n and iodine do not combine directly, but gaseous
hydriodic acid reacts with amorphous boron to form the iodide, BI3,
which can also be obtained by passing boron chloride and hydriodic
acid through a red-hot porcelain tube. It is a white crystalline solid
of melting point 43 C.; it boils at 210 deg. C., and it can be
distilled without decomposition. It is decomposed by water, and with a
solution of yellow phosphorus in carbon bisulphide it gives a red
powder of composition PBI2, which sublimes _in vacuo_ at 210 deg. C.
to red crystals, and when heated in a current of hydrogen loses its
iodine and leaves a residue of boron phosphide PB.

Boron nitride BN is formed when boron is burned either in air or in
nitrogen, but can be obtained more readily by heating to redness in a
platinum crucible a mixture of one part of anhydrous borax with two
parts of dry ammonium chloride. After fusion, the melt is well washed
with dilute hydrochloric acid and then with water, the nitride
remaining as a white powder. It can also be prepared by heating
borimide B2(NH)3; or by heating boron trioxide with a metallic
cyanide. It is insoluble in water and unaffected by most reagents, but
when heated in a current of steam or boiled for some time with a
caustic alkali, slowly decomposes with evolution of ammonia and the
formation of boron trioxide or an alkaline borate; it dissolves slowly
in hydrofluoric acid.

Borimide B2(NH)3 is obtained on long heating of the compound B2S3.6NH2
in a stream of hydrogen, or ammonia gas at 115-120 deg. C. It is a
white solid which decomposes on heating into boron nitride and
ammonia. Long-continued heating with water also decomposes it slowly.

Boron sulphide B2S3 can be obtained by the direct union of the two
elements at a white heat or from the tri-iodide and sulphur at 440
deg. C., but is most conveniently prepared by heating a mixture of the
trioxide and carbon in a stream of carbon bisulphide vapour. It forms
slightly coloured small crystals possessing a strong disagreeable
smell, and is rapidly decomposed by water with the formation of boric
acid and sulphuretted hydrogen. A pentasulphide B2S5 is prepared, in
an impure condition, by heating a solution of sulphur in carbon
bisulphide with boron iodide, and forms a white crystalline powder
which decomposes under the influence of water into sulphur,
sulphuretted hydrogen and boric acid.