The German Genius

thirty-seven) he would have been as surprised as anyone at the direction physics was about to take. Rollo Appleyard says he was in all respects “a Newtonian.” 10
    A N EW K IND OF R AY
     
    In Chapter 17, we saw that gases had claimed physicists’ attention earlier in the century, at first for the light they threw on the conservation of energy, then for the statistical behavior of their atoms and molecules. As part of this, and especially with the growth of interest in electromagnetism and the possibility that the “void” between atoms was filled, as Maxwell said, with an electromagnetic field, a new specialism grew up that entailed the discharge of electricity in gases. As this interest developed, a new piece of apparatus was conceived, eventually known as the cathode ray tube. This was a glass tube with metal plates sealed into either end, and the gas sucked out, leaving a vacuum. If the metal plates were then connected to a battery and a current generated, the empty space, the vacuum inside the glass tube, glowed or fluoresced. This glow was generated from the negative plate, the cathode, and was absorbed into the positive plate, the anode. * The Berlin physicist Eugen Goldstein in 1876 was the first to label the new equipment “cathode ray” tubes.
    William Crookes in Britain hypothesized in 1879 that cathode rays were a “fourth state of matter” (i.e., neither solid, liquid, nor gas), but that wasn’t convincing and most physicists still asked what exactly cathode rays were . The situation remained both confusing and promising, and several physicists started to take a look. One was the professor of physics at the University of Würzburg, Wilhelm Conrad Röntgen.
    Born in the Lower Rhine province, Röntgen grew up in the Netherlands before studying in Zurich under Clausius. At Würzburg, he started investigating cathode rays—in particular their penetrating power—toward the end of 1895. It was by then common to use a barium platino-cyanide screen to detect any fluorescence caused by cathode rays. 11 This screen was not really part of the experiment, more a fail-safe device should there be any anomalies. In Röntgen’s case, the screen was some way from the cathode ray tube which was in fact covered with black cardboard and operated only in a darkened room. On November 8, 1895, now a famous date in the history of science, Röntgen noticed—to his surprise—that the screen, though a good distance from the tube, also fluoresced. This could not possibly have been caused by the cathode rays. But did that mean the apparatus was giving off other rays, invisible to the naked eye? He confirmed his results, noting also that the paper screen covered with barium platino-cyanide fluoresced “whether the treated side or the other be turned towards the discharging tube.” 12
    When his discovery was published, it caused a great stir, well beyond the confines of professional scientists, and he was soon commanded to demonstrate his discovery to the Kaiser. But what exactly were the new rays? 13 In his follow-up studies, Röntgen found they had some of the properties of light, in that they followed straight lines, affected photographic plates, and were not interfered with by magnetic fields. At the same time, they were different from light and from Hertz’s electromagnetic waves in not being either reflected or refracted. For as much as a decade, physicists worked fruitfully with x-rays, as they came to be called ( x for unknown, though they are called Röntgenstrahlen in Germany), without understanding exactly what they were. 14
    Eventually, in the early twentieth century, it was shown that x-rays were a form of electromagnetic wave with an extremely short wavelength, but some confusion remained until the wave-particle duality was clarified with the advent of quantum mechanics (see Chapter 32). In 1912 Max von Laue, a physicist who worked with Planck and Einstein, realized that, with their very small wavelength, x-rays could only be studied (i.e., reflected or refracted) by substances that had a very small grid structure, and that such spacings were to be found in the “inter-atomic distances between the ions of a crystal.” 15 The experiment to test this prediction was carried out by the Munich physicist Walter Friedrich and his student Paul Knipping in the spring of 1912, with this collaboration constituting the first-ever “x-ray diffraction.” This provided proof that x-rays are indeed electromagnetic