the tendency of physico-chemical science is clearly towards
the reduction of the problems of the world of the infinitely little,
as it already has reduced those of the infinitely great world, to
questions of mechanics.[H]

In the meanwhile, the primitive atomic theory, which has served as the
scaffolding for the edifice of modern physics and chemistry, has been
quietly dismissed. I cannot discover that any contemporary physicist
or chemist believes in the real indivisibility of atoms, or in an
interatomic matterless vacuum. 'Atoms' appear to be used as mere
names for physico-chemical units which have not yet been subdivided,
and 'molecules' for physico-chemical units which are aggregates of the
former. And these individualised particles are supposed to move in an
endless ocean of a vastly more subtle matter--the ether. If this ether
is a continuous substance, therefore, we have got back from the
hypothesis of Dalton to that of Descartes. But there is much reason to
believe that science is going to make a still further journey, and, in
form, if not altogether in substance, to return to the point of view
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 ch