became a royal residence. During the middle ages it was a
considerable centre of commerce and shipping, and under the Hohenstaufen
emperors was raised to the rank of a free imperial city. In 1312,
however, the emperor Henry VII. pledged the town to his brother Baldwin,
archbishop-elector of Trier, and it remained in the possession of the
electors until it was absorbed by France during the Revolutionary epoch.
It was assigned by the congress of Vienna in 1815 to Prussia.

BORA, an Italian name for a violent cold northerly and northeasterly
wind, common in the Adriatic, especially on the Istrian and Dalmatian
coasts. There is always a northern tendency in the winds on the north
Mediterranean shores in winter owing to the cold air of the mountains
sliding down to the sea where the pressure is less. When, therefore, a
cyclone is formed over the Mediterranean, the currents in its
north-western area draw the air from the cold northern regions, and
during the passage of the cyclone the bora prevails. The bora also
occurs at Novorossiysk on the Black Sea. It is precisely similar in
character to the mistral which prevails in Provence and along the French
Mediterranean littoral.

BORACITE, a mineral of special interest on account of its optical
anomalies. Small crystals bounded on all sides by sharply defined faces
are found in considerable numbers embedded in gypsum and anhydrite in
the salt deposits at Luneburg in Hanover, where it was first observed in
1787. In external form these crystals are cubic with inclined
hemihedrism, the symmetry being the same as in blende and tetrahedrite.
Their habit varies according to whether the tetrahedron (fig. 1), the
cube (fig. 2). or the rhombic dodecahedron (fig. 3) predominates.
Penetration twins with a tetrahedron face as twin-plane are sometimes
observed. The crystals vary from translucent to transparent, are
possessed of a vitreous lustre, and are colourless or white, though
often tinged with grey, yellow or green. The hardness is as high as 7 on
Mohs' scale; specific gravity 3.0. As first observed by R.J. Hauy in
1791, the crystals are markedly pyroelectric; a cube when heated becomes
positively electrified on four of its corners and negatively on the four
opposite corners. In a crystal such as represented in fig. 3, the
smaller and dull tetrahedral faces s are situated at the analogous poles
(which become positively electrified when the crystal is heated), and
the larger