anium minerals the presence of a body which he named ionium, and
which is so similar to thorium that it cannot be separated from it.
It, however, far exceeds thorium in activity.

The lead which is present in uranium and thorium minerals--apparently
in fairly definite ratio to the amount of uranium and thorium--is
found, on separation and purification, to possess radio-active
properties. This activity is due to the presence of a very small
proportion of an active constituent called radio-lead, which has
chemical properties identical with those of ordinary lead. The bulk of
the lead obtained from radio-active minerals differs in atomic weight
from ordinary lead and appears also to be different according to
whether its source is a thorium or a uranium mineral.

A large number of other radio-active substances have been separated
and some of their properties determined, but these were found by
different means and will be noted in their proper place. They number
in all more than thirty. The sources or parents of these are the
original uranium or thorium, and the products form regular series with
distinctive properties for each member.

CHAPTER II

PROPERTIES OF THE RADIATIONS

The activity of these radio-active bodies consists in the emission of
certain radiations which may be separated into rays and studied
through the phenomena which they cause.

Ionization of Gases

One of these phenomena is the power of forming ions or carriers of
electricity by the passage of the rays through a gas, thus ionizing
the gas. The details of an experiment will serve to make the meaning
of this ionization clear.

[Illustration: FIG. 1.--IONIZATION OF GASES.]

When this apparatus is set up a minute current will be observed
without the introduction of any radio-active matter. This, as
Rutherford says, has been found due mainly to a slight natural
radio-activity of the matter composing the plates. If radio-active
matter is spread on plate _A_, which is connected with one pole of a
grounded battery, and if plate _B_ is connected with an electrometer
which is also connected with the earth, a current is caused which
increases rapidly with the difference of potential between the plates,
then more slowly until a value is reached that changes only slightly
with a larger increase in the voltage.

According to the theory of ionization, the radiation produces ions at
a constant rate. The ions carrying a positive charge are attr