A paper upon the sulphides of boron is communicated by M. Paul
Sabatier to the September number of the _Bulletin de la Societe
Chimique. Nature_ gives the following: Hitherto only one compound of
boron with sulphur has been known to us, the trisulphide, B_{2}S_{3},
and concerning even that our information has been of the most
incomplete description. Berzelius obtained this substance in an impure
form by heating boron in sulphur vapor, but the first practical mode
of its preparation in a state of tolerable purity was that employed by
Wohler and Deville. These chemists prepared it by allowing dry
sulphureted hydrogen gas to stream over amorphous boron heated to
redness. Subsequently a method of obtaining boron sulphide was
proposed by Fremy, according to which a mixture of boron trioxide,
soot, and oil is heated in a stream of the vapor of carbon bisulphide.
M. Sabatier finds that the best results are obtained by employing the
method of Wohler and Deville. The reaction between boron and
sulphureted hydrogen only commences at red heat, near the temperature
of the softening of glass. When, however, the tube containing the
boron becomes raised to the temperature, boron sulphide condenses in
the portion of the tube adjacent to the heated portion; at first it is
deposited in a state of fusion, and the globules on cooling present an
opaline aspect. Further along the tube it is slowly deposited in a
porcelain like form, while further still the sublimate of sulphide
takes the form of brilliant acicular crystals. The crystals consist of
pure B_{2}S_{3}; the vitreous modification, however, is usually
contaminated with a little free sulphur. Very fine crystals of the
trisulphide may be obtained by heating a quantity of the
porcelain-like form to 300° at the bottom of a closed tube whose upper
portion is cooled by water. The crystals are violently decomposed by
water, yielding a clear solution of boric acid, sulphureted hydrogen
being evolved. On examining the porcelain boat in which the boron had
been placed, a non-volatile black substance is found, which appears to
consist of a lower sulphide of the composition B_{4}S. The same
substance is obtained when the trisulphide is heated in a current of
hydrogen; a portion volatilizes, and is deposited again further along
the tube, while the residue fuses, and becomes reduced to the
unalterable subsulphide B_{4}S, sulphureted hydrogen passing away in
the stream of gas.
Two selenides of boron, B_{2}Se_{3} and B_{4}Se, corresponding to the
above described sulphides, have also been prepared by M. Sabatier, by
heating amorphous boron in a stream of hydrogen selenide, H_{2}Se. The
triselenide is less volatile than the trisulphide, and is pale green
in color. It is energetically decomposed by water, with formation of
boric acid and liberation of hydrogen selenide. The liquid rapidly
deposits free selenium, owing to the oxidation of the hydrogen
selenide retained in solution. Light appears to decompose the
triselenide into free selenium and the subselenide B_{4}Se.
Silicon selenide, SiSe_{3}, has likewise been obtained by M. Sabatier
by heating crystalline silicon to redness in a current of hydrogen
selenide. It presents the appearance of a fused hard metallic mass
incapable of volatilization. Water reacts most vigorously with it,
producing silicic acid, and liberating hydrogen selenide. Potash
decomposes it with formation of a clear solution, the silica being
liberated in a form in which it is readily dissolved by alkalies.
Silicon selenide emits a very irritating odor, due to the hydrogen
selenide which is formed by its reaction with the moisture of the
atmosphere. When heated to redness in the air it becomes converted
into silicon dioxide and free selenium.
* * * * *