MARIE SKLODOWSKA CURIE

Marie Sklodowska Curie opened up the science of radioactivity. She is best known as the discoverer of the radioactive elements polonium and radium and as the first person to win two Nobel prizes. For scientists and the public, her radium was a key to a basic change in our understanding of matter and energy. Her work not only influenced the development of fundamental science but also ushered in a new era in medical research and treatment.

Nation and Family

A PRISONER IN CHAINS. That is what Poland seemed like to Maria Sklodowska. Manya, as she was affectionately called, learned to be a Polish patriot from her parents, Bronislawa and Vladislav Sklodowski. At the time of Maria's birth in Warsaw on November 7, 1867, Poland had not been an independent country for most of a century. It had been divided up among Austria, Prussia, and czarist Russia.

Warsaw was in the part of Poland controlled by the czar, who hoped to stamp out Polish nationalism by keeping the people ignorant of their culture and language. But Polish patriots were determined to regain control of their nation. As educators, Maria's parents did their best to overcome restrictions placed on them by their Russian supervisors.

The birth of Manya, her fifth child, led her mother to resign her position as head of a school, where the family had resided until then. They moved to a boys' high school, where Vladislav taught math and physics and earned a good salary. Eventually, however, the Russian supervisor in charge of the school fired him for his pro-Polish sentiments.

“Constantly held in suspicion and spied upon, the children knew that a single conversation in Polish, or an imprudent word, might seriously harm, not only themselves, but also their families.” --Marie Curie

AS HER FATHER WAS FORCED into a series of progressively lower academic posts, the family's economic situation deteriorated. To help make ends meet they had to take in student boarders. Maria was only eight when her oldest sister caught typhus from a boarder and died. That death was followed less than three years later by the death of Madame Sklodowska, who lost a five-year battle with tuberculosis at the age of 42. The surviving family members--Professor Sklodowski; his son Joseph; and his daughters Bronya, Hela, and Maria--drew closer to one another.

Although Sklodowski would never forgive himself for losing the family savings in a bad investment, the children honored him for nurturing them emotionally and intellectually. On Saturday nights he read classics of literature to Maria and her siblings. He also exposed them to the scientific apparatus he had once used in teaching physics but now kept at home, since the Russian authorities had eliminated laboratory instruction from the Polish curriculum.

“I easily learned mathematics and physics, as far as these sciences were taken in consideration in the school. I found in this ready help from my father, who loved science....Unhappily, he had no laboratory and could not perform experiments.”

Manya was the star pupil in her class. Her personal losses did not impede her academic success, but the pleasure of being awarded a gold medal at her high school graduation in 1883 was blunted because it meant shaking the hand of the grandmaster of education in Russian Poland. After graduating at 15, Manya suffered a collapse that doctors thought was due to fatigue or "nervous" problems -- today it might be diagnosed as depression. At her father's urging Manya spent a year with cousins in the country. A merry round of dances and other festivities, it would be the only carefree year of her life.

The Floating University

ARIA HOPED, LIKE HER SIBLINGS, to get an advanced degree. Although Joseph was able to enroll in the medical school at the University of Warsaw, women were not welcome there. Maria and Bronya joined other friends in attending the Floating University. This illegal night school got its name from the fact that its classes met in changing locations, the better to evade the watchful eyes of the czarist authorities. Its students' lofty goal went beyond mere self-improvement. They hoped their grass-roots educational movement would raise the likelihood of eventual Polish liberation.

This fly-by-night education could not match the curriculum at any of the major European universities that admitted women. Although Maria understood this fact, at the Floating University she did get a taste of progressive thought and an introduction to new developments in the sciences.

“It was one of those groups of Polish youths who believed that the hope of their country lay in a great effort to develop the intellectual and moral strength of the nation....we agreed among ourselves to give evening courses, each one teaching what he knew best.”--Marie Curie

Polish Girlhood (1867-1891)

The Governess

MARIA AND BRONYA MADE A PACT: the younger sister, still not 17, would work as a private tutor, setting aside money to pay Bronya's tuition at medical school in Paris and her living expenses there. As soon as Bronya could, she would help subsidize Maria's education.

After two years of teaching various subjects to children from wealthy families, Maria realized she was not saving money efficiently enough. For the next three years she worked as a well-paid governess.

Her charges were the children of an agriculturist who ran a beet-sugar factory in a village 150 kilometers north of Warsaw. Maria felt a kinship with her employer when he permitted her in her spare time to teach the illiterate children of his peasant laborers. He encouraged his older daughter to assist Maria, even though he knew the czarist authorities equated such activity with treason. “Even this innocent work presented danger,” Maria recalled, as all initiative of this kind was forbidden by the government and might bring imprisonment or deportation to Siberia.

When their governess fell in love with their oldest son, however, her employers were none too pleased. As fond as they were of Maria, they did not welcome the knowledge that their beloved Kazmierz, on vacation from his agricultural engineering course in Warsaw, wanted to marry the penniless girl. Although the couple bowed to his parents' wishes and broke off the engagement, their romantic involvement continued for several years more. As difficult as it was to stay under the same roof as a family that clearly did not welcome her as one of their own, Maria remained in their employ because she took her pact with Bronya seriously.

