aracters 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.

Thus the whole series does not run:

_a, b, d, e, f, g, h, i, k,_ &c.,

but

_a, b, c, d_, A, B, C, D, alpha, beta, gamma, delta, &c.;

so that it is said to express a _periodic law_ of recurrent
similarities. Or the relation may be expressed in another way. In each
section of the series, the atomic weight is greater than in the
preceding section, so that if _w_ is the atomic weight of any element
in the first segment, _w+x_ will represent the atomic weight of any
element in the next, and _w+x+y_ the atomic weight of any element in
the next, and so on. Therefore the sections may be represented as
parallel series, the corresponding terms of which have analogous
properties; each successive series starting with a body the atomic
weight of which is greater than that of any in the preceding series,
in the following fashion:

_d_ D delta
_c_ C gamma
_b_ B beta
_a_ A alpha
- ----- ---------
_w_ _w + x_ _w + x + y_

[Sidenote: The possibility of a primary form of matter.]

This is a conception with, which biologists are very familiar, animal
and plant groups constantly appearing as series of parallel
modifications of similar and yet different primary forms. In the
living world, facts of this kind are now understood to mean evolution
from a common prototype. It is difficult to imagine that in the
not-living world they are devoid of significance. Is it not possible,
nay probable that they may mean the evolution of our 'elements' from a
primary undifferentiated form of matter? Fifty years ago, such a
suggestion would have been scouted as a revival of the dreams of the
alchemists. At present, it may be said to be the burning question of
physico-chemical science.

In fact, the so-called 'vortex-ring' hypothesis is a very serious and
remarkable attempt to deal with material units from a point of view
which is consistent with the doctrine of evolution. It supposes the
ether to be a uniform substance, and that the 'elementary' units are,
broadly speaking, permanent whirlpools, or vortices, of this ether,
the properties of which depend on their actual and potential modes of
motion. It is curious and highly interesting to remark that this
hypothesis reminds us not only of the sp