THE DREAM BECOME A REALITY. THE DISCOVERY OF RADIUM
I have already said that in 1897 Pierre Curie was occupied with an
investigation on the growth of crystals. I myself had finished, by the
beginning of vacation, a study of the magnetization of tempered steels
which had resulted in our getting a small subvention from the Society
for the Encouragement of National Industry. Our daughter Irene was born
in September, and as soon as I was well again, I resumed my work in the
laboratory with the intention of preparing a doctor's thesis.
Our attention was caught by a curious phenomenon discovered in 1896 by
Henri Becquerel. The discovery of the X-ray by Roentgen had excited the
imagination, and many physicians were trying to discover if similar rays
were not emitted by fluorescent bodies under the action of light. With
this question in mind Henri Becquerel was studying uranium salts, and,
as sometimes occurs, came upon a phenomenon different from that he was
looking for: the spontaneous emission by uranium salts of rays of a
peculiar character. This was the discovery of radioactivity.
The particular phenomenon discovered by Becquerel was as follows:
uranium compound placed upon a photographic plate covered with black
paper produces on that plate an impression analogous to that which light
would make. The impression is due to uranium rays that traverse the
paper. These same rays can, like X-rays, discharge an electroscope, by
making the air which surrounds it a conductor.
Henri Becquerel assured himself that these properties do not depend on a
preliminary isolation, and that they persist when the uranium compound
is kept in darkness during several months. The next step was to ask
whence came this energy, of minute quantity, it is true, but constantly
given off by uranium compounds under the form of radiations.
The study of this phenomenon seemed to us very attractive and all the
more so because the question was entirely new and nothing yet had been
written upon it. I decided to undertake an investigation of it.
It was necessary to find a place in which to conduct the experiments. My
husband obtained from the director of the School the authorization to
use a glassed-in study on the ground floor which was then being used as
a storeroom and machine shop.
[Illustration: _Henri Manuel, Paris._
Pierre and Marie Curie in their laboratory, where radium was
discovered.]
In order to go beyond the results reached by Becquerel, it was necessary
to employ a precise quantitative method. The phenomenon that best lent
itself to measurement was the conductibility produced in the air by
uranium rays. This phenomenon, which is called _ionization_, is produced
also by X-rays and investigation of it in connection with them had made
known its principal characteristics.
For measuring the very feeble currents that one can make pass through
air ionized by uranium rays, I had at my disposition an excellent method
developed and applied by Pierre and Jacques Curie. This method consists
in counterbalancing on a sensitive electrometer the quantity of
electricity carried by the current with that which a piezo-electric
quartz can furnish. The installation therefore required a Curie
electrometer, a piezo-electric quartz, and a chamber of ionization,
which last was formed by a plate condenser whose higher plate was joined
to the electrometer, while the lower plate, charged with a known
potential, was covered with a thin layer of the substance to be
examined. Needless to say, the place for such an electrometric
installation was hardly the crowded and damp little room in which I had
to set it up.
My experiments proved that the radiation of uranium compounds can be
measured with precision under determined conditions, and that this
radiation is an atomic property of the element of uranium. Its intensity
is proportional to the quantity of uranium contained in the compound,
and depends neither on conditions of chemical combination, nor on
external circumstances, such as light or temperature.
I undertook next to discover if there were other elements possessing the
same property, and with this aim I examined all the elements then known,
either in their pure state or in compounds. I found that among these
bodies, thorium compounds are the only ones which emit rays similar to
those of uranium. The radiation of thorium has an intensity of the same
order as that of uranium, and is, as in the case of uranium, an atomic
property of the element.
It was necessary at this point to find a new term to define this new
property of matter manifested by the elements of uranium and thorium. I
proposed the word radioactivity which has since become generally
adopted; the radioactive elements have been called radio elements.
During the course of my research, I had had occasion to examine not only
simple compounds, salts and oxides, but also a great number of minerals.
Certain ones proved radioactive; these were those containing uranium and
thorium; but their radioactivity seemed abnormal, for it was much
greater than the amount I had found in uranium and thorium had led me to
expect.
This abnormality greatly surprised us. When I had assured myself that it
was not due to an error in the experiment, it became necessary to find
an explanation. I then made the hypothesis that the ores uranium and
thorium contain in small quantity a substance much more strongly
radioactive than either uranium or thorium. This substance could not be
one of the known elements, because these had already been examined; it
must, therefore, be a new chemical element.
I had a passionate desire to verify this hypothesis as rapidly as
possible. And Pierre Curie, keenly interested in the question, abandoned
his work on crystals (provisionally, he thought) to join me in the
search for this unknown substance.
