Curie, Marie . Pierre Curie / By Marie Curie
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   IN spite of our desire to concentrate our entire effort on the work in which we were engaged, and in spite of the fact that our needs were so modest, we were forced to recognize, toward 1900, that some increase in our income was indispensable. Pierre Curie had few illusions about his chances of obtaining an important chair in the University of Paris, which would, even though it meant no large salary, have sufficed for the small needs of our family, and enabled us to live without a supplementary revenue. Since he was neither a graduate of the Normal School nor of the Polytechnic, he lacked the support, often decisive, which these big schools give their pupils; and the posts to which he might justly have aspired, because of his achievements, were given to others, without anyone's even thinking of him as a possible candidate. At the beginning of 1898, he asked,



without success, for the Chair of Physical Chemistry left vacant by the death of Salet, and this failure convinced him that he had no chance of advancement. He was appointed, however, in March, 1900, to the position of assistant professor (répétiteur) in the Polytechnic School, but he kept his post only six months.

    In the spring of 1900, there came an unexpected offer, that of the Chair of Physics in the University of Geneva. The doyen of that University made the invitation in the most cordial manner, and insisted that the University was ready to make an exceptional effort to secure a scientist of such high repute. The advantages of this position were that the salary was larger than the average one, that it carried the promise of the development of a Physics Laboratory adequate to our needs, and that an official position for me would be provided in this laboratory. Such a proposition merited a most careful consideration, so we made a visit to the University of Geneva, where our reception was the most encouraging possible.

    This was a grave decision for us to make. Geneva presented material advantages, and the opportunity of a life comparable in its tranquillity with that in the country. Pierre Curie was, therefore, tempted to accept, and it was


only our immediate interest in our researches in radium that made him finally decide not to. He feared the interruption of our investigations which such a change must involve.

    At this moment the Chair of Physics in the physics, chemistry and natural history course at the Sorbonne, obligatory for students of medicine, and familiarly known as P.C.N., was vacant; he applied, and was appointed, due to the influence of Henri Poincaré, who was anxious to free him from the necessity of quitting France. At the same time I was given charge of the physics lectures in the Normal School for Girls at Sèvres.

    So we remained in Paris, and with our income increased. But we were at the same time working under increasingly difficult conditions. Pierre Curie was doing double teaching; and that in the P.C.N., with its very large number of students, fatigued him greatly. As for myself, I had to give much time to the preparation of my lectures at Sèvres, and to the organization of the laboratory work there, which I found very insufficient.

    Moreover, Pierre Curie's new position did not bring with it a laboratory; a little office and a single work room were all that he had at his disposition in the annex (12 rue Cuvier) of the


Sorbonne, which served as teaching quarters for the P.C.N. And yet he felt it absolutely necessary to go ahead with his own work. In fact, the rapid extension of his investigations in radioactivity had made him determine that in his new position at the Sorbonne he would receive students and start them in research. He therefore took steps to find larger available working quarters. Those who have taken similar steps realize the wall of financial and administrative obstacles against which he was throwing himself, and realize the large number of official letters, visits, and of requests the least success entailed. All this thoroughly wearied and discouraged Pierre Curie. He was obliged, too, constantly, to keep traveling back and forth between the laboratories of the P.C.N. and the hangar of the School of Physics where we still continued our work.

    And besides these difficulties, we found that we could not make further progress without the aid of industrial means of treating our raw material. Fortunately certain expedients and generous assistance solved this question.

    As early as 1899 Pierre Curie succeeded in organizing a first industrial experiment, using for it a chance installation placed at his disposition by the Central Society of Chemical Products, with which he had had relations in connection


with the construction of his balances. The technical details had been arranged very successfully by André Debierne, and the operations brought good results, even though it had been necessary to train a special personnel for this chemical work which demanded special precautions.

