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: