We have now reached the last of the groups as arranged on Sir William Crookes' lemniscates, that forming the "neutral" column; it is headed by helium, which is _sui generis_. The remainder are in the form of a flat star (see Plate IV, 4), with a centre formed of five intersecting and "cigar"-bearing tetrahedra, and six radiating arms. Ten of these have been observed, five pairs in which the second member differs but slightly from the first; they are: Neon, Meta-neon; Argon, Metargon; Krypton, Meta-krypton; Xenon, Meta-xenon; Kalon, Meta-kalon; the last pair and the meta forms are not yet discovered by chemists. These all show the presence of a periodic law; taking an arm of the star in each of the five pairs, we find the number of atoms to be as follows :-- 40 99 224 363 489 47 106 231 370 496 It will be observed that the meta form in each case shows seven more atoms than its fellow. [Illustration: PLATE XX.] HELIUM (Plate III, 5, and Plate XX, 1) shows two "cigar"-bearing tetrahedra, and two hydrogen triangles, the tetrahedra revolving round an egg-shaped central body, and the triangles spinning on their own axes while performing a similar revolution. The whole has an attractively airy appearance, as of a fairy element. HELIUM: Two tetrahedra of 24 atoms 48 Two triangles of 9 atoms 18 Central egg 6 ---- Total 72 ---- Atomic weight 3.94 Number weight 72/18 4.00 NEON (Plate XX, 2 and 6) has six arms of the pattern shown in 2, radiating from the central globe. NEON: Six arms of 40 atoms 240 Central tetrahedra 120 ---- Total 360 ---- Atomic weight 19.90 Number weight 360/18 20.00 META-NEON (Plate XX, 3 and 6) differs from its comrade by the insertion of an additional atom in each of the groups included in the second body within its arm, and substituting a seven-atomed group for one of the triplets in neon. META-NEON: Six arms of 47 atoms 282 Central tetrahedra 120 ---- Total 402 ---- Atomic weight ---- Number weight 402/18 22.33 ARGON (Plate XX, 4, 6 and 7) shows within its arms the _b_ 63 which we met in nitrogen, yttrium, vanadium and niobium, but not the "balloon," which we shall find with it in krypton and its congeners.