certain numerical relations held good between the atomic
weights of elements chemically similar to one another. Thus the weight
(88) of an atom of strontium compared with that of hydrogen as unity,
is about the mean of those of calcium (40) and barium (137). Such
relations, in this and other chemical groups, were illustrated by
Beguyer de Chancourtois in 1862 by the construction of a spiral diagram
in which the atomic weights are placed in order round a cylinder and
elements chemically similar are found to fall on vertical lines.

Newlands seems to have been the first to see the significance of such
a diagram. In his "law of octaves," formulated in 1864, he advanced
the hypothesis that, if arranged in order of rising atomic weight, the
elements fell into groups, so that each eighth element was chemically
similar. Stated thus, the law was too definite; no room was left for
newly-discovered elements, and some dissimilar elements were perforce
grouped together.

But in 1869 Mendeleeff developed Newland's hypothesis in a form that
attracted at once general attention. Placing the elements in order of
rising atomic weight, but leaving a gap where necessary to bring similar
elements into vertical columns, he obtained a periodic table with
natural vacancies to be filled as new elements were discovered, and with
a certain amount of flexibility at the ends of the horizontal lines.
From the position of the vacancies, the general chemical and physical
properties of undiscovered elements could be predicted, and the
success of such predictions gave a striking proof of the usefulness of
Mendeleeff's generalisation.

When the chemical and physical properties of the elements were known
to be periodic functions of their atomic weights, the idea of a common
origin and common substance became much more credible. Differences in
atomic weight and differences in properties alike might reasonably be
explained by the differences in the amount of the primordial substance
present in the various atoms; an atom of oxygen being supposed to be
composed of sixteen times as much stuff as the atom of hydrogen, but to
be made of the same ultimate material. Speculations about the mode of
origin of the elements now began to appear, and put on a certain air
of reality. Of these speculations perhaps the most detailed was that of
Crookes, who imagined an initial chaos of a primordial medium he named
protyle, and a process of periodic change in which the ch