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Hi, I'm Toby and welcome back to another video today
I would like to show you through one of the most influential
PhD theses ever written and that is the work of Marie Curie. In this thesis
Marie talks about the discovery of the new elements radium and polonium
and also describes how she gained one of the first understandings of the new physical phenomenon of
radioactivity. It was for that understanding that she was awarded the Nobel Prize in
1903 the same year this thesis was published and it was a few years later that she gained another Nobel Prize for the discovery
of those new elements this thesis was published at the University of Sorbonne in Paris
When Marie was 35 and she was the first woman in France to be awarded a doctoral degree
Although she was originally from Poland
So with that, let's have a little look at this really important work
The title of the thesis is research on radioactive substances
And it says it's by madam Sklodowska Curie, which is her full name
And in fact Marie is the French version of her original name Maria
This is a reprint of the thesis done in
1904 and this is of course not the original copy
the link is in the description to where I got this from the original documents would probably be quite
contaminated with radioactive substances, so I wouldn't want to be handling them.
This is the table of contents and the original version was in French, but don't worry
I'll show you an English translation soon.
This just shows us what some of the chapters were about
we have first an introduction a look at the history of the science then chapter one about the radioactivity of
uranium and thorium
which are radioactive
materials and
'radioactive' is actually a word that Marie used in this thesis
To first describe the phenomenon that she was seeing. She then talks about some new
radioactive substances that she's found and then the subsequent chapters talk about trying to separate those new radioactive substances
By some really tough chemical methods. Let's get page one of the English version over here
the introduction says that the work presented here has been worked on for the last four years and
That she is being assisted by. Mr. Curie
That's pure her husband who has put aside his own work to help her on this new exciting field the work
establishes the existence of a new element radium.
But the preparation of pure chloride of radium and the determination of the atomic weight of radium
form the main part of the work. This new element they have found has very curious properties and
also a new method of chemical research is established. This work was kick-started by the discovery of
Röntgen rays, which we would call x-rays. These mysterious rays that could produce an image of the bones in your hand
caused a lot of excitement and around this time many new different kinds of mysterious rays were being reported.
Some what kind of a false alarm but some were very legit. We have here that Mr. Becquerel has made similar
experiments on the salts of uranium.
This paragraph here explains that these penetrating uranium rays were not a result of
Phosphorescence that means that even when the uranium compounds were placed in complete darkness
They still continued to act on photographic plates
This was quite peculiar because at this time it was thought that for energy to be conserved
The only way for light to be emitted from a material is for it to have been first absorbed
But if you're keeping your uranium in the dark and you're still seeing these rays then that doesn't seem to fit with the model
What it took many more years to realize is that energy is indeed being stored in these materials
But billions of years ago and stored in the form of mass
chapter one mentions that these uranium rays or Becquerel rays
cause a gas to become a feeble conductor of electricity
with that Marie develops a way to
measure the intensity of radiation of uranium by the effect of this radiation on the
Conductivity of air and down here is where she calls
substances, which generate Becquerel rays
radioactive. This is her experimental set up here where she measures the intensity of radiation
What she does is to put a powder of her radioactive substance on this plate here
And then she applies a potential difference across these two plates
By setting up a circuit. The current in the circuit would be an indication of how well this
material is acting as a conductor and
therefore of how intense the radiation is. And the way that she got a measure of this current was having this
quartz crystal over here
Which is stressed by having some weights placed upon it. This crystal is an example of the piezoelectric effect
Which is when a stressed crystal will cause electricity to flow
Marie's husband Pierre actually discovered the piezoelectric effect in
1883
so he was really knowledgeable about how to use this in the circuit and what they did was to apply a
current that opposed the current in this circuit
and the amount of weight added to this plate here would be a measure of
how strong a current was needed to fully oppose this conductivity
Marie thought that it would be improbable that radioactivity would be a property belonging to only one kind of matter
so she tested a variety of materials. This table contains the results of
uranium with a bunch of different uranium minerals. What is really unexpected and
important about these results is that there are a few uranium minerals that have a higher
radioactivity than pure uranium.
