I could tell you many things about radium and radioactivity and it would
take a long time. But as we can not do that, I shall only give you a
short account of my early work about radium. Radium is no more a baby,
it is more than twenty years old, but the conditions of the discovery
were somewhat peculiar, and so it is always of interest to remember them
and to explain them.
We must go back to the year 1897. Professor Curie and I worked at that
time in the laboratory of the school of Physics and Chemistry where
Professor Curie held his lectures. I was engaged in some work on uranium
rays which had been discovered two years before by Professor Becquerel.
I shall tell you how these uranium rays may be detected. If you take a
photographic plate and wrap it in black paper and then on this plate,
protected from ordinary light, put some uranium salt and leave it a day,
and the next day the plate is developed, you notice on the plate a black
spot at the place where the uranium salt was. This spot has been made by
special rays which are given out by the uranium and are able to make an
impression on the plate in the same way as ordinary light. You can also
test those rays in another way, by placing them on an electroscope. You
know what an electroscope is. If you charge it, you can keep it charged
several hours and more, unless uranium salts are placed near to it. But
if this is the case the electroscope loses its charge and the gold or
aluminum leaf falls gradually in a progressive way. The speed with which
the leaf moves may be used as a measure of the intensity of the rays;
the greater the speed, the greater the intensity.
I spent some time in studying the way of making good measurements of the
uranium rays, and then I wanted to know if there were other elements,
giving out rays of the same kind. So I took up a work about all known
elements, and their compounds and found that uranium compounds are
active and also all thorium compounds, but other elements were not found
active, nor were their compounds. As for the uranium and thorium
compounds, I found that they were active in proportion to their uranium
or thorium content. The more uranium or thorium, the greater the
activity, the activity being an atomic property of the elements, uranium
and thorium.
Then I took up measurements of minerals and I found that several of
those which contain uranium or thorium or both were active. But then the
activity was not what I could expect, it was greater than for uranium or
thorium compounds like the oxides which are almost entirely composed of
these elements. Then I thought that there should be in the minerals some
unknown element having a much greater radioactivity than uranium or
thorium. And I wanted to find and to separate that element, and I
settled to that work with Professor Curie. We thought it would be done
in several weeks or months, but it was not so. It took many years of
hard work to finish that task. There was not one new element, there were
several of them. But the most important is radium which could be
separated in a pure state.
All the tests for the separation were done by the method of electrical
measurements with some kind of electroscope. We just had to make
chemical separations and to examine all products obtained with respect
to their activity. The product which retained the radioactivity was
considered as that one which had kept the new element; and, as the
radioactivity was more strong in some products, we knew that we had
succeeded in concentrating the new element. The radioactivity was used
in the same way as a spectroscopical test.
The difficulty was that there is not much radium in a mineral; this we
did not know at the beginning. But we now know that there is not even
one part of radium in a million parts of good ore. And too, to get a
small quantity of pure radium salt, one is obliged to work up a huge
quantity of ore. And that was very hard in a laboratory.
We had not even a good laboratory at that time. We worked in a hangar
where there were no improvements, no good chemical arrangements. We had
no help, no money. And because of that the work could not go on as it
would have done under better conditions. I did myself the numerous
crystalizations which were wanted to get the radium salt separated from
the barium salt with which it is obtained out of the ore. And in 1902 I
finally succeeded in getting pure radium chloride and determining the
atomic weight of the new element radium, which is 226 while that of
barium is only 137.
Later I could also separate the metal radium, but that was a very
difficult work; and, as it is not necessary for the use of radium to
have it in this state, it is not generally prepared that way.
Now, the special interest of radium is in the intensity of its rays
which is several million times greater than the uranium rays. And the
effects of the rays make the radium so important. If we take a practical
point of view, then the most important property of the rays is the
production of physiological effects on the cells of the human organism.
These effects may be used for the cure of several diseases. Good results
have been obtained in many cases. What is considered particularly
important is the treatment of cancer. The medical utilization of radium
makes it necessary to get that element in sufficient quantities. And so
a factory of radium was started to begin with in France, and later in
America where a big quantity of ore named carnotite is available.
America does produce many grams of radium every year but the price is
still very high because the quantity of radium contained in the ore is
so small. The radium is more than a hundred thousand times dearer than
gold.
But we must not forget that when radium was discovered no one knew that
it would prove useful in hospitals. The work was one of pure science.
And this is a proof that scientific work must not be considered from the
point of view of the direct usefulness of it. It must be done for
itself, for the beauty of science, and then there is always the chance
that a scientific discovery may become like the radium a benefit for
humanity.
But science is not rich, it does not dispose of important means, it does
not generally meet recognition before the material usefulness of it has
been proved. The factories produce many grams of radium every year, but
the laboratories have very small quantities. It is the same for my
laboratory and I am very grateful to the American women who wish me to
have more of radium and give me the opportunity of doing more work with
it.
The scientific history of radium is beautiful. The properties of the
rays have been studied very closely. We know that particles are expelled
from radium with a very great velocity near to that of the light. We
know that the atoms of radium are destroyed by expulsion of these
particles, some of which are atoms of helium. And in that way it has
been proved that the radioactive elements are constantly disintegrating
and that they produce at the end ordinary elements, principally helium
and lead. That is, as you see, a theory of transformation of atoms which
are not stable, as was believed before, but may undergo spontaneous
changes.
Radium is not alone in having these properties. Many having other
radioelements are known already, the polonium, the mesothorium, the
radiothorium, the actinium. We know also radioactive gases, named
emanations. There is a great variety of substances and effects in
radioactivity. There is always a vast field left to experimentation and
I hope that we may have some beautiful progress in the following years.
It is my earnest desire that some of you should carry on this scientific
work and keep for your ambition the determination to make a permanent
contribution to science.