J. J. Thomson’s 1897 discovery of the electron was the third of three rapid-fire developments at the end of the nineteenth century that launched the campaign, still in progress, to understand the inner structure of atoms. The other two were complete surprises: X-rays, discovered by W. K. Roentgen in 1895, and natural radioactivity, discovered in uranium by H. Becquerel in 1896. Because the X-rays seemed to come from a glowing spot on the glass wall of Roentgen’s apparatus, Becquerel immediately investigated whether phosphorescent materials could be induced to emit similar rays following exposure to the Sun. His discovery that uranium did indeed emit penetrating radiation, without benefit of sunlight, is surely the most profound purely accidental discovery in physics. Until Einstein’s discovery that mass is energy, it was also among the most puzzling: Where did the enormous radiant energy come from? (See Note 27 in Chapter 2.)