    Our investigations had started a general scientific movement, and similar work was being undertaken in other countries. Toward these efforts Pierre Curie maintained a most disinterested and liberal attitude. With my agreement he refused to draw any material profit from our discovery. We took no copyright, and published without reserve all the results of our research, as well as the exact processes of the preparation of radium. In addition, we gave to those interested whatever information they asked of us. This was of great benefit to the radium industry, which could thus develop in full freedom, first in France, then in foreign countries, and furnish to scientists and to physicians the products which they needed. This industry still employs to-day, with scarcely any modifications, the processes indicated by us.9


    Even though our industrial experiment yielded good results, again our slender resources made it difficult to make further progress. Inspired by our attempt, a French industrial, Armet de Lisle, had the idea, which seemed daring at that epoch, of founding a veritable radium factory that would furnish this product to physicians, whose interest in the biological effects of radium and its possible therapeutic applications had been aroused by the publication of various investigations. The project proved a success because he could employ men already trained by us in the delicate processes of this manufacture. Radium was then regularly placed on sale, at a high price, it is true, because of the special conditions under which it had to be made, and because, too, of the immediate rise in the cost of the minerals necessary to its production.10

    I should like to express, here, our appreciation of the spirit in which Armet de Lisle offered to coöperate with us. In an entirely disinterested


manner he placed at our disposition a little working place in his factory and a part of the means necessary for us to use it. Other funds were added either by ourselves, or came through subventions, of which the most important, accorded in 1902 by the Academy of Sciences, amounted to 20,000 francs.

    It was in this way that we were able to utilize the ore we had acquired little by little in the preparation of a certain quantity of radium, which we used constantly in our research. The radium-bearing barium was extracted in the factory, and I carried on its purification and fractional crystallization in the laboratory. In 1902 I succeeded in preparing a decigramme of chloride of pure radium which gave only the spectrum of the new element, radium. I made a first determination of the atomic weight of this new element, an atomic weight much higher than that of barium. Thus the chemical individuality of radium was completely established, and the reality of radioelements was a known fact about which there could be no further controversy.

    I based my doctor's thesis, presented in 1903, on these investigations.

    Later, the quantity of radium extracted for the laboratory was increased, and in 1907 I


was able to make a second and more precise determination of the atomic weight as 225.35 -- one accepts now the number 226. I succeeded, too, jointly with André Debierne, in obtaining radium in the state of metal. The total quantity of radium I prepared and gave to the laboratory, in agreement with Pierre Curie's desire, amounted to more than a gramme of radium element.

    The activity of pure radium exceeded all our expectations. For equal weights this substance emits a radiation more than a million times more intense than uranium. To offset this, the quantity of radium contained in uranium minerals is scarcely more than three decigrammes of radium to the ton of uranium. There is a very close relation between these two substances. In fact, we know to-day that radium is produced in the minerals at the expense of uranium.

    The years that followed his nomination to the P.C.N. were hard for Pierre Curie. He had to face the many anxieties incident to the organization of a complicated system of work when his happiness depended on his being able to concentrate his efforts on a single determined subject. The physical fatigue due to the numerous courses he was obliged to give was so great that he suffered from attacks of acute pain, which in


his overtaxed condition became more and more frequent.

    It was therefore vitally important, if he was to spare his energy and keep his health, that the burden of his professional duties be lightened. He decided to apply for the Chair of Mineralogy, which was vacant, at the Sorbonne, for which he was entirely qualified because of his profound knowledge and his important publications on the theories of the physics of crystals. Yet his candidacy failed.

    During this painful period he nevertheless managed, by a truly superhuman effort, successfully to complete and publish several investigations that he had made either alone or in collaboration:

Investigations on induced radioactivity (in collaboration with A. Debierne).

Investigations on the same subject (in collaboration with J. Danne).

Investigations on the conductibility provoked in dielectric liquids by the rays of radium and the Roentgen rays.

Investigations on the law of the decrease of the emanation of radium and on the radioactive constants that characterize this emanation and its active deposit.

Discovery of the liberation of heat produced by radium (in collaboration with A. Laborde).


Investigations on the diffusion of the emanation of radium in the air (in collaboration with J. Danne).

Investigation on the radioactivity of gases from thermal springs (in collaboration with A. Laborde).

Investigation on the physiological effects of radium rays (in common with Henri Becquerel).

Investigation on the physiological action of the radium emanation (in common with Bouchard and Balthazard).

Notes on the apparatus for the determination of magnetic constants (in common with C. Cheneveau).

