of Aristotle.

[Sidenote: Elementary bodies]

The greater number of the so-called 'elementary' bodies, now known,
had been discovered before the commencement of our epoch; and it had
become apparent that they were by no means equally similar or
dissimilar, but that some of them, at any rate, constituted groups,
the several members of which were as much like one another as they
were unlike the rest. Chlorine, iodine, bromine, and fluorine thus
formed a very distinct group; sulphur and selenium another; boron and
silicon another; potassium, sodium, and lithium another; and so on. In
some cases, the atomic weights of such allied bodies were nearly the
same, or could be arranged in series, with like differences between
the several terms. In fact, the elements afforded indications that
they were susceptible of a classification in natural groups, such as
those into which animals and plants fall.

[Sidenote: fall into different series.]

Recently this subject has been taken up afresh, with a result which
may be stated roughly in the following terms: If the sixty-five or
sixty-eight recognised 'elements' are arranged in the order of their
atomic weights--from hydrogen, the lightest, as unity, to uranium, the
heaviest, as 240--the series does not exhibit one continuous
progressive modification in the physical and chemical characters of
its several terms, but breaks up into a number of sections, in each of
which the several terms present analogies with the corresponding terms
of the other series.