monads and
paramagnetic; lithium, potassium and rubidium are positive, while fluorine
and manganese are negative. We seem thus to have a pair, corresponding with
each other, as in other cases, and the interperiodic group is left
interperiodic and congruous within itself.

[Illustration: PLATE XIX.]

LITHIUM (Plate IV, 2 and Plate XIX, 1) is a striking and beautiful form,
with its upright cone, or spike, its eight radiating petals (_x_) at the
base of the cone, and the plate-like support in the centre of which is a
globe, on which the spike rests. The spike revolves swiftly on its axis,
carrying the petals with it; the plate revolves equally swiftly in the
opposite direction. Within the spike are two globes and a long ovoid; the
spheres within the globe revolve as a cross; within the ovoid are four
spheres containing atoms arranged on tetrahedra, and a central sphere with
an axis of three atoms surrounded by a spinning wheel of six.

LITHIUM: Spike of 63 atoms 63
8 petals of 6 atoms 48
Central globe of 16 atoms 16
----
Total 127
----
Atomic weight 6.98
Number weight 127/18 7.05
POTASSIUM (Plate XIX, 2) consists of nine radiating lithium spikes, but has
not petals; its central globe contains one hundred and thirty-four atoms,
consisting of the "nitrogen balloon," encircled by six four-atomed spheres.

POTASSIUM: 9 bars of 63 atoms 567
Central globe 134
----
Total 701
----
Atomic weight 38.94
Number weight 701/18 38.85
(The weight, as determined by Richards [_Nature_, July 18, 1907] is
39.114.)

RUBIDIUM: (Plate XIX, 3) adds an ovoid, containing three spheres--two
triplets and a sextet--to the lithium spike, of which it has sixteen, and
its central globe is composed of three "balloons."

RUBIDIUM: 16 spikes of 75 atoms 1200
Central globe 330
----
Total 1530
----
Atomic weight 84.85
Number weight 1530/18