“If [men] don't want to marry impecunious young girls, let them go to the devil! Nobody is asking them anything. But why do they offend by troubling the peace of an innocent creature?” --letter of Marie Curie to her cousin Henrietta Michalowska, April 4, 1887

TO FILL HER LONELY HOURS she began a course of self-study. Unsure at first where her academic interests lay, she read sociological studies and works of literature along with physics and chemistry textbooks. By mail she also took the equivalent of an advanced math course with her father. When it became clear that math and the physical sciences were her forte, she took chemistry lessons from a chemist in the beet-sugar factory.

After returning to Warsaw in 1889, Maria worked as a live-in governess for another year before resuming life with her father and work as a private tutor. During her absence Sklodowski had become director of a reform school, and the new position paid well enough for him to send a monthly subsidy to Bronya in Paris. By arrangement with Bronya, he began to set aside a portion of that subsidy to compensate Maria for the sums she had been sending her sister. Eventually it became clear that by fall 1891, Maria would have enough money to begin studies at the University of Paris--the famous Sorbonne.

“During these years of isolated work, trying little by little to find my real preferences, I finally turned towards mathematics and physics, and resolutely undertook a serious preparation for future work.”

MARIA STILL LACKED real laboratory experience, and she hoped to gain some before her departure. This was no easy task, given the czarist ban on such work. The ingenuity of her cousin Joseph Boguski helped her achieve her illicit goal. A former assistant of Russian chemist Dmitri Mendeleev , Boguski ran the so-called Museum of Industry and Agriculture, which was actually a laboratory aimed at training Polish scientists. One of Boguski's colleagues there gave Maria an intensive chemistry course on Sundays and evenings. More often than not, however, she struggled through experiments on her own, often failing to duplicate the expected results.

Finally, in autumn 1891, Maria Sklodowska set out for Paris. Traveling as economically as possible, she carried not only enough food and reading for the trip but also a folding chair and a blanket: fourth-class travelers through Germany were not provided with seating. “So it was in November, 1891,” she recalled, “at the age of 24, that I was able to realize the dream that had been constantly in my mind for several years.”

A Student in Paris (1891-1897)

Working Wife and Mother

JUGGLING HOUSEHOLD AND PROFESSIONAL responsibilities was something Marie had to learn from the outset of her married life. In addition to the two master's degrees she held by the time of her marriage, she decided to earn a certificate that would permit her to teach science to young women. Meanwhile, she continued to conduct her research on the magnetic properties of steel. The director of the Municipal School of Industrial Physics and Chemistry granted her permission to complete that work on the school premises, although even Pierre had no private laboratory there. The school did not help to subsidize her studies, but she received complimentary steel samples from several metallurgical firms. For the rest of her life she would continue this three-cornered arrangement of mutual assistance among research, industry, and the government's educational system.

After submitting the results of her research to the Society for the Encouragement of National Industry in the summer of 1897, she used part of her payment to return the scholarship money she had received four years earlier. She was not expected to do so, of course, but she wanted to contribute to the education of some other worthy Polish student.

“Having grown up in an atmosphere of patriotism kept alive by the oppression of Poland, I wished, like many other young people of my country, to contribute my effort toward the conservation of our national spirit.”

PARENTHOOD SOON CHANGED the Curies' lives. In September 1897 their first child, Irène, was born. Pierre's father, a physician, delivered the baby. Just as she had done with the household budget from the time of their marriage, Marie now began keeping records of every stage of her daughter's development with the same meticulous care that she used to keep track of her experimental work

Only a few weeks after Irène's birth Dr. Curie lost his wife to breast cancer, and he moved into a house at the edge of Paris with his son, daughter-in-law, and granddaughter. With their expanded family the Curies had to hire a servant to tend to chores. Marie, who remained in charge of her child's care, found in Dr. Curie an ideal babysitter. She could carry out her lab work fully confident that Irène was in excellent hands. Over the years grandfather and granddaughter would forge a very close bond.

“It became a serious problem how to take care of our little Irène and of our home without giving up my scientific work. Such a renunciation would have been very painful to me, and my husband would not even think of it...So the close union of our family enabled me to meet my obligations.”

Work and Family

AS BUSY YOUNG PARENTS the Curies had time, money, and energy for only two commitments, work and family. They maintained warm ties with the family of Pierre's older brother, Jacques, who taught mineralogy at the University of Montpellier. They socialized infrequently, and then only with other scientists who gathered at the Curie home on the rue Kellerman or in its garden -- colleagues and students who shared their liberal views and intellec tual interests. Despite the satisfaction Marie took in her busy and fulfilling life, she missed the Sklodowski family, particularly after Bronya and her husband returned to Poland. (The Dluskis opened a tuberculosis sanatorium in the Carpathians of Austrian Poland.)

“It was under this mode of quiet living, organized according to our desires, that we achieved the great work of our lives, work begun about the end of 1897 and lasting for many years.”

