cathode-ray tubes finally made everything clear and set modern physics on its triumphant course. In the first place, in November 1895, Wilhelm Rontgen, at Wurzburg, observed that when the cathode rays hit the glass wall of a cathode-ray tube, highly penetrating rays were emitted, which he called X-rays (because x , for a mathematician, signified the unknown). The X-rays caused various metals to fluoresce and, most amazingly, were found to pass through the soft tissue of his hand, to reveal the bones within. A year later, Henri Becquerel, intrigued by the fluorescing that Rontgen had observed, decided to see whether naturally-fluorescing elements had the same effect. In a famous but accidental experiment, he put some uranium salt on a number of photo-electric plates, and left them in a closed (light-tight) drawer. Four days later, he found images on the plates, given off by what we now know was a radio-active source. Becquerel had discovered that 'fluorescing' was naturally-occurring radio-activity.9 But it was Thomson's 1897 discovery which capped everything, produced the first of the Cavendish's great successes and gave modern physics its lift-off, into arguably the most exciting and important intellectual adventure of the modern world. In a series of experiments J. J. pumped different gases into the glass tubes, passed an electric current, and then surrounded them either with electrical fields or with magnets. As a result of this systematic manipulation of conditions, Thomson convincingly demonstrated that cathode 'rays' were in fact infinitesimally minute particles erupting from the cathode and drawn to the anode. Thomson further found that the particles' trajectory could be altered by an electric field and that a magnetic field shaped them into a curve.10 More important still, he found that the particles were lighter than hydrogen atoms, the smallest known unit of matter, and exactly the same whatever the gas through which the discharge passed. Thomson had clearly identified something fundamental-this was in fact the first experimental establishment of the particulate theory of matter. The 'corpuscles', as Thomson called these particles at first, are today known as electrons. It was the discovery of the electron, and Thomson's systematic examination of its properties, that led directly to Ernest Rutherford's further breakthrough, a decade later, in conceiving the configuration of the atom as a miniature 'solar system', with the tiny electrons orbiting the massive nucleus like stars around the sun. In doing this, Rutherford demonstrated experimentally what Einstein discovered inside his head and revealed in his famous calculation, E = mc 2 (1905), that matter and energy are essentially the same.11 The consequences of these insights and experimental results-which included thermonuclear weapons, and the ensuing political stand-off known as the Cold War-fall outside the time-frame of this book.* But Thomson's work is important for another reason that does concern us here. He achieved the advances that he did by systematic experimentation . At the beginning of this book, in the Introduction, it was asserted that the three most influential ideas in history have been the soul, the idea of Europe, and the experiment. It is now time to support this claim. It is most convincingly done by taking these ideas in reverse order. It is surely beyond reasonable doubt that, at the present time, and for some considerable time in the past, the countries that make up what we call the West-traditionally western Europe and northern America in particular, but with outposts such as Australia-have been the most successful and prosperous societies on earth, in terms of both the material advantages enjoyed by their citizens and the political and therefore moral freedoms they have. (This situation is changing now but these sentiments are true as far as they go.) These advantages are linked, intertwined, in so far as many material advances-medical innovations, printing and other media, travel technology, industrial processes-bring with them social and political freedoms in a general process of democratisation. And these are the fruit, almost without exception, of scientific innovations based on observation, experimentation, and deduction. Experimentation is all-important here as an independent, rational (and therefore democratic) form of authority . And it is this, the authority of the experiment, the authority of the scientific method , independent of the status of the individual scientist, his proximity to God or to his king, and as revealed and reinforced via myriad technologies, which we can all share, that underlies the modern world. The cumulative nature of science also makes it a far less fragile form of knowledge. This is what makes the experiment such an important idea. The scientific method, apart from its other attractions, is probably the purest form of democracy