-activity. The underlying principle was unknown
and even the statement of what was sometimes erroneously called the
Periodic Law was manifestly incorrect and its terms were ignored.

Basis of the Periodic System

The ordinary statement of the fundamental principle of the Periodic
System has been that the properties of the elements were periodic
functions of the atomic weights, and that when the elements were
arranged in the order of their atomic weights they fell into a natural
series, taking their places in the proper related groups.

In accepting this, the interpretation of function was both
unmathematical and vague, and the order of the atomic weights was not
strictly adhered to but unhesitatingly abandoned to force the group
relationship. Wherever consideration of the atomic weight would have
placed an element out of the grouping with other elements to which it
was clearly related in physical and chemical properties, the guidance
of these properties was accepted and that of the atomic weights
disregarded. Such shiftings are noted in the cases of tellurium and
iodine; cobalt and nickel; argon and potassium. It was most helpful
that, following the order of atomic weights, the majority of the
elements fell naturally into their places. Otherwise the
generalization known as the Periodic System might have remained for a
long time undiscovered and the progress of chemistry would have been
greatly retarded.

Influence of Positive Nucleus

It is evident that the order of the elements is determined by
something else than their atomic weights. From the known facts of
radio-activity it would seem that this determining factor is the
positive nucleus. And this nucleus also determines the mass or weight
of the atom. Taking the elements in their order in the Periodic Series
and numbering the positions held by them in this series as 1, 2, 3,
etc., we get the position number or what is called the atomic number.
This designates the order or position of the element in the series.
We must learn that this number marks a position rather than a single
element, a statement which will be explained later.

Determination of the Atomic Number

Since the atomic weight is unreliable as a means of settling the
position of an element in the series and so fixing its atomic number,
how is this number to be determined? Of course, one answer to this
question is that we may rely upon a consideration of the general
properties, as has been