Radium has the form of a tetrahedron, and it is in the tetrahedral groups (see article IV) that we shall find its nearest congeners; calcium, strontium, chromium, molybdenum resemble it most closely in general internal arrangements, with additions from zinc and cadmium. Radium has a complex central sphere (Plate XXII), extraordinarily vivid and living; the whirling motion is so rapid that continued accurate observation is very difficult; the sphere is more closely compacted than the centre-piece in other elements, and is much larger in proportion to the funnels and spikes than is the case with the elements above named; reference to Plate VIII will show that in these the funnels are much larger than the centres, whereas in radium the diameter of the sphere and the length of the funnel or spike are about equal. Its heart consists of a globe containing seven atoms, which assume on the proto level the prismatic form shown in cadmium, magnesium and selenium. This globe is the centre of two crosses, the arms of which show respectively three-atomed and two-atomed groups. Round this sphere are arranged, as on radii, twenty-four segments, each containing five bodies--four quintets and a septet--and six loose atoms, which float horizontally across the mouth of the segment; the whole sphere has thus a kind of surface of atoms. On the proto level these six atoms in each segment gather together and form a "cigar." In the rush of the streams presently to be described one of these atoms is occasionally torn away, but is generally, if not always, replaced by the capture of another which is flung into the vacated space. Each of the four funnels opens, as usual, on one face of the tetrahedron, and they resemble the funnels of strontium and molybdenum but contain three pillars instead of four (Plate XXIII). They stand within the funnel as though at the angles of a triangle, not side by side. The contained bodies, though numerous, contain forms which are all familiar. The spikes alternate with the funnels, and point to the angles of the tetrahedron as in zinc and cadmium; each spike contains three "lithium spikes" (see Plate XIX) with a ten-atomed cone or cap at the top, floating above the three (Plate XXIV). The "petals" or "cigars" of lithium exist in the central globe in the floating atoms, and the four-atomed groups which form the lithium "plate" may be seen in the funnels, so that the whole of lithium appears in radium.