The deep interest and importance of the research which this book describes will best be appreciated if introduced by an account of the circumstances out of which it arose. The first edition, consisting mainly of articles reprinted from the Theosophist, dealt at once with the later phases of the research in a way which, though intelligible to the occult student, must have been rather bewildering to the ordinary reader. These later phases, however, endow the earlier results with a significance that in the beginning could only be vaguely conjectured. I am the better entitled to perform the task that has been assigned to me-that of preparing the present edition-by reason of the fact that it was in my presence and at my instigation that the first efforts were made to penetrate the mystery previously enshrouding the ultimate molecule of matter. I remember the occasion vividly.