which is an analogue
of argon and belongs to the zero group. Other instances might be cited
which go to show that in all cases the loss of an alpha particle makes
a change of two places toward the left or positive side of the System.

Changes from Loss of Beta Particles

The loss of a beta particle causes no change in the atomic weight but
does cause a shift for each beta particle of one group toward the
right or negative side of the System. Two such losses, then, will
counterbalance the loss of an alpha particle and bring the new element
back to the group originally occupied by its progenitor. Thus uranium
in the sixth group loses an alpha particle and the product UX_{1}
falls in the fourth group. One beta particle is then lost and UX_{2}
belonging to the fifth group is formed. With the loss of one more beta
particle the new element returns to the sixth group from which the
transformation began.

The table on page 48, as adapted from Soddy, affords a general view of
these changes.

Isotopes

An examination of the table will show a number of different elements
falling in the same position in a group of the Periodic System
irrespective of their atomic weights. These are chemically inseparable
so far as the present limitations of chemical analysis are concerned.
Even the spectra of these elements seem to be identical so far as
known. This identity extends to most of the physical properties, but
this demands much further investigation. For this new phenomenon Soddy
has suggested the word isotope for the element and isotopic for the
property, and these names have come into general use.

[Illustration: RADIO-ACTIVE ELEMENTS FROM URANIUM AND THORIUM
PLACED IN THE PERIODIC SYSTEMS Adapted from Soddy]

Manifestly, we have come across a phenomenon here which quite
eliminates the atomic weight as a determining factor as to position in
the Periodic or Natural System or of the elemental properties in
general. All of the properties of the bodies which we call elements,
and consequently of their compounds and hence of matter in general,
seem to depend upon the balance maintained between the charges of
negative and positive electricity which, according to Rutherford's
theory, go to make up the atom.

It is evident that any study of chemical phenomena and chemical theory
is quite incomplete without a study of radio-activity and the
transformations which it produces.

Radio-activity in Nature

In c