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 When the Th X, by precipitation with ammonia, is removed from a quantity of thorium nitrate, the precipitated thorium hydroxide does not at first possess appreciable emanating power. This loss of emanating power is not due, as in the case of the de-emanated oxide, to a retardation in the rate of escape of the emanation produced; for the hydroxide, when dissolved in acid, still gives off no emanation. On the other hand, the solution, containing the Th X, possesses emanating power to a marked degree. When the precipitated hydroxide and the Th X is left for some time, it is found that the Th X decreases in emanating power, while the hydroxide gradually regains its emanating power. After about a month's interval, the emanating power of the hydroxide has nearly reached a maximum, while the emanating power of the Th X has almost disappeared.

The curves of decay and recovery of emanating power with time are found to be exactly the same as the curves of decay and recovery of activity of Th X and the precipitated hydroxide respectively, shown in Fig. 47. The emanating power of Th X, as well as its activity, falls to half value in four days, while the hydroxide regains half its final emanating power as well as half its lost activity in the same interval.

It follows from these results that the emanating power of Th X is directly proportional to its activity, i.e. that the rate of production of emanating particles is always proportional to the number of α particles, projected from the Th X per second. The radiation from Th X thus accompanies the change of the Th X into the emanation. Since the emanation has chemical properties distinct from those of the Th X, and also a distinctive rate of decay, it cannot be regarded as a vapour of Th X, but it is a distinct chemical substance, produced by the changes occurring in Th X. On the view advanced in section 136, the atom of the emanation consists of the part of the atom of Th X left behind after the expulsion of one or more α particles. The atoms of the emanation are unstable, and in turn expel α particles. This projection of α particles constitutes the radiation from the emanation, which serves as a measure of the amount of emanation present. Since the activity of the emanation falls to half value in one minute while that of Th X falls to half value in four days, the emanation