become more precise (see Fig. 2).

Fig. 2

There are four known radioactive families in 1904, together with the actinium family. The number of “metabolons” has nearly tripled [109]

Considerably more work was required to complete the radioactive families. The “metabolons,” as Rutherford calls them, need to be identified. Some are radium A,B,C, and others thorium A,B,C, denominations which simply indicating their origin. But the path is clear.

Rutherford stresses what he considers an essential point: each radioactive substance is characterized by its half-life (as it is called today) by means of which it can be unambiguously identified.

Rutherford remained in Montreal for three more months during which he confirmed and consolidated his results. His reputation grew to the extent that he had to refuse most invitations. The laboratory in McGill attracted many researchers among whom a young German chemist, Otto Hahn who will soon appear on the scene.

In September 1906, Rutherford received a letter from Arthur Schuster, physics professor at the University of Manchester. He was of German origin (born in Frankfurt in 1851 into a well-to-do Jewish family) and, since 1887, he was Langworthy Professor of Experimental Physics at Owens College in Manchester, where he acquired a reputation in the fields of spectroscopy and the conduction of electricity by gases. At the age of 55, he wanted to retire, enjoy his fortune and write a book. He wished to be succeeded by whom he considered to be the greatest physics experimentalist, namely Ernest Rutherford. After exchanging letters for 6 months, Rutherford finally accepted the offer much to the dismay of his colleagues in McGill. It was an occasion for him to return to England, and to be closer to the best physics laboratories. When he retired, Arthur Schuster created a scholarship allowing to invite a young mathematical physicist. The scholarship will be used later by a young Danish physicist, named Niels Bohr. On May 17, 1907, Ernest Rutherford embarks on a boat headed to the old continent.

Consecrations and Mourning: The End of an Era

First Henri Becquerel, Pierre and Marie Curie, then Ernest Rutherford are awarded the Nobel Prize. The deaths of Pierre Curie and of Henri Becquerel mark the end of an era.

On November 27, 1895, the Swedish industrialist Alfred Bernhard Nobel, aged 63, signed his last will in Paris. He endowed most of his fortune to a fund

… the interest on which shall be annually distributed in the form of prizes to those who, during the preceding year, shall have conferred the greatest benefit on mankind. The said interest shall be divided into five equal parts […]: one part to the person who shall have made the most important discovery or invention within the field of physics; one part to the person who shall have made the most important chemical discovery or improvement; one part to the person who shall have made the most important discovery within the domain of physiology or medicine; one part to the person who shall have produced in the field of literature the most outstanding work in an ideal direction; and one part to the person who shall have done the most or best work for fraternity between nations, for the abolition or reduction of standing armies and for holding the promotion of peace congresses […] It is my express wish that in awarding the prizes, no consideration whatever shall be given to the nationality of the candidates, but that the most worthy shall receive the prize, whether he be a Scandinavian or not [110].

Alfred Nobel died on December 10, 1896. The first Nobel prize in physics was attributed in 1901 to Conrad Röntgen for his discovery of X-rays. The 1902 Nobel Prize was attributed to Hendrik Lorentz and Pieter Zeeman. The Nobel Prize quickly acquired a fame unequalled by any other.

1903: Henri Becquerel Shares the Nobel Prize with Pierre and Marie Curie

On November 1903, telegrams sent from Stockholm announce that the Nobel Prize in Physics is shared: one half is attributed to Henri Becquerel “in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity”; the other half to Pierre and Marie Curie “in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel” [111].

Just a few months earlier, on June 25, 1903, Marie Curie defended her Ph.D. thesis in front of a jury composed of Gabriel Lippmann, Edmond Bouty and Henri Moisan. The thesis bore a simple title: “Research on radioactive substances.” It was the first Ph.D. thesis defended by a woman in France. That same evening, Paul Langevin invited the Curie couple to dinner, together with Jean Perrin, his wife as well as Ernest Rutherford and his wife, who were passing through Paris. Paul Langevin had met Rutherford at the Cavendish where he spent the academic year 1897–1898, after graduating from the École Normale Supérieure. That was to be the only encounter between Rutherford and Pierre Curie.

The Nobel Prize was officially awarded on December 11. According to the statutes of the Nobel Fund, the laureate was invited to deliver a lecture to the Swedish Academy, within 6 months after receiving the prize. Becquerel went to Stockholm with his wife and, in his lecture, he gave a very precise account of his observations. However, Pierre and Marie Curie were unable to go. For some time already they felt very tired and they feared the 2-day train trip to Stockholm. A few months before, Marie gave birth to a premature child who died within a few hours. Pierre suffered from pain in his joints, which he thought were due to rheumatism. They thought they had worked too hard, which was true, but they did not suspect that a continuous exposure to radioactive substances could be the cause.

