370 THE EMPTY QUARTER

by Mr. Philby as * accretions/ evidently without any knowledge
of their real nature. They are fulgurites or lightning-tubes,
produced when discharges of lightning strike the ground, the
intense heat of the powerful spark fusing the quartz sand along
the path of the electric current. The inside of such tubes of
silica-glass is always smooth and glazed, and the outside is rough
with adhering grains of partly fused sand. The walls are usually
very thin and friable, and specimens can be collected only as
fragments. Though roughly cylindrical, the tubes taper away
downwards and are sometimes branched. When flattened they
often have thin flanges of fused material projecting from the sides
of the tube : these have been called * winged fulgurites.' The most
complete specimen in the Mineral Collection of the British Museum
is a portion 3 feet in length of a tube 1-1 £ inches in diameter, from
Maldonado, Uruguay ; and the longest, but made up of pieced
fragments, is nearly 16 feet in length, from near Dresden. A
fulgurite in the sand-dunes at Drigg on the coast of Cumberland
was traced for 30 feet without reaching the end. Specimens are
very rarely found and are not common in collections.

The specimens collected by Mr. Philby are fragments found
loose on the surface of the sand. They were found in hollows
between the sand-dunes, and an interesting fact is that they are
taken by the Arabs as indications of the presence of water. No
doubt the lightning would strike in the wetter and more conduct-
ing parts of the ground.

(1). From Bard Jallab (collected 9/2/32).—Narrow tubes | cm,
in diameter, the longest fragment 7 cm. in length. These are
cylindrical and straight and are remarkable in having very
thick walls, there being only a thread-like central cavity. Nearly
the whole of the fulgurite here consists of a grey silica-glass full of
small bubbles. This suggests that the lightning struck dry sand,
and that there was very little radial expansion of the tube caused
by the pressure of water-vapour. One piece shows a division
into two branches.

(2). Bani Jafnan (18/2/32).—Fragments of irregular shape
with spine-like projections, Here the walls are thin and the
central cavity large with inside glazing. The largest fragment is
3J cm. long with a flattened cross-section of about 1'J x J cm.

(3). Eibaka Qa'amiyat (24/2/32), about 40 miles west of
Shanna.—Broken fragments of flattened tubes with projecting
spines and wings. The walls are thick but with a well-marked
central cavity, being intermediate between those in (1) and (2).
The largest fragment is 4 cm. long with a cross-section of about
2x1 cm.