The crystals shown in the accompanying photographic reproductions, Figs. 24 to 33 (Plates IV. to VIII.), as well as Fig. 4 (Plate II.), already described, were thus photographed in the very act of slow growth, employing a one-inch objective very much stopped down. Such photographs are infinitely sharper and more beautifully and delicately shaded than those taken of dry crystals. Fig. 24, Plate IV., represents cubic octahedra of the double cyanide of potassium and cadmium, 2KCN.Cd(CN)_{2}, a salt which crystallises out in relatively large and wonderfully transparent and well-formed single octahedra on a micro-slip, and is particularly suitable for demonstrating the character of this highest system, the cubic, of crystal symmetry. Special development of the pair of faces of the octahedron parallel to the glass surfaces has occurred, owing to greater freedom of growth at the boundaries of these faces, as is usual in such circumstances of deposition, but the other pairs of faces are quite large enough to show their nature clearly. Fig. 25, on the same Plate IV., shows a slide of cæsium alum, Cs_{2}SO_{4}.Al_{2}(SO_{4})_{3}.24H_{2}O, in which the octahedra are smaller, and some of them, notably one in the centre of the field, are perfectly proportioned. [Illustration: _PLATE V._ FIG. 26.—Octahedra of Ammonium Iron Alum crystallising on a Hair. ] [Illustration: FIG. 27.—Tetragonal Crystals of Potassium Ferrocyanide. CRYSTALS GROWING FROM SOLUTION. ] [Illustration: _PLATE VI._ FIG. 28.--Rhombic Crystals of Potassium Hydrogen Tartrate. ] [Illustration: FIG. 29.--Rhombic Crystals of Ammonium Magnesium Phosphate, showing Special Growth along Line of Scratch. RHOMBIC CRYSTALS GROWING BY SLOW PRECIPITATION. ] Fig. 26, Plate V., represents octahedra of ammonium iron alum (formula like that of cæsium alum, but with NH_{4} replacing Cs and Fe replacing Al) crystallising on a hair. It illustrates the interesting manner in which crystallisation will sometimes occur, under conditions of quietude, when some object or other on which the crystals can readily deposit themselves is present or introduced, such as a silk or cotton thread, or a hair as in this case. Fig. 27, on the same Plate V., represents tetragonal crystals of potassium ferrocyanide, K_{4}Fe(CN)_{6}, composed of tabular crystals parallel to the basal pinakoid, bounded by faces of one order, first or second, of tetragonal prism, the corners being modified at 45° by smaller faces of the other order of tetragonal prism. Fig. 28, Plate VI., is a photograph of large rhombic crystals of hydrogen potassium tartrate, HKC_{4}H_{4}O_{6}, obtained by addition of tartaric acid to a dilute solution of potassium chloride. They are rectangular rhombic prisms capped by pyramidal forms, and also modified by other prismatic and domal forms.