With her household in order and the results of her first research published, it was time for Marie to choose a topic for her doctoral research. Although an unmarried German woman's doctoral research in electrochemistry was at an advanced stage, no woman anywhere in the world had yet been awarded a doctorate in science.

Research Breakthroughs (1897-1904)

X-rays and Uranium Rays

MARIE CURIE'S CHOICE of a thesis topic was influenced by two recent discoveries by other scientists. In December 1895, about six months after the Curies married, German physicist Wilhelm Roentgen discovered a kind of ray that could travel through solid wood or flesh and yield photographs of living people's bones. Roentgen dubbed these mysterious rays X-rays, with X standing for unknown. In recognition of his discovery, Roentgen in 1901 became the first Nobel laureate in physics.

In early 1896, only a few of months after Roentgen's discovery, French physicist Henri Becquerel reported to the French Academy of Sciences that uranium compounds, even if they were kept in the dark, emitted rays that would fog a photographic plate. He had come upon this discovery accidentally. Despite Becquerel's intriguing finding, the scientific community continued to focus its attention on Roentgen's X-rays, neglecting the much weaker Becquerel rays or uranium rays.

THE IGNORED URANIUM RAYS appealed to Marie Curie. Since she would not have a long bibliography of published papers to read, she could begin experimental work on them immediately. The director of the Paris Municipal School of Industrial Physics and Chemistry, where Pierre was professor of physics, permitted her to use a crowded, damp storeroom there as a lab.

A clever technique was her key to success. About 15 years earlier, Pierre and his older brother, Jacques, had invented a new kind of electrometer, a device for measuring extremely low electrical currents. Marie now put the Curie electrometer to use in measuring the faint currents that can pass through air that has been bombarded with uranium rays. The moist air in the storeroom tended to dissipate the electric charge, but she managed to make reproducible measurements.

“Instead of making these bodies act upon photographic plates, I preferred to determine the intensity of their radiation by measuring the conductivity of the air exposed to the action of the rays.”

You can exit this site to an exhibit on the discovery of the electron

With numerous experiments Marie confirmed Becquerel's observations that the electrical effects of uranium rays are constant, regardless of whether the uranium was solid or pulverized, pure or in a compound, wet or dry, or whether exposed to light or heat. Likewise, her study of the rays emitted by different uranium compounds validated Becquerel's conclusion that the minerals with a higher proportion of uranium emitted the most intense rays. She went beyond Becquerel's work, however, in forming a crucial hypothesis: the emission of rays by uranium compounds could be an atomic property of the element uranium--something built into the very structure of its atoms.

MARIE'S SIMPLE HYPOTHESIS would prove revolutionary. It would ultimately contribute to a fundamental shift in scientific understanding. At the time scientists regarded the atom--a word meaning undivided or indivisible -- as the most elementary particle. A hint that this ancient idea was false came from the discovery of the electron by other scientists around this same time. But nobody grasped the complex inner structure or the immense energy stored in atoms. Marie and Pierre Curie themselves were not convinced that radioactive energy came from within atoms--maybe, for example, the earth was bathed in cosmic rays, whose energy certain atoms somehow caught and radiated? Marie's real achievement was to cut through the complicated and obscure observations with a crystal-clear analysis of the set of conclusions that, however unexpected, were logically possible.

Marie tested all the known elements in order to determine if other elements or minerals would make air conduct electricity better, or if uranium alone could do this. In this task she was assisted by a number of chemists who donated a variety of mineral samples, including some containing very rare elements. In April 1898 her research revealed that thorium compounds, like those of uranium, emit Becquerel rays. Again the emission appeared to be an atomic property. To describe the behavior of uranium and thorium she invented the word “radioactivity” --based on the Latin word for ray.

Radium and Radioactivity
By Mme. Sklodowska Curie, Discoverer of Radium
from Century Magazine (January 1904), pp. 461-466

The discovery of the phenomena of radioactivity adds a new group to the great number of invisible radiations now known, and once more we are forced to recognize how limited is our direct perception of the world which surrounds us, and how numerous and varied may be the phenomena which we pass without a suspicion of their existence until the day when a fortunate hazard reveals them.

The radiations longest known to us are those capable of acting directly upon our senses; such are the rays of sound and light. But it has also long been recognized that, besides light itself, warm bodies emit rays in every respect analogous to luminous rays, though they do not possess the power of directly impressing our retina. Among such radiations, some, the infra-red, announce themselves to us by producing a measurable rise of temperature in the bodies which receive them, while others, the ultra-violet, act with specially great intensity upon photographic plates. We have here a first example of rays only indirectly accessible to us.

Yet further surprises in this domain of invisible radiations were reserved for us. The researches of two great physicists, Maxwell and Hertz, showed that electric and magnetic effects are propagated in the same manner as light, and that there exist “electromagnetic radiations,” similar to luminous radiations, which are to the infra-red rays what these latter are to light. These are the electromagnetic radiations which are used for the transmission of messages in wireless telegraphy. They are present in the space around us whenever an electric phenomenon is produced, especially a lightning discharge. Their presence may be established by the use of special apparatus, and here again the testimony of our senses appears only in an indirect manner. If we consider these radiations in their entirety - the ultra-violet, the luminous, the infra-red, and the electromagnetic - we find that the radiations we see constitute but an insignificant fraction of those that exist in space. But it is human nature to believe that the phenomena we know are the only ones that exist, and whenever some chance discovery extends the limits of our knowledge we are filled with amazement. We cannot become accustomed to the idea that we live in a world that is revealed to us only in a restricted portion of its manifestations.