This was a massive clue to the existence of a substance which was more radioactive than uranium.
Considering this she is able to identify
two new strongly radioactive substances
polonium which she's named after her homeland of Poland and
radium. They do a
spectral analysis to confirm that these elements are different from elements that have already been studied
But they haven't got a pure sample of these new
elements yet and the thesis then moves on to using some pretty intense chemical methods to get pure
samples. A pure sample would be needed to measure things like the atomic weight and
officially add these new substances to the periodic table. Here Marie writes that after working for
several years to try and isolate radium and polonium,
she's only succeeded in isolating radium. She failed with polonium for a few reasons
including the fact that
polonium was only present in the uranium ore in very tiny trace amounts the method for
Isolating radium that she describes over the next few pages in the thesis
Demonstrates that this was a gruelling task done on an enormous scale.
It shows her courage and stamina in chemistry. On this page
We see the description of beautiful crystals which have formed and that's one of only three or four
subjective aesthetic judgments
Which have snuck their way in to the thesis.
After starting with millions of grams of raw material, Marie ends up with a sample of radium chloride
which is about 1/10 of a gram. On this page here
we see her finding the atomic weight of the sample and this is the purest sample here with only a trace of
Barium present. She lists the atomic weight as being 225
And that's actually really close to the current accepted value of 226
That's really impressive for work that was done in a very humble shed
using very difficult raw materials and
chemical processes that had never been done before
All to understand a mysterious property that no one had even measured before
Marie mentions that for radium the strength of the radioactivity is
On the order of 10^6 higher than that of uranium
And again, this is in the correct magnitude of our currently accepted numbers
And again is a really great result.
I found it interesting that Marie did know about the different kinds of radiation
Alpha, beta and gamma rays as described by Ernest Rutherford around the same time and she has a little diagram here
showing these three different rays, that the gamma are the most penetrating, that alpha would be stopped after only a short
distance and that these have
opposite effect when acted on by a magnetic field.
They didn't know what these types of radiation were
And in fact, the Curie's are trying to formulate an explanation of their radioactivity
Almost a decade before the atomic nucleus would be discovered. Another page from the thesis that I found interesting
was this one about the physiological
effects. Now it says here that radium rays exert an action upon the epidermis
So they knew that radioactivity was having some kind of biological effect.
You may know that Marie Curie ultimately died from a disease
likely caused by her exposure to radiation and that
Made me think that they had no idea of the effect of radiation on the body
Perhaps it is true that they didn't know this effect would be detrimental
But the experiments on this page at least show that they knew it affected the skin.
In one experiment Mr. Curie
caused a relatively weak radioactive product to act upon his arm for ten hours.
The redness appeared immediately and later a wound was caused which took four months to heal.
She then talks about some use of radioactive materials in the hospital for probably the treatment of
cancers and tumors. And actually that became something very useful about these properties and we still do use
Radiation to treat cancers. So when used carefully it can actually save lives
But we're not treated carefully it can also cost you lives.
Chapter four moves on to explaining what was ultimately a wrong hypothesis that
radioactivity could be
communicated from one substance to another
this idea was caused by a few of their strange results, maybe explained by the fact that their
Samples were disappearing by radioactive decay or being absorbed into the glass
Radium is produced by the decay of uranium. And that is why they are always found together
But Marie never commented on this fact of why these two
seemingly different
elements would always be found together and maybe if she had
it would have been more of a clue to explaining what was really going on.
We will finish by going through Marie's
conclusions. The only sentence that she chooses to put in italics is that
Radium is a new chemical element
So she was really proud of this discovery of a new element and that is what she is often known for today
It's true that this is a very impressive contribution
But even more so is their discovery of the property of radioactivity and she sums it up at the bottom here
our researchers upon the new radioactive bodies have given rise to a
scientific movement
Indeed. This would be the starting point for a lot more science and a lot more
Discoveries about the nature of the atom, why radiation occurs and how we can use it.
So that has been a look at Marie Curie's work
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