but there is no trace of it in hydrogen, the head of the group. We have not
met it elsewhere. It may be remarked that, in Sir William Crookes' scheme,
in which they are all classed as monads, these two groups are the nearest
to the neutral line, on the ingoing and outgoing series, and are
respectively positive and negative.

II and IIa. _The Tetrahedron._--The characteristics of this form are four
funnels, containing ovoid bodies, opening on the face of a tetrahedron. The
funnels generally, but not always, radiate from a central globe. We give
beryllium (glucinum) as the simplest example (2 on Plate III), and to this
group belong calcium and strontium. The tetrahedron is the form of chromium
and molybdenum, but not that of the head of their group, oxygen, which is,
like hydrogen, _sui generis_. These two groups are marked in orthodox
chemistry as respectively positive and negative, and are closely allied.
Another pair of groups show the same tetrahedral form: magnesium, zinc and
cadmium, positive; sulphur, selenium and tellurium, negative. Selenium is a
peculiarly beautiful element, with a star floating across the mouth of each
funnel; this star is extremely sensitive to light, and its rays tremble
violently and bend if a beam of light falls on it. All these are dyads.

The tetrahedron is not confined to the external form of the above atoms; it
seems to be one of the favourite forms of nature, and repeatedly appears in
the internal arrangements. There is one tetrahedron within the unknown
element occultum; two appear in helium (3 on Plate III); yttrium has also
two within its cube, as has germanium; five, intersecting, are found in
neon, meta-neon, argon, metargon, krypton, meta-krypton, xenon, meta-xenon,
kalon, meta-kalon, tin, titanium and zirconium. Gold contains no less than
twenty tetrahedra.

III. _The Cube._--The cube appears to be the form of triads. It has six