s go into solution, and the insoluble boron remains
as a brown powder, which may by filtered off and dried. H. Moissan
(_Ann. Chim. Phys._, 1895, 6, p. 296) heats three parts of the oxide
with one part of magnesium powder. The dark product obtained is washed
with water, hydrochloric acid and hydrofluoric acid, and finally
calcined again with the oxide or with borax, being protected from air
during the operation by a layer of charcoal. Pure amorphous boron is a
chestnut-coloured powder of specific gravity 2.45; it sublimes in the
electric arc, is totally unaffected by air at ordinary temperatures, and
burns on strong ignition with production of the oxide B2O3 and the
nitride BN. It combines directly with fluorine at ordinary temperature,
and with chlorine, bromine and sulphur on heating. It does not react
with the alkali metals, but combines with magnesium at a low red heat to
form a boride, and with other metals at more or less elevated
temperatures. It reduces many metallic oxides, such as lead monoxide and
cupric oxide, and decomposes water at a red heat. Heated with sulphuric
acid and with nitric acid it is oxidized to boric acid, whilst on fusion
with alkaline carbonates and hydroxides it gives a borate of the alkali
metal. Like silicon and carbon, very varying values had been given for
its specific heat, until H.F. Weber showed that the specific heat
increases rapidly with increasing temperature. By strongly heating a
mixture of boron trioxide and aluminium, protected from the air by a
layer of charcoal, F. Wohler and H. Sainte-Claire Deville obtained a
grey product, from which, on dissolving out the aluminium with sodium
hydroxide, they obtained a crystalline product, which they thought to be
a modification of boron, but which was shown later to be a mixture of
aluminium borides with more or less carbon. Boron dissolves in molten
aluminium, and on cooling, transparent, almost colourless crystals are
obtained, possessing a lustre, hardness and refractivity near that of
the diamond. In 1904 K.A. Kuhne (D.R.P. 147,871) described a process in
which external heating is not necessary, a mixture of aluminium
turnings, sulphur and boric acid being ignited by a hot iron rod, the
resulting aluminium sulphide, formed as a by-product, being decomposed
by water.

Boron hydride has probably never been isolated in the pure condition;
on heating boron trioxide with magnesium filings, a magnesium boride
Mg3B2 is obtained, an