We chose, for our work, the ore pitchblende, a uranium ore, which in its
pure state is about four times more active than oxide of uranium.
Since the composition of this ore was known through very careful
chemical analysis, we could expect to find, at a maximum, 1 per cent of
new substance. The result of our experiment proved that there were in
reality new radioactive elements in pitchblende, but that their
proportion did not reach even a millionth per cent!
The method we employed is a _new method in chemical research based on
radioactivity_. It consists in inducing separation by the ordinary means
of chemical analysis, and of measuring, under suitable conditions, the
radioactivity of all the separate products. By this means one can note
the chemical character of the radioactive element sought for, for it
will become concentrated in those products which will become more and
more radioactive as the separation progresses. We soon recognized that
the radioactivity was concentrated principally in two different chemical
fractions, and we became able to recognize in pitchblende the presence
of at least two new radioactive elements: polonium and radium. We
announced the existence of polonium in July, 1898, and of radium in
December of the same year.[7]
In spite of this relatively rapid progress, our work was far from
finished. In our opinion, there could be no doubt of the existence of
these new elements, but to make chemists admit their existence, it was
necessary to isolate them. Now, in our most strongly radioactive
products (several hundred times more active than uranium), the polonium
and radium were present only as traces. The polonium occurred associated
with bismuth extracted from pitchblende, and radium accompanied the
barium extracted from the same mineral. We already knew by what methods
we might hope to separate polonium from bismuth and radium from barium;
but to accomplish such a separation we had to have at our disposition
much larger quantities of the primary ore than we had.
It was during this period of our research that we were extremely
handicapped by inadequate conditions, by the lack of a proper place to
work in, by the lack of money and of personnel.
Pitchblende was an expensive mineral, and we could not afford to buy a
sufficient quantity. At that time the principal source of this mineral
was at St. Joachimsthal (Bohemia) where there was a mine which the
Austrian government worked for the extraction of uranium. We believed
that we would find all the radium and a part of the polonium in the
residues of this mine, residues which had so far not been used at all.
Thanks to the influence of the Academy of Sciences of Vienna, we secured
several tons of these residues at an advantageous price, and we used it
as our primary material. In the beginning we had to draw on our private
resources to pay the costs of our experiment; later we were given a few
subventions and some help from outside sources.
The question of quarters was particularly serious; we did not know where
we could conduct our chemical treatments. We had been obliged to start
them in an abandoned storeroom across a court from the workroom where we
had our electrometric installation. This was a wooden shed with a
bituminous floor and a glass roof which did not keep the rain out, and
without any interior arrangements. The only objects it contained were
some worn pine tables, a cast-iron stove, which worked badly, and the
blackboard which Pierre Curie loved to use. There were no hoods to carry
away the poisonous gases thrown off in our chemical treatments, so that
it was necessary to carry them on outside in the court, but when the
weather was unfavorable we went on with them inside, leaving the windows
open.
[Illustration: A view of the extraction of radium in the old
shed where the first radium was obtained.]
In this makeshift laboratory we worked practically unaided during two
years, occupying ourselves as much with chemical research as with the
study of the radiation of the increasingly active products we were
obtaining. Then it became necessary for us to divide our work. Pierre
Curie continued the investigations on the properties of radium, while I
went ahead with the chemical experiments which had as their objective
the preparation of pure radium salts. I had to work with as much as
twenty kilogrammes of material at a time, so that the hangar was filled
with great vessels full of precipitates and of liquids. It was
exhausting work to move the containers about, to transfer the liquids,
and to stir for hours at a time, with an iron bar, the boiling material
in the cast-iron basin. I extracted from the mineral the radium-bearing
barium and this, in the state of chloride, I submitted to a fractional
crystallization. The radium accumulated in the least soluble parts, and
I believed that this process must lead to the separation of the chloride
of radium. The very delicate operations of the last crystallizations
were exceedingly difficult to carry out in that laboratory, where it was
impossible to find protection from the iron and coal dust. At the end of
a year, results indicated clearly that it would be easier to separate
radium than polonium; that is why we concentrated our efforts in this
direction. We examined the radium salts we obtained with the aim of
discovering their powers and we loaned samples of the salts to several
scientists,[8] in particular to Henri Becquerel.
During the years 1899 and 1900, Pierre Curie published with me a memoir
on the discovery of the induced radioactivity produced by radium. We
published another paper on the effects of the rays: the luminous
effects, the chemical effects, etc.; and still another on the electric
charge carried by certain of the rays. And, finally, we made a general
report on the new radioactive substances and their radiations, for the
Congress of Physics which met in Paris in 1900. My husband published,
besides, a study of the action of a magnetic field on radium rays.
