The object of the present work is the publication of
researches which I have been carrying on for more than four years
on radio-active bodies. I began these researches by a study of the
phosphorescence of uranium, discovered by M. Becquerel. The results
to which I was led by this work promised to afford so interesting a
field that M. Curie put aside the work on which he was engaged, and
joined me, our object being the extraction of new radio-active
substances and the further study of their properties.
Since the commencement of our research we thought it well to
hand over specimens of the substances, discovered and prepared by
ourselves, to certain physicists, in the first place to M.
Becquerel, to whom is due the discovery of the uranium rays. In
this way we ourselves facilitated the research by others besides
ourselves on the new radio-active bodies. At the termination of our
first publications, M. Giesel, in Germany, also began to prepare
these substances, and passed on specimens of them to several German
scientists. Finally, these substances were placed on sale in France
and Germany, and the subject growing in importance gave rise to a
scientific movement, such that numerous memoirs have appeared, and
are constantly appearing on radio-active bodies, principally
abroad. The results of the various French and foreign researches
are necessarily confused, as is the case with all new subjects in
course of investigation, the aspect of the question becoming
modified from day to day.
From the chemical point of view, however, one point is
definitely established:— i.e. ,
the existence of a new element, strongly radio-active, viz.,
radium. The preparation of the pure chloride of radium and the
determination of the atomic weight of radium form the chief part of
my own work. Whilst this work adds to the elements actually known
with certainty a new element with very curious properties, a new
method of chemical research is at the same time established and
justified. This method, based on the consideration of
radio-activity as an atomic property of matter, is just that which
enabled M. Curie and myself to discover the existence of
radium.
If, from the chemical point of view, the question that we
undertook primarily may be looked upon as solved, the study of the
physical properties of the radio-active bodies is in full
evolution. Certain important points have been established, but a
large number of the conclusions are still of a provisional
character. This is not surprising when we consider the complexity
of the phenomena due to radio-activity, and the differences
existing between the various radio-active substances. The
researches of physicists on these substances constantly meet and
overlap. Whilst endeavouring to keep strictly to the limits of this
work and to publish my individual research only, I have been
obliged at the same time to mention results of other researches,
the knowledge of which is indispensable.
I desired, moreover, to make this work an inclusive survey of
the actual position of the question.
I indicate at the end the particular questions with which I
am specially concerned, and those which I investigated in
conjunction with M. Curie.
I carried on the work in the laboratories of the School of
Physics and Chemistry in Paris, with the permission of
Schützenberger, late Director of the School, and M. Lauth, actual
Director. I take this opportunity of expressing my gratitude for
the kind hospitality received in this school.
Historical.
The discovery of the phenomena of radio-activity is connected
with researches followed, since the discovery of the Röntgen rays,
upon the photographic effects of phosphorescent and fluorescent
substances.
The first tubes for producing Röntgen rays were without the
metallic anticathode. The source of the Röntgen rays was the glass
surface impinged upon by the cathode rays; this surface was at the
same time actively fluorescent. The question then was whether the
emission of Röntgen rays necessarily accompanied the production of
fluorescence, whatever might be the cause of the latter. This idea
was first enunciated by M. Henri Poincaré.
Shortly afterwards, M. Henry announced that he had obtained
photographic impressions through black paper by means of
phosphorescent zinc sulphide. M. Niewenglowski obtained the same
phenomenon with calcium sulphide exposed to the light. Finally, M.
Troost obtained strong photographic impressions with zinc sulphide
artificially phosphorescent acting across black paper and thick
cardboard.
The experiences just cited have not been reproduced, in spite
of numerous attempts to this end. It cannot therefore be considered
as proved that zinc sulphide and calcium sulphide are capable of
emitting, under the action of light, invisible rays which traverse
black paper and act on photographic plates.
M. Becquerel has made similar experiments on the salts of
uranium, some of which are fluorescent.
He obtained photographic impressions through black paper with
the double sulphate of uranium and potassium.
M. Becquerel at first believed that this salt, which is
fluorescent, behaved like the sulphides of zinc and calcium in the
experiments of MM. Henry, Niewenglowski, and Troost. But the
conclusion of his experiments showed that the phenomenon observed
was in no way related to the fluorescence. It is not necessary that
the salt should be fluorescent; further, uranium and all its
compounds, fluorescent or not, act in the same manner, and metallic
uranium is the most active. M. Becquerel finally found that by
placing uranium compounds in complete darkness, they continue
acting on photographic plates through black paper for years. M.
Becquerel allows that uranium and its compounds emit peculiar
rays—uranium rays. He proved that these rays can penetrate thin
metallic screens, and that they discharge electrified bodies. He
also made experiments from which he concluded that uranium rays
undergo reflection, refraction, and polarisation.
The work of other physicists (Elster and Geitel, Lord Kelvin,
Schmidt, Rutherford, Beattie, and Smoluchowski) confirms and
extends the results of the researches of M. Becquerel, with the
exception of those relating to the reflection, refraction, and
polarisation of uranium rays, which in this respect behave like
Röntgen rays, as has been recognised first by Mr. Rutherford and
then by M. Becquerel himself.