Among recent scientific achievements which have attracted most attention must be placed the discovery of cathode rays, and in even greater measure that of Roentgen rays. These rays are produced in vacuum-tubes when an electric discharge is passed through the rarefied gas. The prevalent opinion among physicists is that cathode rays are formed by extremely small material particles, charged with negative electricity, and thrown off with great velocity from the cathode, or negative electrode, of the tube. When the cathode rays meet the glass wall of the tube they render it vividly fluorescent. These rays can be deflected from their straight path by the action of a magnet. Whenever they encounter a solid obstacle, the emission of Roentgen rays is the result. These latter can traverse the glass and propagate themselves through the outside air. They differ from cathode rays in that they carry no electric charge and are not deflected from their course by the action of a magnet. Everyone knows the effect of Roentgen rays upon photographic plates and upon fluorescent screens, the radiographs obtainable from them, and their application in surgery.

The discovery of Becquerel rays dates from a few years after that of Roentgen rays. At first they were much less noticed. The world, attracted by the sensational discovery of Roentgen rays, was less inclined to astonishment. On all sides a search was instituted by similar processes for new rays, and announcements of phenomena were made that have not always been confirmed. It has been only gradually that the positive existence of a new radiation has been established. The merit of this discovery belongs to M. Becquerel, who succeeded in demonstrating that uranium and its compounds spontaneously emit rays that are able to traverse opaque bodies and to affect photographic plates.

It was at the close of the year 1897 that I began to study the compounds of uranium, the properties of which had greatly attracted my interest. Here was a substance emitting spontaneously and continuously radiations similar to Roentgen rays, whereas ordinarily Roentgen rays can be produced only in a vacuum-tube with the expenditure of energy. By what process can uranium furnish the same rays without expenditure of energy and without undergoing apparent modification? Is uranium the only body whose compounds emit similar rays? Such were the questions I asked myself, and it was while seeking to answer them that I entered into the researches which have led to the discovery of radium.

First of all, I studied the radiation of the compounds of uranium. Instead of making these bodies act upon photographic plates, I preferred to determine the intensity of their radiation by measuring the conductivity of the air exposed to the action of the rays. To make this measurement, one can determine the speed with which the rays discharge an electroscope, and thus obtain data for a comparison. I found in this way that the radiation of uranium is very constant, varying neither with the temperature nor with the illumination. I likewise observed that all the compounds of uranium are active, and that they are more active the greater the proportion of this metal which they contain. Thus I reached the conviction that the emission of rays by the compounds of uranium is a property of the metal itself—that it is an atomic property of the element uranium independent of its chemical or physical state. I then began to investigate the different known chemical elements, to determine whether there exist others, besides uranium, that are endowed with atomic radioactivity—that is to say, all the compounds of which emit Becquerel rays. It was easy for me to procure samples of all the ordinary substances—the common metals and metalloids, oxides and salts. But as I desired to make a very thorough investigation, I had recourse to different chemists, who put at my disposal specimens—in some cases the only ones in existence—containing very rare elements. I thus was enabled to pass in review all the chemical elements and to examine them in the state of one or more of their compounds. I found but one element undoubtedly possessing atomic radioactivity in measurable degree. This element is thorium. All the compounds of thorium are radioactive, and with about the same intensity as the similar compounds of uranium. As to the other substances, they showed no appreciable radioactivity under the conditions of the test.

I likewise examined certain minerals. I found, as I expected, that the minerals of uranium and thorium are radioactive; but to my great astonishment I discovered that some are much more active than the oxides of uranium and of thorium which they contain. Thus a specimen of pitch-blende (oxide of uranium ore) was found to be four times more active than oxide of uranium itself. This observation astonished me greatly. What explanation could there be for it? How could an ore, containing many substances which I had proved inactive, be more active than the active substances of which it was formed? The answer came to me immediately: The ore must contain a substance more radioactive than uranium and thorium, and this substance must necessarily be a chemical element as yet unknown; moreover, it can exist in the pitch-blende only in small quantities, else it would not have escaped the many analyses of this ore; but, on the other hand, it must possess intense radioactivity, since, although present in small amount, it produces such remarkable effects. I tried to verify my hypothesis by treating pitch-blende by the ordinary processes of chemical analysis, thinking it probable that the new substance would be concentrated in passing through certain stages of the process. I performed several experiments of this nature, and found that the ore could in fact be separated into portions some of which were much more radioactive than others.

To try to isolate the supposed new element was a great temptation. I did not know whether this undertaking would be difficult. Of the new element I knew nothing except that it was radioactive. What were its chemical properties? In what quantity did it appear in pitch-blende? I began the analysis of pitch-blende by separating it into its constituent elements, which are very numerous. This task I undertook in conjunction with M. Curie. We expected that perhaps a few weeks would suffice to solve the problem. We did not suspect that we had begun a work which was to occupy years and which was brought to a successful issue only after considerable expenditure.