[Illustration: Pierre Curie with the quartz piezo-electroscope
he invented, by which rays of radium are measured.]
The main result of our investigations and of those of other scientists
during these years, was to make known the nature of the rays emitted by
radium, and to prove that they belonged to three different categories.
Radium emits a stream of active corpuscles moving with great speed.
Certain of them carry a positive charge and form the Alpha rays; others,
much smaller, carry a negative charge and form Beta rays. The movements
of these two groups are influenced by a magnet. A third group is
constituted by the rays that are insensible to the action of a magnet,
and that, we know to-day, are a radiation similar to light and to
X-rays.
We had an especial joy in observing that our products containing
concentrated radium were all spontaneously luminous. My husband who had
hoped to see them show beautiful colorations had to agree that this
other unhoped-for characteristic gave him even a greater satisfaction
than that he had aspired to.
The Congress of 1900 offered us an opportunity to make known, at closer
range, to foreign scientists, our new radioactive bodies. This was one
of the points on which the interest of this Congress chiefly centered.
We were at this time entirely absorbed in the new field that opened
before us, thanks to the discovery so little expected. And we were very
happy in spite of the difficult conditions under which we worked. We
passed our days at the laboratory, often eating a simple student's lunch
there. A great tranquillity reigned in our poor, shabby hangar;
occasionally, while observing an operation, we would walk up and down
talking of our work, present and future. When we were cold, a cup of hot
tea, drunk beside the stove, cheered us. We lived in a preoccupation as
complete as that of a dream.
Sometimes we returned in the evening after dinner for another survey of
our domain. Our precious products, for which we had no shelter, were
arranged on tables and boards; from all sides we could see their
slightly luminous silhouettes, and these gleamings, which seemed
suspended in the darkness, stirred us with ever new emotion and
enchantment.
Actually, the employees of the School owed Pierre Curie no service. But
nevertheless the laboratory helper whom he had had to aid him when he
was laboratory chief had always continued to help him as much as he
could in the time at his disposal. This good man, whose name was Petit,
felt a real affection and solicitude for us, and many things were made
easier because of his good will and the interest he took in our success.
[Illustration: A view of the extraction of radium in the old
shed where the first radium was obtained.]
We had begun our research in radioactivity quite alone, but because of
the magnitude of the undertaking, we were more and more convinced of the
utility of inviting collaboration. Already in 1898, one of the
laboratory chiefs of the School, G. Bemont, had given us temporary aid.
And toward 1900 Pierre Curie associated with him a young chemist, André
Debierne, preparator under Friedel, who held him in high esteem. André
Debierne gladly accepted Pierre Curie's proposal that he occupy himself
with the investigation of radioactivity; and he undertook, in
particular, the search for a new radio element, which we suspected
existed in the iron group and in rare earths. He discovered the element
actinium. Even though he carried on his work in the laboratory of
physical chemistry at the Sorbonne, directed by Jean Perrin, he
frequently came to visit us in our storeroom, and was soon an intimate
friend of ours, and of Doctor Curie and the children.
About this same time, George Sagnac, a young physicist engaged in the
study of X-rays, often came to discuss with my husband the analogies one
could expect to find between these rays, and their secondary rays, and
the radiations of radioactive bodies. They worked together on the
investigation of the electric charge carried by the secondary rays.
Besides our collaborators we saw very few persons in the laboratory;
however, from time to time some physicist or chemist came to see our
experiments, or to ask Pierre Curie for advice or information; for his
authority in several branches of physics was very well recognized. And
then there were discussions before the blackboard,--discussions which
are pleasantly remembered to-day, because they stimulated an interest in
science and an ardor for work without interrupting any course of
reflection, and without troubling that atmosphere of peace and
contemplation which is the true atmosphere of the laboratory.
[Footnote 7: This last publication was issued in common with G.
Bemont, who had collaborated with us in our experiments.]
[Footnote 8: I quote, as an example, a letter addressed to Pierre Curie
by A. Paulsen, thanking him for radioactive products loaned him in 1899:
"Den Damke Nordl's Expedition
Akureyi, 16 Oct. 1899.
Monsieur, and most honored colleague,
"I thank you warmly for your letter of August 1, which I have just
received in the north of Iceland.
"We have abandoned all the methods hitherto employed to establish in a
fixed conductor the potential that exists at certain points in the mass
of air that surrounds it, and are using only your radiant powder.
"Accept, Monsieur, and most honored colleague, my respectful salutations
and my renewed thanks for the great services you have rendered my
expedition.
"ADAM PAULSEN."]