In fact, biological effects of radioactive substances had been observed as early as 1900 by Giesel [112], who noted a red spot which developed into a wound on his arm which had been exposed to a radioactive source. One day in 1901, Pierre Curie lent Henri Becquerel some barium chloride enclosed in a test tube, itself wrapped in a cardboard box. After keeping it for about 6 h in his waistcoat, Becquerel noticed that his skin had reddened and that a wound formed which took a month to heal. Then Pierre Curie voluntarily attached a sample to his arm. The resulting wound took about 52 days to heal. In a communication to the Académie des Sciences, Becquerel and Pierre Curie wrote:

The rays of radium act strongly on the skin. The effect is similar to the one produced by the rays of Röntgen [113].

They stress the effect produced on the hands of experimentalists who handle radioactive substances:

In addition to these strong effects, various other effects are noticeable on our hands while we manipulated strongly radioactive substances. The hands have a tendency towards desquamation; the tips of fingers, which held test tubes or capsules containing very radioactive substances, harden and become occasionally very painful; the inflammation of the finger tips of one of us lasted two weeks after which the skin pealed off; the pain persisted for 2 months.

Pierre and Marie Curie must have believed that they were only local injuries. Pierre wrote to the Swedish Academy of Sciences that his teaching duties made it difficult for him to travel to Sweden. It was the French ambassador in Stockholm who received the Nobel Prize from the King of Sweden. After postponing the trip to Sweden several times, they finally went in June 1905 and the trip was very pleasant. In his talk, Pierre exhibited several experiments using radium and other apparatus brought from Paris. He also recalled what opposed him to Rutherford. In order to account for the huge energy released by radioactivity, Pierre and Marie Curie made the hypothesis that radioactive atoms captured an as yet undiscovered radiation which uniformly permeated space. Pierre Curie then recognized that Rutherford made a better hypothesis which he considered bold and even somewhat daring at first. This is an example of two outstanding physicists, with different temperaments and different cultures, who have a different vision. Pierre Curie preferred his hypothesis because, in the absence of convincing experimental evidence, it seemed more plausible, natural and the least daring. The opposite was true for Rutherford. In all fairness, Pierre Curie admitted that the second hypothesis explained more results, a touchstone of any theory. Pierre Curie ended his talk with premonitory considerations marked by his idealism and faith in science:

It is conceivable that, placed in the wrong hands, radium could become very dangerous and we can wonder whether humanity is ripe enough to benefit from the knowledge of nature’s secrets, or if this knowledge will be harmful. The discoveries made by Nobel are characteristic in this respect. His powerful explosives enable wonderful constructions to be made. They are also means of terrible destruction in the hands of criminals who lead people to war. I belong to those who believe, as Nobel did, that humanity will derive more good than evil from new discoveries [111].

The Death of Pierre Curie

There remained less than a year for Pierre Curie to live. On April 19, 1906, while crossing the rue Dauphine,

… he was struck by a truck coming from the Pont Neuf and fell under its wheels. A concussion of the brain brought instantaneous death.

So perished the hope founded on the wonderful being who thus ceased to be. In the study room to which he was never to return, the water buttercups he had brought from the country were still fresh [114].

He was buried in intimacy on Saturday, April 21. On April 23, Henri Poincaré presided the session of the Académie des Sciences and opened the session with an eulogy:

You all know what a kind and reliable person he was; you are all familiar with the delicate charm which emanated, so to speak, from his gentle modesty. One would not have believed that, behind this gentleness, hid an uncompromising mind. He made no compromise with the generous principles under which he was brought up, with the moral ideal which he conceived, this ideal of absolute sincerity, probably too elevated for the world we live in [115].

Upon which Poincaré closed the session, an exceptional step, in sign of mourning.

1908: Rutherford is Awarded the Nobel Prize

A telegram sent in the end of September 1908 informed Rutherford and the world that the Swedish Royal Academy had attributed to him the Nobel Prize for Chemistry “for his investigation into the disintegration of the elements and the chemistry of radioactive substances.”

He was surprised to be awarded the Nobel Prize for chemistry, instead of physics, since his whole work was that of a physicist and he joked about that for some time. But it is true that he modified the way chemists conceived atoms. He travelled to Stockholm with his wife Mary and he delivered the traditional Nobel lecture on “The chemical nature of α-particles emitted by radioactive substances.” Like Pierre Curie, he illustrated his lecture with experimental demonstrations.

The Death of Henri Becquerel

A few months before, on the 29th of June, 1908, Henri Becquerel was elected Perpetual Secretary of the Académie des Sciences, and he set off, as he did each year, to his summer residence at Le Croisic. He died there on the 29th of August, after a short illness.

The first page of the history of radioactivity was turned.