    All these investigations in radioactivity are fundamental and touch very varied subjects. Several have as their aim the study of the emanation, that strange gaseous body that radium produces and which is largely responsible for the intense radiation commonly attributed to the radium itself. Pierre Curie demonstrated by a searching examination the rigorous and invariable law according to which the emanation destroys itself, no matter what the conditions are in which it finds itself. To-day the emanation of radium, harvested in tiny phials, is commonly employed by physicians as a therapeutic agent. Technical considerations


make its employment preferable to the direct use of radium, and in this case no physician can proceed without consulting the numerical chart which tells how much of this emanation has disappeared each day, despite the fact that it is cloistered in its little glass prison. It is this same emanation that is found in small quantities in mineral waters, and that plays a part in their curative effects.

    More striking still was the discovery of the discharge of heat from radium. Without any alteration in appearance this substance releases each hour a quantity of heat sufficient to melt its own weight of ice. When well protected against this external loss, radium heats itself. Its temperature can rise 10 degrees or more above that of the surrounding atmosphere. This defied all contemporary scientific experience.

    Finally, I cannot pass in silence, because of their various repercussions, the experiments connected with the physiological effects of radium.

    In order to test the results that had just been announced by F. Giesel, Pierre Curie, voluntarily exposed his arm to the action of radium during several hours. This resulted in a lesion resembling a burn, that developed progressively and required several months to heal. Henri


Becquerel had by accident a similar burn as a result of carrying in his vest pocket a glass tube containing radium salt. He came to tell us of this evil effect of radium, exclaiming in a manner at once delighted and annoyed: "I love it, but I owe it a grudge!"

    Since he realized the interest in these physiological effects of radium, Pierre Curie undertook, in collaboration with physicians, the investigations to which I have just referred, submitting animals to the action of radium emanation. These studies formed the point of departure in radium therapy. The first attempts at treatment with radium were made with products loaned by Pierre Curie, and had as their object the cure of lupus and other skin lesions. Thus radium therapy, an important branch of medicine, and frequently designated as Curie-therapie, was born in France, and was developed first through the investigations of French physicians (Danlos, Oudin, Wickham, Dominici, Cheron, Degrais, and others).11


    In the meantime the great impetus given to the study of radioactivity abroad led to a rapid succession of new discoveries. Many scientists engaged in the search for other radio elements, using the new method of chemical analysis, with the aid of radiation, that we had inaugurated. Thus were found the mesothorium now used by physicians and manufactured industrially, radio-thorium, ionium, protoactinium, radio-lead, and other substances. At present we know, in all, about thirty radio elements (among which three are gases, or emanations), but among them all radium still plays the most important part, because of the great intensity of its radiation, which diminishes only extremely slowly during the course of years.

    The year 1903 was especially important in the development of the new science. In this year the investigation of radium, the new chemical element, was achieved, and Pierre Curie demonstrated the astonishing discharge of heat by this element, which nevertheless remained unaltered in appearance. In England, Ramsay and Soddy announced a great discovery. They proved that radium continually produces helium gas and


under conditions that force one to believe in an atomic transformation. If, indeed, radium salt heated to its melting point is confined for some time in a sealed glass tube, entirely emptied of air, one can, in reheating it, make it throw off a small quantity of helium, easy to measure and to recognize from the character of its spectrum. This fundamental experiment has received numerous confirmations. It furnished us the first example of a transformation of atoms, independent, it is true, of our will, but at the same time it reduces to nothing the theory of the absolute fixity of the atomic edifice.

    All these facts, along with others formerly known, were made the object of a synthesis of the highest value, in a work by E. Rutherford and F. Soddy, who proposed a theory of radioactive transformations, to-day universally adopted. According to this theory, each radio element, even when it appears unchanged, is undergoing a spontaneous transformation, and the more rapid the transformation, the more intense is the radiation.12

    A radioactive atom can transform itself in two ways: it can expel from itself an atom of helium,


which, thrown off at an enormous speed and with a positive charge, constitutes an Alpha ray. Or, instead, it can detach from its structure a much smaller fragment, one of those electrons to which we have become accustomed in modern physics, and whose mass, 1800 times smaller than that of an atom of hydrogen when its speed is moderate, grows excessively when its speed approaches that of light. These electrons, which carry a negative charge, form the Beta rays. Whatever the detached fragment, the residual atom no longer resembles the primitive atom. Thus when the atom of radium has expelled an atom of helium, the residue is an atom of gnseous emanation. This residue changes in its turn, and the process is not arrested until the attainment of a last residue which is stable and does not give off any radiation. This stable matter is inactive matter.

