By heating a mixture of cobalt oxalate and sal-ammoniac in air, it is obtained in the form of minute hard octahedra, which are not magnetic, and are only soluble in concentrated sulphuric acid.
This solution, on standing, deposits octahedra of the composition CoC1 2.6NH 3.
By sublimation in a current of hydrogen it can be crystallized in the form of regular octahedra; it is slightly harder than tin, but is softer than zinc, and like tin, emits a crackling sound when bent.
It forms grey coloured octahedra of specific gravity 5.49 6 at 20° C., melting at 900° C.; it burns at a red heat, is insoluble in hydrochloric acid, but dissolves in aqua regia, and is also soluble in molten alkalis.
Titanium is often present, and occasionally the mineral contains magnesium, nickel, &c. It is always strongly magnetic. Magnetite crystallizes in the cubic system, usually in octahedra, less commonly in rhombic dodecahedra, and not infrequently in twins of the " spinel type " (fig.
Many of them contain large octahedra of magnetite.
It crystallizes in octahedra isomorphous with stannic oxide.
Potash alum, K 2 SO 4 ï¿½Al 2 (SO 4)aï¿½24H 2 O, crystallizes in regular octahedra and is very soluble in water.
It may be obtained as jet black octahedra (isomorphous with thoria) by fusion with borax.
Stannic iodide, Sn14, forms red octahedra and is prepared similarly to stannic bromide.
It crystallizes in octahedra which melt at 120.5° C. and boil at 290° C. Its vapour burns with a red flame.
It crystallizes in dark red octahedra which are almost insoluble in cold water.
It may be obtained crystallized in quadratic octahedra of a greenish-blue colour, by melting in a sealed tube containing an inert gas, and inverting the tube when the metal has partially solidified.
The salt K2S03 2H20 is obtained as oblique rhombic octahedra by crystallizing the solution over sulphuric acid.
On evaporating the solution dark purple octahedra of the alum :are obtained.
It forms red octahedra and is less soluble in water than the corresponding potassium compound.
It crystallizes in red octahedra and dyes silk and wool yellow.
Octahedra having triangular faces other than equilateral occur as crystal forms. See Polyhedron and Crystallography.
It crystallizes in the cubical system, often in beautiful octahedra and rhombic dodecahedra.
It crystallizes in the cubic system, usually in cubes, pentagonal dodecahedra or octahedra, often of great beauty and perfection.
There are also twins of diamond in which two octahedra (fig.
On account of their resemblance to the twins of the mineral spinel (which crystallizes in octahedra) these are ?i D known as " spinel twins."
Halite or rock-salt crystallizes in the cubic system, usually in cubes, rarely in octahedra; the cubes being solid, unlike the skeleton-cubes obtained by rapid evaporation of brine.
Thallous fluoride, T1F, forms white glistening octahedra; it is obtained by crystallizing a solution of the carbonate in hydrofluoric acid.
Gallium crystallizes in greyish-white octahedra which melt at 30.15° C. to a silvery-white liquid.
Ann., 1830, 96, p. 155), or in the form of blue octahedra by evaporating a solution of nickel chloride in aqueous ammonia.
It forms yellow octahedra, which become anhydrous at 100°, and are converted into the trioxide on ignition.
It crystallizes in colourless octahedra which melt at 125-126° C., and is easily soluble in water.
It crystallizes in octahedra, having a specific gravity of 3.2, and melts at 597° C. (T.
Fine yellow fluor-spar occurs in some of the Saxon mines, and beautiful rose-red octahedra are found in the Alps, near Goschenen.
Silver peroxide, AgO, appears under certain conditions as minute octahedra when a solution of silver nitrate is electrolysed, or as an amorphous crust in the electrolysis of dilute sulphuric acid between silver electrodes.
Silver fluoride, AgF, is obtained as quadratic octahedra, with one molecule of water, by dissolving the oxide or carbonate in hydrofluoric acid.
By cooling the aqueous solution, hyacinth-red octahedra of a crystalline hydrate of composition Br 4H 2 O or Br2.8H20 are obtained (Bakhuis Roozeboom, Zeits.