emed to show no decrease in its
enormous activity. Polonium, however, was found to lose most of its
activity in a year, and later it appeared that some radio-active
substances lost most of their activity in the course of a few minutes
or hours.

Induced Activity

A phenomenon called induced or secondary radio-activity was also
observed. Thus a metal plate or wire exposed to the action of thorium
oxide for some hours became itself active. This induced activity was
not permanent but decreased to half its value in about eleven hours
and practically disappeared within a week. Similar phenomena were
observed when radium was substituted for thorium.

Discovery of Uranium X

In 1900 Crookes precipitated a solution of an active uranium salt with
ammonium carbonate. The precipitate was dissolved so far as possible
in an excess of the reagent, leaving an insoluble residue. This
residue was many hundred times more active, weight for weight, than
the original salt, and the solution containing the salt was
practically inactive. At the end of a year the uranium salt had
regained its activity while the residue had become inactive.

Another method of obtaining the same result is to dissolve
crystallized uranium nitrate in ether. Two layers of solution are
formed, one ether and the other water coming from the water of
crystallization. The aqueous layer is active, while the water layer is
inactive. Similarly, by adding barium chloride solution to a solution
of a salt of uranium and then precipitating the barium as sulphate,
the activity is transferred to this precipitate. These experiments
give proof of the formation and separation of a radio-active body by
ordinary chemical operations.

So, too, in the case of thorium salts a substance can be obtained by
means of ammonium hydroxide which is several thousand times more
active than an equal weight of the original salt. After standing a
month, the separated material has lost its activity and the thorium
salt has regained it. Here, again, there is the formation, separation,
and loss of a radio-active body.

Conclusions Drawn

Now, these are ordinary chemical processes for the separation of
distinct chemical individuals. The results, therefore, lead naturally
to the conclusions: (1) it would seem that uranium and thorium are
themselves inactive and the activity is due to some other substance
formed by these elements; (2) this active substance is produced by
some transformat