    Thus the Alpha and Beta rays result from the fragmentation of atoms. Gamma-rays are a radiation analogous to light, which accompanies the cataclysm of the atomic transformation. They are very penetrating, and are the ones most used in the therapeutic methods so far developed.13


    We can see in all this that radio elements form families, in which each member derives from a preceding member by direct descent the primary elements being uranium and thorium. We can in particular prove that radium is a descendant of uranium, and that polonium is a descendant of radium. Since each radio element, at the same time that it is formed by the mother substance, destroys itself, it cannot accumulate in the presence of this mother substance beyond a determined proportion, which explains why the relation between radium and uranium remains constant in the very ancient unaltered minerals.

    The spontaneous destruction of radio elements takes place according to a fundamental law, called the exponential law, according to which the quantity of each radio element diminishes by one-half in a time always the same, called a period, this time-period making it possible to determine without ambiguity the element under consideration. These periods, which can be measured by diverse methods, vary greatly. The period of uranium is several billions of years; that of radium is about 1600 years; that of its emanation a little less than four days; and there are among the following descendants some whose period is the small


fraction of a second. The exponential law has a profound philosophic bearing; it indicates that the transformation is produced according to the laws of probability. The causes that determine the transformation are a mystery to us, and we do not yet know if they derive from causal conditions outside the atom, or from conditions of internal instability. In many cases, up to the present, no exterior action has shown itself effective in influencing the transformation.

    This rapid succession of discoveries which overthrew familiar scientific conceptions long held in physics and chemistry did not fail to meet, at first, with doubts and incredulity. But the great part of the scientific world received them with enthusiasm. At the same time Pierre Curie's fame grew in France and in foreign countries. Already in 1901 the Academy of Sciences had awarded him the Lacaze prize. In 1902, Mascart, who had many times given him most valuable aid, decided to propose him as a member of the Academy of Sciences. It was not easy for Pierre Curie to agree to this, believing, as he did, that the Academy should elect its members without the necessity of any preliminary solicitation or paying of calls. Nevertheless, because of the friendly insistence of Mascart, and above all because the Physics


Section of the Academy had already declared itself unanimously in his favor, he presented himself. In spite of this, however, he failed of election, and it was only in 1905 that he became a member of the Institute, a membership which did not last even a year. He was also elected to several academies and scientific societies in other countries, and given an honorary doctor's degree by several universities.

    During 1903 we went to London at the invitation of the Royal Institution, before which my husband was to lecture on radium. On this occasion he had a most enthusiastic reception. He was especially happy to see here again Lord Kelvin, who had always expressed an affection for him, and who, despite his advanced age, preserved an interest, perennially young, in science. The illustrious scientist showed, with touching satisfaction, a glass vial containing a grain of radium salt that Pierre Curie had given him. We met here also other celebrated scientists, as Crookes, Ramsay, and J. Dewar In collaboration with the latter, Pierre Curie published investigations on the discharge of heat by radium at very low temperatures, and upon the formation of helium in radium salt.

    A few months later the Davy medal was conferred upon him (and also upon me) by the


Royal Society of London, and at almost the same time, we received, together with Henri Becquerel, the Nobel prize for physics. Our health prevented us from attending the ceremony for the awarding of this prize in December, and it was only in June, 1905, that we were able to go to Stockholm where Pierre Curie gave his Nobel lecture. We were most cordially received and had the felicity of seeing the admirable Swedish nature in its most brilliant aspect.

    The award of the Nobel prize was an important event for us because of the prestige carried by the Nobel foundation, only recently founded (1901). Also, from a financial point of view, the half of the prize represented an important sum. It meant that in the future Pierre Curie could turn over his teaching in the School of Physics to Paul Langevin, one of his former students, and a physicist of great competence, He could also engage a preparator to aid him in his work.

