The Forces that Shape the Earth: Convection and Plates

Anybody who, like me, has spent some significant amount of time in the city of Paris knows that the weather, there, is very often overcast. It was so in the last days of February, 1896, as Henri Becquerel ran an experiment in his laboratory involving uranium and X-rays. Becquerel’s lab was in the museum of natural history, literally next door from old Buffon’s apartments at the Jardin des Plantes, and—today—from the new site of the Institut de Physique du Globe. If you ever pass by, you might see a marble plaque that says, “dans le laboratoire de physique appliquée du museum Henri Becquerel a découvert la radioactivité le 1 \(^{\text{ er }}\) mars 1896”. Becquerel had thought that what uranium did was absorb energy from the sun and then release it—in the form of high-energy electromagnetic radiation, i.e., X-rays. To see more precisely how that happened, he decided to expose to sunlight some material that contained uranium, then place it near photographic plates, to record the radiation that was emitted. The way Becquerel saw things, no sunlight meant the experiment couldn’t be done; but for some reason, after that one overcast day, he left the uranium near a photographic plate anyway. And it turned out that, even though the uranium could not have absorbed any energy from the sun, the imprint of radiation still showed up in the plate.