(_J.
pr. Chem._, 1905, ii. p. 196).

Barium sulphate, BaSO_4, is the most abundant of the naturally occurring
barium compounds (see BARYTES) and can be obtained artificially by the
addition of sulphuric acid or any soluble sulphate to a solution of a
soluble barium salt, when it is precipitated as an amorphous white powder
of specific gravity 4.5. It is practically insoluble in water, and is only
very slightly soluble in dilute acids; it is soluble to some extent, when
freshly prepared, in hot concentrated sulphuric acid, and on cooling the
solution, crystals of composition BaSO_4 . H_2SO_4 are deposited. It is
used as a pigment under the name of "permanent white" or _blanc fixe_.

Barium nitride, Ba_3N_2, is obtained as a brownish mass by [v.03 p.0403]
passing nitrogen over heated barium amalgam. It is decomposed by water with
evolution of hydrogen, and on heating in a current of carbonic oxide forms
barium cyanide (L. Maquenne). Barium amide, Ba(NH_2)_2, is obtained from
potassammonium and barium bromide.

Barium nitrate, Ba(NO_3)_2, is prepared by dissolving either the carbonate
or sulphide in dilute nitric acid, or by mixing hot saturated solutions of
barium chloride and sodium nitrate. It crystallizes in octahedra, having a
specific gravity of 3.2, and melts at 597deg C. (T. Carnelley). It is
decomposed by heat, and is largely used in pyrotechny for the preparation
of green fire. Barium carbonate, BaCO_3, occurs rather widely distributed
as witherite (_q.v._), and may be prepared by the addition of barium
chloride to a hot solution of ammonium carbonate, when it is precipitated
as a dense white powder of specific gravity 4.3; almost insoluble in water.

Barium and its salts can be readily detected by the yellowish-green colour
they give when moistened with hydrochloric acid and heated in the
Bunsenflame, or by observation of their spectra, when two characteristic
green lines are seen. In solution, barium salts may be detected by the
immediate precipitate they give on the addition of calcium sulphate (this
serves to distinguish barium salts from calcium salts), and by the yellow
precipitate of barium chromate formed on the addition of potassium
chromate. Barium is estimated quantitatively by conversion into the
sulphate. The atomic weight of the element has been determined by C.
Marignac by the conversion of barium chloride into barium sulphate, and
also by a determination of the amount of silver required to prec