Whilst practical men were turning the work of Faraday and Maxwell to account by exploiting the mechanical working of electricity in power-production, and its similarity to light in the wireless communication of thought, a new field of research, with entirely new practical possibilities, was suddenly opened up in the last third of the nineteenth century through the discovery of how electricity behaves in rarefied air. This brings us to the discovery of cathode rays and the phenomena accompanying them, from which the latest stage in the history of electricity originated. And here once more, as in the history of Galvani's discoveries, we encounter certain undercurrents of longing and expectation in the human soul which seemed to find an answer through this sudden, great advance in the knowledge of electricity - an advance which has again led to practical applications of the utmost significance for human society, though not at all in the way first hoped for. Interest in the phenomena arising when electricity passes through gases with reduced pressure had simultaneously taken hold of several investigators in the seventies of the nineteenth century. But the decisive step in this sphere of research was taken by the English physicist, William Crookes. He was led on by a line of thought which seems entirely irrelevant; yet it was this which first directed his interest to the peculiar phenomena accompanying cathode rays; and they proved to be the starting-point of the long train of inquiry which has now culminated in the release of atomic energy.3 In the midst of his many interests and activities, Crookes was filled from his youth with a longing to find by empirical means the bridge leading from the world of physical effects to that of superphysical causes. He himself tells how this longing was awakened in him by the loss of a much-beloved brother. Before the dead body he came to the question, which thereafter was never to leave him, whether there was a land where the human individuality continues after it has laid aside its bodily sheath, and how that land was to be found. Seeing that scientific research was the instrument which modern man had forged to penetrate through the veil of external phenomena to the causes producing them, it was natural for Crookes to turn to it in seeking the way from the one world into the other.