ion in those elements, for on standing the activity
is regained. This latter conclusion is startling, for it indicates a
change in the atom which, up to the time of this discovery, was deemed
unchangeable under the influence of such physical and chemical changes
as were known to us.

Search for New Radio-active Bodies

The search for new radio-active bodies and the study of their
characteristics has been systematically and successfully carried on.
The bodies obtained in the above experiments were named uranium _X_
and thorium _X_, respectively. Further, it became clear from the
investigation of uranium minerals that radium, polonium, actinium, and
ionium originated from uranium. From thorium minerals a body was
separated called mesothorium, which was analogous to radium. Both
thorium and radium were found to give off a radio-active gas. The
first lost half of its activity in less than one minute. The second
was more stable and lost half of its activity in about four days. The
name radium emanation was given to the latter and it was found
chemically and physically to belong to the class of monatomic or noble
gases, such as helium, argon, neon, etc., which had been discovered by
Ramsay. In some cases the chemical action was determined and these new
bodies were found analogous to well-known elements, as radium to
barium, polonium to bismuth. The physical properties were investigated
and, where possible, spectra were mapped and atomic weights
determined.

It is clear, therefore, that these bodies are elemental in character
and as such are made up of distinct, similar atoms, just as the
commonly recognized elements are believed to be. In this way more than
thirty new elements have been added to the list. These new elements
are called radio-active elements, but it is an open question whether
all atoms do not possess this property in greater or less degree.
Certainly, it is possessed in varying degree by four of the old
elements widely separated in the Periodic System, namely, uranium,
thorium, rubidium, and potassium. The last two, while feebly active
themselves, do not form any secondary radio-active substance so far as
is known. Only two of the elements, then, can definitely be said to go
through these transformations. It is just possible that radio-activity
may be found to be a common property of all atoms and of all matter.

Methods of Investigation

It is important to know how these new bodies were discovered a