Geoscience Reference
In-Depth Information
Serendipity
The decade of the 1890s was a time of great progress in science. No year in that
decadewasmoreeventfulthan1895.InSweden,SvanteArrheniuswascalculating
the effect of atmospheric CO
2
on global temperature, while in Britain John Perry
was exposing the fallacy of Kelvin's assumptions. The Scottish chemist William
Ramsay discovered helium, previously known only from the Sun's spectrum, in an
earthly mineral. Helium would not only help explain what had eluded Kelvin—the
true source of the Sun's energy—but help refute his claims about the age of the
Earth. In the town hall at Sceaux, France, a young Polish chemist named Marie
Sklodowska married her sweetheart, Pierre Curie.
That same year, at the University of Wurzburg in Germany, Wilhelm Röntgen
(1845-1923) was investigating the properties of cathode rays. These mysterious
beams, discovered in 1876, appeared when scientists applied a voltage across the
electrodes in an evacuated tube. As sometimes happens in science, Röntgen made
a serendipitous discovery. After enclosing the cathode-ray tube in a black box to
exclude all light, then turning off the lights in the laboratory, Röntgen switched on
the current in the vacuum tube. To his surprise, a spectral shimmer appeared on his
benchtop a few meters away. He lit a match and saw that the glow had come from
a paper plate that he had coated with a barium compound. When he switched off
the current, the glow disappeared. He recognized that the cathode-ray tube must
give off invisible rays that had somehow excited the barium. Röntgen named them
X-rays, X for unknown. He found that when he placed a solid object between the
cathode-ray tube and a photographic plate, a ghostly image of the object appeared
on the plate. Röntgen used X-rays to make a famous image of his wife's hand, her
bones and ring clearly visible.
Some substances, like Röntgen's barium compound, glow or “fluoresce” when
irradiated with X-rays. Others, including some uranium minerals, “phosphoresce”:
they continue to glow for a while even after the irradiation is turned off. In 1896,
the French physicist Henri Becquerel (1852-1908), suspecting that sunlight might
also trigger phosphorescence, wrapped a photographic plate in black paper so that
no light could reach it, placed a specimen of a phosphorescent uranium mineral
called pitchblende on top of the wrapped plate, and sat the whole thing in the sun-
