On a New, Strongly Radioactive Substance Contained in Pitchblende
Pitchblende glowed far brighter than its uranium could explain — so the Curies tracked down a new element: radium.
A lump of black ore was pouring out more energy than the uranium inside it could possibly account for — so two people set out to find what else was hiding in the rock.
The big idea
In 1896 Henri Becquerel found that uranium quietly gives off invisible rays, entirely on its own. Marie and Pierre Curie set out to measure those rays carefully — and noticed something strange. The natural ore called pitchblende was several times more ‘active’ than its uranium content should allow. Something else in the rock, present in only the faintest trace, had to be far more powerfully radioactive than uranium itself.
By dissolving and re-crystallising the ore over and over — following the rays at every step instead of weighing anything — they concentrated that hidden activity again and again. Out came not one new element but two: polonium, which Marie named for her homeland of Poland, and then a second, hundreds of times more active still, which they called radium, from the Latin for ‘ray’.
How it came about
Marie Curie chose radioactivity for her doctoral research precisely because almost no one was working on it. Pierre, already an accomplished physicist, set his own work aside to join her; their laboratory was a leaky, unheated shed. Their key tool was an electrometer Pierre had built, sensitive enough to read the faint electric current the rays stir up in air — so they could ‘see’ radioactivity as a number on a dial.
When pitchblende read far too high, they trusted the number over expectation and chased it. The chemist Gustave Bémont helped pull radium from the ore, and the spectroscopist Eugène Demarçay clinched it by finding a brand-new colour in the substance's glow — a spectral line no known element could claim. Radium was still mixed with barium when they announced it in December 1898; teasing out a pure, weighable speck would cost Marie four more years of brutal labour.
Why it mattered
Until then, atoms were thought to be permanent and unbreakable. Radioactivity was the first hint that some atoms are restless — that they hurl energy out from within. That single fact cracked the atom open. It led straight to the discovery of the atomic nucleus, to nuclear energy, and to a hidden clock in the rocks that tells us the Earth is billions of years old. And it handed medicine a new way to fight cancer.
A way to picture it
Imagine ten thousand sealed jars you can never open — but each holds a tiny bell that rings exactly once, at a moment you can't predict, the instant it cracks. You'll never know which jar rings next. Yet if you simply count the rings per minute, you can say precisely how fast the whole batch is failing, and which kind of jar fails soonest. The Curies couldn't see single atoms either. They counted the ‘rings’ — the rays — and from that ringing alone they tracked down an element no one had ever laid eyes on.
Where it sits
Becquerel found the rays; the Curies found the elements behind them and made radioactivity something you could measure. From here the thread runs straight to Rutherford's nuclear atom (1911), to Einstein's E = mc² — which he suggested might first be tested with ‘radium salts’ — and on into the whole nuclear century. It is a landmark in another way, too: Marie Curie became the first woman to win a Nobel Prize, and the only person ever to win in two different sciences.
Les diverses raisons que nous venons d'énumérer nous portent à croire que la nouvelle substance radioactive renferme un élément nouveau, auquel nous proposons de donner le nom de radium.