Fig. 29, also on Plate VI., represents another rhombic substance, ammonium magnesium phosphate, NH_{4}MgPO_{4}.6H_{2}O, obtained by very slow precipitation of a dilute solution of magnesium sulphate containing ammonium chloride and ammonia with hydrogen disodium phosphate. It illustrates in an interesting manner how, when a saturated solution is kept quiet, and then the surface of the vessel containing it is scratched by a needle point, a line of small crystals at once starts forming along the line of scratch, even although the latter has made no actual impression on the glass itself. Such a line of crystals will be observed running across the middle of the slide. Fig. 30, Plate VII., shows a monoclinic substance, ammonium magnesium sulphate (NH_{4})_{2}Mg(SO_{4})_{2}.6H_{2}O, which crystallises out splendidly on a micro-slip. The field includes several very well-formed typical crystals of the salt, which is one of the same exceedingly important isomorphous series to which potassium nickel sulphate, Fig. 15, belongs; it is obtained by mixing solutions containing molecularly equivalent quantities of ammonium and magnesium sulphates. The primary monoclinic prism is the chief form, terminated by clinodome faces and smaller strip-faces of the basal plane, the latter, however, being occasionally the chief end form. Small pyramid faces are also seen here and there modifying the solid angles. Another beautifully crystallising monoclinic substance is shown in the next slide, Fig. 31, on the same Plate VII., namely, potassium sodium carbonate, KNaCO_{3}.6H_{2}O, obtained from a solution of molecular proportions of potassium and sodium carbonates. Numerous forms of the monoclinic system are developed, on relatively large and perfectly transparent and delicately shaded individuals. A triclinic substance is represented in Fig. 32, Plate VIII., potassium ferricyanide, K_{6}F_{2}(CN)_{12}. The triply oblique nature of the symmetry is clearly exhibited by this salt, the absence of any right angles being very marked. [Illustration: _PLATE VII._ FIG. 30.—Monoclinic Crystals of Ammonium Magnesium Sulphate. ] [Illustration: FIG. 31.—Monoclinic Crystals of Sodium Potassium Carbonate. MONOCLINIC CRYSTALS GROWING FROM SOLUTION. ] [Illustration: _PLATE VIII._ FIG. 32.—Triclinic Crystals of Potassium Ferricyanide. ] [Illustration: FIG. 33.—Tetrahedral Crystals of Sodium Sulphantimoniate, Cubic Class 28. CRYSTALS GROWING FROM SOLUTION. ]