    But at the same time the publicity this very happy event entailed bore very heavily on a man who was neither prepared for it, nor accustomed to it. There followed an avalanche of visits, of letters, of demands for articles and lectures, which meant a constant enervation, fatigue, and loss of time. He was kind and did not like to


refuse a request; but on the other hand, he had to recognize that he could not accede to the solicitations that overwhelmed him without disastrous results to his health, as well as to his peace of mind, and his work. In a letter to Ch. Ed. Guillaume, he said:

"People ask me for articles and lectures, and after a few years are passed, the very persons who make these demands will be astonished to see that we have not accomplished any work."

    And in other letters of the same period, written to E. Gouy, he expressed himself as follows:

"20 March 1902
"As you have seen, fortune favors us at this moment; but these favors of fortune do not come without many worries. We have never been less tranquil than at this moment. There are days when we scarcely have time to breathe. And to think that we dreamed of living in the wild, quite removed from human beings!"
"22 January 1904
"I have wanted to write to you for a long time; excuse me if I have not done so. The cause is the stupid life which I lead at present. You have seen this sudden infatuation for radium, which has resulted for us in all the advantages of a moment of popularity.


We have been pursued by journalists and photographers from all countries of the world; they have gone even so far as to report the conversation between my daughter and her nurse, and to describe the black-and-white cat that lives with us. . . . Further, we have had a great many appeals for money. . . . Finally, the collectors of autographs, snobs, society people, and even at times, scientists, have come to see us -- in our magnificent and tranquil quarters in the laboratory -- and every evening there has been a voluminous correspondence to send off. With such a state of things I feel myself invaded by a kind of stupor. And yet all this turmoil will not perhaps have been in vain, if it results in my getting a chair and a laboratory. To tell the truth, it will be necessary to create the chair, and I shall not have the laboratory at first. I should have preferred the reverse, but Liard wishes to take advantage of the present moment to bring about the creation of a new chair that will later be acquired for the university. They are to establish a chair without a fixed program, which will be something like a course in the Collège de France, and I believe I shall be obliged to change my subject each year, which will be a great trial to me."

   "31 January 1905
". . . I have had to give up going to Sweden. We are, as you see, most irregular in our relations with the Swedish Academy; but, to tell the truth, I can only keep up by avoiding all physical fatigue. And my wife is in the same condition; we can no longer dream of the great work days of times gone by.


   "As to research, I am doing nothing at present. With my course, my students, apparatus to install, and the interminable procession of people who come to disturb me without serious reason, the days pass without my having been able to achieve anything useful at this end."

   "25 July 1905
"We have regretted so much being deprived of your visit this year, but hope to see you in October. If we do not make an effort from time to time, we end by losing touch with our best and most congenial friends, and in keeping company with others for the simple reason that it is easy to meet them.

    "We continue to lead the same life of people who are extremely occupied, without being able to accomplish anything interesting. It is now more than a year since I have been able to engage in any research, and I have no moment to myself. Clearly I have not yet discovered a means to defend ourselves against this frittering away of our time which is nevertheless extremely necessary. Intellectually, it is a question of life or death."

   "7 November 1905
"I begin my course tomorrow but under very bad conditions for the preparation of my experiments. The lecture room is at the Sorbonne, and my laboratory is in the rue Cuvier. Besides, a great number of other courses are given in the same lecture room, and I can use it only one morning for the preparation of my own.


    "I am neither very well, nor very ill; but I am easily fatigued, and I have left but very little capacity for work. My wife, on the contrary, leads a very active life, between her children, the School at Sèvres, and the laboratory. She does not lose a minute, and occupies herself more regularly than I can with the direction of the laboratory in which she passes the greater part of the day."

    To sum up: despite these outside complications, our life, by a common effort of will, remained as simple and as retired as formerly. Toward the close of 1904 our family was increased by the birth of a second daughter. Eve Denise was born in the modest house in Boulevard Kellermann, where we still lived with Doctor Curie, seeing only a few friends.

    As our elder daughter grew up, she began to be a little companion to her father, who took a lively interest in her education and gladly went for walks with her in his free times, especially on his vacation days. He carried on serious conversations with her, replying to all her questions and delighting in the progressive development of her young mind. From their early age, his children enjoyed his tender affection, and he never wearied of trying to understand these little beings, in order to be able to give them the best he had to give.