We readily proved that pitch-blende contains very radioactive substances, and that there were at least three. That which accompanies the bismuth extracted from pitch-blende we named Polonium; that which accompanies barium from the same source we named Radium; finally, M. Debierne gave the name of Actinium to a substance which is found in the rare earths obtained from the same ore.

Radium was to us from the beginning of our work a source of much satisfaction. Demarçay, who examined the spectrum of our radioactive barium, found in it new rays and confirmed us in our belief that we had indeed discovered a new element.

The question now was to separate the polonium from the bismuth, the radium from the barium. This is the task that has occupied us for years, and as yet we have succeeded only in the case of radium. The research has been a most difficult one. We found that by crystallizing out the chloride of radioactive barium from a solution we obtained crystals that were more radioactive, and consequently richer in radium, than the chloride that remained dissolved. It was only necessary to make repeated crystallizations to obtain finally a pure chloride of radium.

But although we treated as much as fifty kilograms of primary substance, and crystallized the chloride of radiferous barium thus obtained until the activity was concentrated in a few minute crystals, these crystals still contained chiefly chloride of barium; as yet radium was present only in traces, and we saw that we could not finish our experiments with the means at hand in our laboratory. At the same time the desire to succeed grew stronger; for it became evident that radium must possess most intense radioactivity, and that the isolation of this body was therefore an object of the highest interest.

Fortunately for us, the curious properties of these radium-bearing compounds had already attracted general attention and we were assisted in our search.

A chemical factory in Paris consented to undertake the extraction of radium on a large scale. We also received certain pecuniary assistance, which allowed us to treat a large quantity of ore. The most important of these grants was one of twenty thousand francs, for which we are indebted to the Institute of France.

We were thus enabled to treat successively about seven tons of a primary substance which was the residue of pitch-blende after the extraction of uranium. Today we know that a ton of this residue contains from two to three decigrams (from four to seven ten-thousandths of a pound) of radium. During this treatment, and as soon as I had in my possession a decigram of chloride of radium recognized as pure by the spectroscope, I determined the atomic weight of this new element, finding it to be 225, while that of barium is 137.

The properties of radium are extremely curious. This body emits with great intensity all of the different rays that are produced in a vacuum-tube. The radiation, measured by means of an electroscope, is at least a million times more powerful than that from an equal quantity of uranium. A charged electroscope placed at a distance of several meters can be discharged by a few centigrams of a radium salt. One can also discharge an electroscope through a screen of glass or lead five or six centimeters thick. Photographic plates placed in the vicinity of radium are also instantly affected if no screen intercepts the rays; with screens, the action is slower, but it still takes place through very thick ones if the exposure is sufficiently long. Radium can therefore be used in the production of radiographs.

The compounds of radium are spontaneously luminous. The chloride and bromide, freshly prepared and free from water, emit a light which resembles that of a glow-worm. This light diminishes rapidly in moist air; if the salt is in a sealed tube, it diminishes slowly by reason of the transformation of the white salt, which becomes colored, but the light never completely disappears. By redissolving the salt and drying it anew, its original luminosity is restored.

A glass vessel containing radium spontaneously charges itself with electricity. If the glass has a weak spot,—for example, if it is scratched by a file,—an electric spark is produced at that point, the vessel crumbles like a Leiden jar when overcharged, and the electric shock of the rupture is felt by the fingers holding the glass.

Radium possesses the remarkable property of liberating heat spontaneously and continuously. A solid salt of radium develops a quantity of heat such that for each gram of radium contained in the salt there is an emission of one hundred calories per hour. Expressed differently, radium can melt in an hour its weight in ice. When we reflect that radium acts in this manner continuously, we are amazed at the amount of heat produced, for it can be explained by no known chemical reaction.The radium remains apparently unchanged. If, then, we assume that it undergoes a transformation, we must therefore conclude that the change is extremely slow; in an hour it is impossible to detect a change by any known methods.

As a result of its emission of heat, radium always possesses a higher temperature than its surroundings. This fact may be established by means of a thermometer, if care is taken to prevent the radium from losing heat.

Radium has the power of communicating its radioactivity to surrounding bodies. This is a property possessed by solutions of radium salts even more than by the solid salts. When a solution of a radium salt is placed in a closed vessel, the radioactivity in part leaves the solution and distributes itself through the vessel, the walls of which become radioactive and luminous. The radiation is therefore in part exteriorized. We may assume, with Mr. Rutherford, that radium emits a radioactive gas and that this spreads through the surrounding air and over the surface of neighboring objects. This gas has received the name emanation. It differs from ordinary gas in the fact that it gradually disappears. [The modern name for this element is radon.]

Certain bodies—bismuth, for instance—may also be rendered active by keeping them in solution with the salts of radium. These bodies then become atomically active, and keep this radioactivity even after chemical transformations. Little by little, however, they lose it, while the activity of radium persists.

