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 after the removal of Th X is put through a series of precipitations with ammonia at short intervals, the Th X is removed almost as fast as it is formed, and, at the same time, the activity of thorium B in the thorium decays.

The following table indicates the results obtained. A portion of the precipitated hydroxide was removed after each series of precipitations and its activity tested in the usual way.

Activity of                                                         hydroxide per cent. After 1 precipitation                                            46 After 3 precipitations at intervals of 24 hours                  39 After 3 more precipitations at intervals of 24 hours and 3 at intervals of 8 hours                                    22 After 3 more each of 8 hours                                     24 After 6 more each of 4 hours                                     25

Fig. 79.

The differences in the last three numbers are not significant, for it is difficult to make accurate comparisons of the activity of thorium compounds which have been precipitated under slightly different conditions. It is thus seen that as a result of successive precipitations, the activity is reduced to a minimum of about 25 per cent. The recovery curve of the activity of this 23 times precipitated