ecessary to study selenium and tellurium so closely,
for most of their properties can be predicted from the relation which
they sustain to sulphur.

2. _It predicts new elements._ When the periodic law was first
formulated there were a number of vacant places in the table which
evidently belonged to elements at that time unknown. From their position
in the table, Mendeleeff predicted with great precision the properties
of the elements which he felt sure would one day be discovered to fill
these places. Three of them, scandium, germanium, and gallium, were
found within fifteen years, and their properties agreed in a remarkable
way with the predictions of Mendeleeff. There are still some vacant
places in the table, especially among the heavier elements.

3. _It corrects errors._ The physical constants of many of the elements
did not at first agree with those demanded by the periodic law, and a
further study of many such cases showed that errors had been made. The
law has therefore done much service in indicating probable error.

~Imperfections of the law.~ There still remain a good many features which
must be regarded as imperfections in the law. Most conspicuous is the
fact that the element hydrogen has no place in the table. In some of the
groups elements appear in one of the families, while all of their
properties show that they belong in the other. Thus sodium belongs with
lithium and not with copper; fluorine belongs with chlorine and not with
manganese. There are two instances where the elements must be
transposed in order to make them fit into their proper group. According
to their atomic weights, tellurium should follow iodine, and argon
should follow potassium. Their properties show in each case that this
order must be reversed. The table separates some elements altogether
which, in many respects have closely agreeing properties. Iron,
chromium, and manganese are all in different groups, although they are
similar in many respects.

The system is therefore to be regarded as but a partial and imperfect
expression of some very important and fundamental relation between the
substances which we know as elements, the exact nature of this relation
being as yet not completely clear to us.

EXERCISES

1. Suppose that an element were discovered that filled the blank in
Group O, Period 5; what properties would it probably have?

2. Suppose that an element were discovered that filled the blank in
Group VI, Period 9