The nature of radium radiations is very complex. They may be divided into three distinct groups, according to their properties. One group is composed of radiations absolutely analogous to cathode rays, composed of material particles called electrons, much smaller than atoms, negatively charged, and projected from the radium with great velocity—a velocity which for some of these rays is very little inferior to that of light.

The second group is composed of radiations which are believed to be formed by material particles the mass of which is comparable to that of atoms, charged with positive electricity, and set in motion by radium with a great velocity, but one that is inferior to that of the electrons. Being larger than electrons and possessing at the same time a smaller velocity, these particles have more difficulty in traversing obstacles and form rays that are less penetrating.

Finally, the radiations of the third group are analogous to Roentgen rays and do not behave like projectiles.

The radiations of the first group are easily deflected by a magnet; those of the second group, less easily and in the opposite direction; those of the third group are not deflected. From its power of emitting these three kinds of rays, radium may be likened to a complete little Crookes tube acting spontaneously.

Radium is a body which gives out energy continuously and spontaneously. This liberation of energy is manifested in the different effects of its radiation and emanation, and especially in the development of heat. Now, according to the most fundamental principles of modern science, the universe contains a certain definite provision of energy, which can appear under various forms, but cannot be increased.

Without renouncing this conception, we cannot believe that radium creates the energy which it emits; but it can either absorb energy continuously from without, or possess in itself a reserve of energy sufficient to act during a period of years without visible modification. The first theory we may develop by supposing that space is traversed by radiations that are as yet unknown to us, and that radium is able to absorb these radiations and transform their energy into the energy of radioactivity. Thus in a vacuum-tube the electric energy is utilized to produce cathode rays, and the energy of the latter is partly transformed, by the bodies which absorb them into the energy of Roentgen rays. It is true that we have no proof of the existence of radiations which produce radioactivity; but, as indicated at the beginning of this article, there is nothing improbable in supposing that such radiations exist about us without our suspecting it.

If we assume that radium contains a supply of energy which it gives out little by little, we are led to believe that this body does not remain unchanged, as it appears to, but that it undergoes an extremely slow change. Several reasons speak in favor of this view. First, the emission of heat, which makes it seem probable that a chemical reaction is taking place in the radium. But this can be no ordinary chemical reaction, affecting the combination of atoms in the molecule. No chemical reaction can explain the emission of heat due to radium. Furthermore, radioactivity is a property of the atom of radium; if, then, it is due to a transformation this transformation must take place in the atom itself. Consequently, from this point of view, the atom of radium would be in a process of evolution, and we should be forced to abandon the theory of the invariability of atoms, which is at the foundation of modern chemistry.

Moreover, we have seen that radium acts as though it shot out into space a shower of projectiles, some of which have the dimensions of atoms, while others can only be very small fractions of atoms. If this image corresponds to a reality, it follows necessarily that the atom of radium breaks up into subatoms of different sizes, unless these projectiles come from the atoms of the surrounding gas, disintegrated by the action of radium; but this view would likewise lead us to believe that the stability of atoms is not absolute.

Radium emits continuously a radioactive emanation which, from many points of view, possesses the properties of a gas. Mr. Rutherford considers the emanation as one of the results of the disintegration of the atom of radium, and believes it to be an unstable gas which is itself slowly decomposed.

Professor Ramsay has announced that radium emits helium gas continuously. If this very important fact is confirmed, it will show that a transformation is occurring either in the atom of radium or in the neighboring atoms, and a proof will exist that the transmutation of the elements is possible. [In fact radium does emit helium, as alpha particles.]

When a body that has remained in solution with radium becomes radioactive, the chemical properties of this body are modified, and here again it seems as though we have to deal with a modification of the atom. It would be very interesting to see whether, by thus giving radioactivity to bodies, we can succeed in causing an appreciable change in their atoms. We should thus have a means of producing certain transformations of elements at will. [These observations were misleading. True artificial radioactivity was not produced until the work of Irène and Frédéric Joliot-Curie in 1934.]

It is seen that the study of the properties of radium is of great interest. This is true also of the other strongly radioactive substances, polonium and actinium, which are less known because their preparation is still more difficult. All are found in the ores of uranium and thorium, and this fact is certainly not the result of chance, but must have some connection with the manner of formation of these elements. Polonium, when it has just been extracted from pitch-blende, is as active as radium, but its radioactivity slowly disappears; actinium has a persistent activity. These two bodies differ from radium in many ways; their study should therefore be fertile in new results. Actinium lends itself readily to the study of the emanation and of the radioactivity produced in inactive bodies, since it gives out emanation in great quantity. It would also be interesting, from the chemical point of view, to prove that polonium and actinium contain new elements. Finally, one might seek out still other strongly radioactive substances and study them.

But all these investigations are exceedingly difficult because of the obstacles encountered in the preparation of strongly radioactive substances. At the present time we possess only about a gram of pure salts of radium. Research in all branches of experimental science—physics, chemistry, physiology, medicine—is impeded, and a whole evolution in science is retarded, by the lack of this precious and unique material, which can now be obtained only at great expense. We must now look to individual initiative to come to the aid of science, as it has so often done in the past, and to facilitate and expedite by generous gifts the success of researches the influence of which may be far-reaching.