    With his great success in other countries, the complete appreciation of Pierre Curie in France, however tardily, did at last follow. At forty-five he found himself in the first rank of French scientists and yet, as a teacher, he occupied an inferior position. This abnormal state of affairs aroused public opinion in his favor, and under the influence of this wave of feeling, the director of the Academy of Paris, L. Liard, asked Parliament to create a new professorship in the Sorbonne, and at the beginning of the academic year 1904-05 Pierre Curie was named titular professor of the Faculty of Sciences of Paris. A year later he definitely quitted the School of Physics where his substitute, Paul Langevin, succeeded him.

    This new professorship was not established without a few difficulties. The first project had provided for a new chair, but not for a laboratory. And Pierre Curie felt that he could not accept a situation which involved the risk of losing even the mediocre means of work that he then had, instead of offering better ones. He wrote, therefore, to his chiefs, that he had decided to remain at the P.C.N. His firmness won the day. To the new chair was added a fund for a laboratory and personnel for the new work (a chief of laboratory, a preparator, and a laboratory


boy). The position of chief of laboratory was offered to me, which was a cause of very great satisfaction to my husband.

    It was not without regret that we left the School of Physics, where we had known such happy work days, despite their attendant difficulties. We had become particularly attached to our hangar, which continued to stand, though in a state of increasing decay, for several years, and we went to visit it from time to time. Later it had to be pulled down to make way for a new building for the Physics School, but we have preserved photographs of it. Warned of its approaching destruction by the faithful Petit, I made my last pilgrimage there, alas, alone. On the blackboard there was still the writing of him who had been the soul of the place; the humble refuge for his research was all impregnated with his memory. The cruel reality seemed some bad dream; I almost expected to see the tall figure appear, and to hear the sound of the familiar voice.

    Even though Parliament had voted the creation of a new chair, it did not go so far as to consider the simultaneous founding of a laboratory which was, nevertheless, necessary to the development of the new science of radioactivity. Pierre Curie therefore kept the little workroom


at the P.C.N., and secured as a temporary solution of his difficulty the use of a large room, then not being used by the P.C.N. He arranged, too, to have a little building consisting of two rooms and a study set up in the court.

    One cannot help feeling sorrow in realizing that this was a last concession, and that actually one of the first French scientists never had an adequate laboratory to work in, and this even though his genius had revealed itself as early as his twentieth year. Without doubt if he had lived longer, he would have had the benefit of satisfactory conditions for his work, but he, was still deprived of them at his death at the premature age of forty-eight. Can we fully imagine the regret of an enthusiastic and disinterested worker in a great work, who is retarded in the realization of his dream by the constant lack of means? And can we think without a feeling of profound grief of the waste -- the one irreparable one -- of the nation's greatest asset: the genius, the powers, and the courage of its best children?

    Pierre Curie had always in mind his urgent need for a good laboratory. When, because of his great reputation, his chiefs felt obliged to try to induce him, in 1903, to accept the decoration of the Légion d'Honneur, he declined that distinction,


remaining true to the opinion already referred to in a preceding chapter. And the letter he wrote on this occasion was inspired by the same feeling as that in the one previously quoted, when he wrote to his director to refuse the palmes académiques. I quote an extract:

"I pray you 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."

    After he was named professor at the Sorbonne, Pierre Curie had to prepare a new course. The position had been given a very personal character and a very general scope. He was left great freedom in the choice of the matter he would present. Taking advantage of this freedom he returned to a subject that was dear to him, and devoted part of his lectures to the laws of symmetry, the study of fields of vectors and tensors, and to the application of these ideas to the physics of crystals. He intended to carry these lessons further, and to work out a course that would completely cover the physics of crystallized matter which would have been especially useful because this subject was so little known in France. His other lessons dealt with radioactivity, set forth the discoveries made in this


new domain, and the revolution they had caused in science.

    Even though he was very much absorbed in the preparation of his course, and often ill, my husband continued, nevertheless, to work in the laboratory, which was becoming better and better organized. He had a little more space now, and could receive a few students. In collaboration with A. Laborde, he carried on investigations in mineral waters and gases discharged from springs. This was the last work he published.

