A few months later the German, Lothar Meyer, independently suggested the
same ideas. This arrangement brought to light a great generalization,
now known as the _periodic law_. An exact statement of the law will be
given after the method of arranging the elements has been described.

[Illustration: DMITRI IVANOVITCH MENDELEEFF (Russian) (1834-1907)

Author of the periodic law; made many investigations on the physical
constants of elements and compounds; wrote an important book entitled
"Principles of Chemistry"; university professor and government
official]

~Arrangement of the periodic table.~ The arrangement suggested by
Mendeleeff, modified somewhat by more recent investigations, is as
follows: Beginning with lithium, which has an atomic weight of 7, the
elements are arranged in a horizontal row in the order of their atomic
weights, thus:

~Li (7.03), Be (9.1), B (11), C (12), N (14.04), O (16), F (19).~

These seven elements all differ markedly from each other. The eighth
element, sodium, is very similar to lithium. It is placed just under
lithium, and a new row follows:

~Na(23.05), Mg (24.36), Al (27.1), Si (28.4), P (31), S (32.06),
Cl(35.45).~

When the fifteenth element, potassium, is reached, it is placed under
sodium, to which it is very similar, and serves to begin a third row:

~K (39.15), Ca (40.1), Sc (44.1,) Ti (48.1), V (51.2), Cr (52.1), Mn(55).~

Not only is there a strong similarity between lithium, sodium, and
potassium, which have been placed in a vertical row because of this
resemblance, but the elements in the other vertical rows exhibit much of
the same kind of similarity among themselves, and evidently form little
natural groups.

The three elements following manganese, namely, iron, nickel, and
cobalt, have atomic weights near together, and are very similar
chemically. They do not strongly resemble any of the elements so far
considered, and are accordingly placed in a group by themselves,
following manganese. A new row is begun with copper, which somewhat
resembles the elements of the first vertical column. Following the fifth
and seventh rows are groups of three closely related elements, so that
the completed arrangement has the appearance represented in the table on
page 168.

THE PERIODIC ARRANGEMENT OF THE ELEMENTS

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Periods | GROUP | GROUP | GROUP | GROUP | GROUP |
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