its compounds were the only known source of these
new radiations, but many other substances were examined and two years
later thorium and its compounds were added to the list. In general the
discharging action seemed about the same. Other elements and ordinary
substances show a minute activity. Only potassium and rubidium have a
greater activity than this, and theirs is only about one-thousandth
that of uranium.

An Atomic Property

In the examination of uranium and thorium compounds it was found that
the activity was determined by the uranium and thorium present; it was
proportto the amount ofional these elements present and independent of
the nature of the other elements composing the compound. The
conclusion was, therefore, that the activity was an inherent property
of the atoms of uranium and thorium, that is, an atomic property. This
was a long step forward and introduced into science the conception of
a new property of matter, or at least of certain forms of matter.

Discovery of New Radio-active Bodies

In examining a large number of minerals containing uranium and
thorium, Mme. Curie made the important observation that many of these
were more active than the elements themselves. In measuring the
activity she made use of the electrical method which will be described
later. In the following table giving her results for uranium minerals
the numbers under _i_ give the maximum current in amperes. They serve
simply for comparison.

_i_
Pitchblende from Joachimsthal 7.0 x 10^{-11}
Clevite 1.4 x 10^{-11}
Chalcolite 5.2 x 10^{-11}
Autunite 2.7 x 10^{-11}
Carnotite 6.2 x 10^{-11}
Uranium 2.3 x 10^{-11}
Uranium and potassium sulphate 0.7 x 10^{-11}
Uranium and copper phosphate 0.9 x 10^{-11}

The last three are pure uranium and compounds of that element given
for comparison with the first five, which are naturally occurring
minerals. The last compound has the same composition as chalcolite and
is simply the artificially prepared mineral. It has the activity which
would be calculated from the proportion of uranium present, the copper
and phosphoric acid contributing no activity.

Since the activity is not dependent upon the composition but upon the
amount of uranium present, the activity in all of the minerals