    His intellectual faculties were at this time at their height. One could but admire the surety and rigor of his reasoning on the theories of physics, his clear comprehension of fundamental principles, and a certain profound sense of phenomena which he had by instinct, but which he perfected during the course of a life entirely consecrated to research and reflection. His skill in experiment, remarkable from the beginning, was increased by practice. He experienced the pleasure of an artist when he succeeded with a delicate installation. He enjoyed, too, devising and constructing new apparatus, and I used jokingly to tell him that he would not be happy unless he made at least an attempt of this kind once every six months. His natural curiosity


and vivid imagination pushed him to undertakings in very varied directions; he could change the object of his research with surprising ease.

    He was scrupulously careful of scientific probity and of complete accuracy in his publications. These are very perfect in form, and none the less so in those parts where he applies the critical spirit to himself, expressing his determination never to affirm anything that does not seem entirely clear. He expresses his thought on this point in the following words:

"In the study of unknown phenomena, one can make very general hypotheses and then advance step by step with the help of experience. This method of progress is sure but necessarily slow. One can, on the contrary, make daring hypotheses in which he specifies the mechanism of phenomena. Such a method of procedure has the advantage of suggesting certain experiments, and, above all, of facilitating reasoning by rendering it less abstract through the employment of an image. But on the other hand, one cannot hope thus to conceive a complex theory in accord with experiment. The precise hypothesis almost certainly includes a portion of error along with a portion of truth. And this last portion, if it exists, forms only a part of a more general proposition to which it will be necessary in the end to return."

    Moreover, even though he never hesitated to make hypotheses, he never permitted their premature


publication. He could never accustom himself to a system of work which involved hasty publications, and was always happier in a domain in which but a few investigators were quietly working. The considerable vogue of radioactivity made him wish to abandon this field of research for a time, and to return to his interrupted studies of the physics of crystals. He dreamed also of making an examination of diverse theoretical questions.

    He gave much thought to his teaching, which constantly improved, and which suggested to him ideas on the general orientation of studies and on methods of teaching, which he believed should be based on contact with experience and nature. He hoped to see his views adopted by the Association of Professors as soon as it was formed, and to obtain the declaration "that the teaching of the sciences must be the dominant teaching of both the boys' and girls' lycées."

    "But," he said, "such a notion would have little chance of success."

    But this last period of his life, so fecund, was, alas, soon to end. His admirable scientific career was to be suddenly broken at the very moment when he could hope that the years of work to come would be less hard than those which had preceded.


    In 1906, quite ill and tired, he went with me and the children to spend Easter in the Chevreuse Valley. Those were two sweet days under a mild sun, and Pierre Curie felt the weight of weariness lighten in a healing repose near to those who were dear to him. He amused himself in the meadows with his little girls, and talked with me of their present and their future.

    He returned to Paris for a reunion and dinner of the Physics Society. There he sat beside Henri Poincaré and had a long conversation with him on methods of teaching. As we were returning on foot to our house, he continued to develop his ideas on the culture that he dreamed of, happy in the consciousness that I shared his views.

    The following day, the 19th of April, 1906, he attended a reunion of the Association of Professors of the Faculties of the Sciences, where he talked with them very cordially about the aims which the Association might adopt. As he went out from this reunion and was 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.


[9] During my recent visit to America, where a gramme of radium was generously offered me by American women, the Buffalo Society of Natural Sciences presented me, as a souvenir, with a publication reviewing the development of the radium industry in the United States. This included photographic reproductions of letters from Pierre Curie in which he replied in as complete a manner as possible to the questions asked by American engineers. (1902 and 1903.)

[10] The price of a milligramme of the element ofradium was then fixed at about 750 francs.

[11] These physicians were aided by the manufacturer, Armet de Lisle, who placed at their disposition the radium needed for their first undertakings. He founded, besides, in 1906, a laboratory for clinical study, provided with a supply of radium. And he subventioned the first special publication devoted to radioactivity and its applications, as a journal under the name Radium, edited by J. Danne. This is an example of generous support of science by industry, in reality still very rare but which one wishes might become general, in the common interest of these two branches of human activity.

[12] The hypothesis according to which radioactivity is bound up with the atomic transformation of elements was first envisaged by Pierre Curie and by me, along with other possible hypotheses, before it was utilized by E. Rutherford. (See Revue Scientifique, 1900, Mme. Curie, etc.)

[13] By using the unusual energy of Alpha-rays E.Rutherford has obtained recently the rupture of certain light atoms, like those of nitrogen.