Radioactivity: The Unstable Nucleus and its Uses

WHEN THE FRENCH PHYSICIST Henri Becquerel (1852-1908) discovered “his” uranium rays in 1896 and when Marie Curie began to study them, one of the givens of physical science was that the atom was indivisible and unchangeable. The work of Becquerel and Curie soon led other scientists to suspect that this theory of the atom was untenable.

Scientists soon learned that some of the mysterious “rays” emanating from radioactive substances were not rays at all, but tiny particles. Radioactive atoms emit three different kinds of radiation. One kind of radiation is a particle of matter, called the alpha particle. It has a positive electric charge and about four times the mass of a hydrogen atom. (We now know that it consists of two protons and two neutrons, the same as the nucleus of the helium atom.) Alpha particles exit radioactive atoms with high energies, but they lose this energy as they move though matter. An alpha particle can pass through a thin sheet of aluminum foil, but it is stopped by anything thicker. Beta “rays,” a second form of radiation, turned out to be electrons, very light particles with a negative electric charge. The beta particles travel at nearly the speed of light and can make their way through half a centimeter of aluminum. Gamma rays, a third type of radiation, are true rays, electromagnetic waves--the same kind of thing as radio waves and light, with no mass and no electrical charge. They are similar to, but more energetic than, the X-rays, an energetic form of electromagnetic radiation discovered by the German physicist Wilhelm Conrad Roentgen (1845-1923) in 1895. Gamma rays emitted by radioactive atoms can penetrate deeper into matter than alpha or beta particles. A small fraction of gamma rays can pass through even a meter of concrete.

The point was that radioactivity was no more nor less than the emission of tiny particles and energetic waves from the atom. Building on the research of Marie Curie and others, scientists soon realized that if atoms emitted such things they could not be indivisible and unchangeable. Atoms are made up of smaller particles, and these can be rearranged.

It began with a vexing puzzle--in any laboratory where people worked with radium or other radioactive minerals, radioactivity tended to spread around, turning up in unexpected corners. In fact the labs were being contaminated by a radioactive gas. In 1900 Ernest Rutherford (1871-1937) found that the radioactivity of the “emanation” (as he called it) from thorium diminished with time. This decay of radioactivity was a vital clue.

Rutherford, working in Canada with the chemist Frederick Soddy (1877-1956), developed a revolutionary hypothesis to explain the process. They realized that radioactive elements can spontaneously change into other elements. As they do so, they emit radiation of one type or another. The spontaneous decay process continues in a chain of emissions until a stable atom is formed. It was, as Rutherford and Soddy boasted, the transmutation of elements that had eluded alchemists for thousands of years. They recognized at once that the ceaseless emissions pointed to a vast store of energy within atoms--energy that might someday be released for useful power or terrible weapons, however people chose.

TO UNDERSTAND WHAT HAPPENS when radioactive atoms emit radiation, scientists had to understand how the atom is built. As Rutherford first explained in 1911, each atom is made of a small, massive nucleus, surrounded by a swarm of light electrons. It is from the nucleus that the radioactivity, the alpha or beta or gamma rays, shoot out. By around 1932 Rutherford's colleagues had found that the nucleus is built of smaller particles, the positively charged protons and the electrically neutral neutrons. A proton or a neutron each has about the mass of one hydrogen atom. All atoms of a given element have a given number of protons in their nuclei, called the atomic number. To balance this charge they have an equal number of electrons swarming around the nucleus. It is these shells of electrons that give the element its chemical properties.

However, it turned out that atoms of a given element can have different numbers of neutrons, and thus different atomic mass. Soddy named the forms of an element with different atomic masses the isotopes of the element. For example, the lightest element, hydrogen, has the atomic number 1. Its nucleus normally is made of one proton and no neutrons, and thus its atomic mass is also 1. But hydrogen has isotopes with different atomic masses. "Heavy" hydrogen, called deuterium, has one proton and one neutron in its nucleus, and thus its atomic mass is 2. Hydrogen also has a radioactive isotope, tritium. Tritium has one proton and two neutrons, and thus its atomic mass is 3. The three forms of hydrogen each have one electron, and thus the same chemical properties.

When a radioactive nucleus gives off alpha or beta particles, it is in the process of changing into a different nucleus--a different element, or a different isotope of the same element. For example, radioactive thorium is formed when uranium-238--an isotope of uranium with 92 protons and 146 neutrons--emits an alpha particle. Since the alpha particle consists of two protons and two neutrons, when these are subtracted what is left is a nucleus with 90 protons and 144 neutrons. Thorium is the element of atomic number 90, and this isotope of thorium has an atomic mass of 234. The results of decay may themselves be unstable, as is the case with thorium-234. The chain of decays continues until a stable nucleus forms, in this case the element lead.

Rutherford and Soddy discovered that every radioactive isotope has a specific half-life. Half the nuclei in a given quantity of a radioactive isotope will decay in a specific period of time. The half-life of uranium-238 is 4.5 billion years, which means that over that immense period of time half the nuclei in a sample of uranium-238 will decay (in the next 4.5 billion years, half of what is left will decay, leaving one quarter of the original, and so forth). The isotopes produced by the decay of uranium themselves promptly decay in a long chain of radiations. Radium and polonium are links in this chain.

