The very first step into this newly discovered territory made the riddle still more bewildering. As we have said, Maxwell's use of a material analogy as a means of formulating mathematically the properties of electro-magnetic fields of force had led to results which brought electricity into close conjunction with light. In his own way Crookes focused, to begin with, his attention entirely on the light-like character of electric effects in a vacuum. It was precisely these observations, however, as continued by Lenard and others, which presently made it necessary to see in electricity nothing else than a special manifestation of inert mass. The developments leading up to this stage are recent and familiar enough to be briefly summarized. The first step was once more an accident, when Röntgen (or rather one of his assistants) noticed that a bunch of keys, laid down by chance on top of an unopened box of photographic plates near a cathode tube, had produced an inexplicable shadow-image of itself on one of the plates. The cathode tube was apparently giving off some hitherto unknown type of radiation, capable of penetrating opaque substances. Röntgen was an experimentalist, not a theorist; his pupils used to say privately that in publishing this discovery of X-rays he attempted a theoretical explanation for the first and only time in his life - and got it wrong! However, this accidental discovery had far-reaching consequences. It drew attention to the fluorescence of minerals placed in the cathode tube; this inspired Becquerel to inquire whether naturally fluorescent substances gave off anything like X-rays, and eventually - yet again by accident - he came upon certain uranium compounds. These were found to give off a radiation similar to X-rays, and to give it off naturally and all the time. Soon afterwards the Curies succeeded in isolating the element, radium, an element which was found to be undergoing a continuous natural disintegration. The way was now clear for that long series of experiments on atomic disintegration which led finally to the splitting of the nucleus and the construction of the atomic bomb. * A typical modern paradox emerges from these results. By restricting his cognitive powers to a field of experience in which the concept of force as an objective reality was unthinkable, man has been led on a line of practical investigation the pursuit of which was bound to land him amongst the force-activities of the cosmos. For what distinguishes electric and sub-electric activities from all other forces of physical nature so far known to science, is that for their operation they have no need of the resistance offered by space-bound material bodies; they represent a world of pure dynamics into which spatial limitations do not enter.