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 completely disappeared. The loss of activity of uranium was thus only temporary in character. In the above experiments, the activity of uranium was examined by the photographic method. The photographic action produced by uranium is due almost entirely to the β rays. The α rays, in comparison, have little if any effect. Now the radiation from Ur X consists entirely of β rays, and is consequently photographically very active. If the activity of uranium had been measured electrically without any screen over it, the current observed would have been due very largely to the α rays, and little change would have been observed after the removal of Ur X, since only the constituent responsible for the β rays was removed. This important point is discussed in more detail in section 205. 128. Thorium X. Rutherford and Soddy, working with thorium compounds, found that an intensely active constituent could be separated from thorium by a single chemical operation. If ammonia is added to a thorium solution, the thorium is precipitated, but a large amount of the activity is left behind in the filtrate, which is chemically free from thorium. This filtrate was evaporated to dryness, and the ammonium salts driven off by ignition. A small residue was obtained which, weight for weight, was in some cases several thousand times more active than the thorium from which it was obtained, while the activity of the precipitated thorium was reduced to less than one half of its original value. This active constituent was named Th X from analogy to Crookes' Ur X. The active residue was found to consist mainly of impurities from the thorium; the Th X could not be examined chemically, and probably was present only in minute quantity. It was also found that an active constituent could be partly separated from thorium oxide by shaking it with water for some time. On filtering the water, and evaporating down, a very active residue was obtained which was analogous in all respects to Th X. On examining the products a month later, it was found that the Th X was no longer active, while the thorium had completely