Radium caught Marie Curie's attention because its half-life is 1600 years. That's long enough so that there was a fair amount of radium mixed with uranium in her pitchblende. And it was short enough so that its radioactivity was quite intense. A long-lived isotope like uranium-238 emits radiation so slowly that its radioactivity is scarcely noticeable. By contrast, the half-life of the longest-lived polonium isotope, polonium-210, is only 138 days. This short half-life helps explain why Marie Curie was unable to isolate polonium. Even as she performed her meticulous fractional crystallizations, the polonium in her raw material was disappearing as a result of its rapid radioactive decay.

Uses of Radioactivity

THE EARLY WORK OF MARIE AND PIERRE CURIE led almost immediately to the use of radioactive materials in medicine. In many circumstances isotopes are more effective and safer than surgery or chemicals for attacking cancers and certain other diseases. Over the years, many other uses have been found for radioactivity. Until electrical particle accelerators were invented in the 1930s, scientists used radiation from isotopes to bombard atoms, uncovering many of the secrets of atomic structure. To this day radioactive isotopes, used as "tracers" to track chemical changes and the processes of life, are an almost indispensable tool for biologists and physiologists. Isotopes are crucial even for geology and archeology. As soon as he understood radioactive decay, Pierre Curie realized that it could be used to date materials. Soon the age of the earth was established by uranium decay at several billion years, far more than scientists had supposed. Since the 1950s radioactive carbon has been used to pin down the age of plant and animal remains, for example in ancient burials back to 50,000 years ago.

The Radium Institute (1919-1934)

The Marie Curie Radium Campaign

SHE NEVER OVERCAME STAGE FRIGHT as a professor, though she taught for nearly 30 years. Yet in order to turn the Radium Institute into a world-class institution, Curie shamelessly sought out assistance, just as she had done during the war years to create the radiological service. Throughout her career Curie had benefitted from the subsidies of wealthy French benefactors. Now, thanks to the interest of an American woman, U.S. citizens also became involved in filling the needs of the Radium Institute.

“[Curie], who handles daily a particle of radium more dangerous than lightning, was afraid when confronted by the necessity of appearing before the public.”--Stéphane Lauzanne, editor-in-chief of Le Matin

Despite her distrust of journalists, in May 1920 Curie agreed to give an interview to Mrs. William Brown Meloney, editor of an American women's magazine. In the interview Curie emphasized the needs of her institution, where research was just resuming following the devastating war.

Thanks to her alliance with industry, few labs in the world if any were better equipped with radium than Curie's. But Curie succeeded in shocking Meloney by emphasizing the fact that research and therapy centers in the United States together had about 50 times as much radium as the single gram she--the scientist who had discovered the element--had in her laboratory. When Meloney learned that Curie's most fervent wish was for a second gram for her laboratory, the editor organized a “Marie Curie Radium Campaign.”

Led by a committee of wealthy American women and distinguished American scientists, the campaign succeeded by soliciting contributions in the United States. Meloney also arranged for Curie to write an autobiographical work for an American publisher. The book would provide royalty income over the years. Equally important, it would capture in simple and moving prose the romantic and heroic image of science that was so helpful for public support and fund-raising.

THE LANGEVIN AFFAIR could not be mentioned in print. On this condition Curie agreed to travel to the United States to drum up support for her institute. Meloney wrested from editors across the country a promise to suppress the old story. When word got out that the President of the United States himself would present Curie with the gift of radium, French officials looked for a way to make up for past oversights. Curie refused the Legion of Honor award (as Pierre had refused it nearly two decades earlier). But she agreed to attend a benefit for the Radium Institute at the Paris Opera shortly before setting sail.

“I pray to thank the Minister, and to inform him that I do not in the least feel the need of a decoration, but that I do feel the greatest need for a laboratory.” --Pierre Curie refusing the Legion of Honor, 1903

Her right hand was in a sling before she had been in the United States many days. So many people wanted to shake hands with the woman who had given humanity the gift of radium. Curie was grateful that her daughters were willing to stand in for her when she felt she could not bear another public function. Irène, for example, accepted some of the many honorary degrees granted to her mother by universities and colleges.

In 1920 Curie and a number of her colleagues created the Curie Foundation, whose mission was to provide both the scientific and the medical divisions of the Radium Institute with adequate resources. Over the next two decades the Curie Foundation became a major international force in the treatment of cancer.

CAMPAIGNS TO RAISE MONEY from governments as well as from individuals, were launched throughout the 1920s in many countries including France itself. Marie's scientist friends were especially active. Insisting that the quickest way to a progressive future was to foster research, they formed partnerships with liberal and socialist politicians, and they supported political parties that would increase government funding. Despite her shyness Marie helped in the work of lobbying, going with her friends from office to office. She could argue fervently, but her appearance alone was the strongest argument. A frail and aging woman dressed in black, already a legendary figure, she had--as one observer put it--the appearance and moral force of a Buddhist monk.

- American Institute of Physics