The Diameter Of The Whole Tube Was Nearly Equal,
And Therefore We Must Suppose That Originally It Extended To
A Much Greater Depth.
These dimensions are however small,
compared to those of the tubes from Drigg, one of which
was traced to a depth of not less than thirty feet.
The internal surface is completely vitrified, glossy, and
smooth. A small fragment examined under the microscope
appeared, from the number of minute entangled air or perhaps
steam bubbles, like an assay fused before the blowpipe.
The sand is entirely, or in greater part, siliceous; but some
points are of a black colour, and from their glossy surface
possess a metallic lustre. The thickness of the wall of the
tube varies from a thirtieth to a twentieth of an inch, and
occasionally even equals a tenth. On the outside the grains
of sand are rounded, and have a slightly glazed appearance:
I could not distinguish any signs of crystallization. In a
similar manner to that described in the Geological Transactions,
the tubes are generally compressed, and have deep
longitudinal furrows, so as closely to resemble a shrivelled
vegetable stalk, or the bark of the elm or cork tree. Their
circumference is about two inches, but in some fragments,
which are cylindrical and without any furrows, it is as much
as four inches. The compression from the surrounding loose
sand, acting while the tube was still softened from the
effects of the intense heat, has evidently caused the creases
or furrows. Judging from the uncompressed fragments, the
measure or bore of the lightning (if such a term may be used)
must have been about one inch and a quarter. At Paris, M.
Hachette and M. Beudant [11] succeeded in making tubes, in
most respects similar to these fulgurites, by passing very
strong shocks of galvanism through finely-powdered glass:
when salt was added, so as to increase its fusibility, the tubes
were larger in every dimension, They failed both with
powdered felspar and quartz. One tube, formed with
pounded glass, was very nearly an inch long, namely .982,
and had an internal diameter of .019 of an inch. When we
hear that the strongest battery in Paris was used, and that
its power on a substance of such easy fusibility as glass was
to form tubes so diminutive, we must feel greatly astonished
at the force of a shock of lightning, which, striking the sand
in several places, has formed cylinders, in one instance of at
least thirty feet long, and having an internal bore, where not
compressed, of full an inch and a half; and this in a material
so extraordinarily refractory as quartz!
The tubes, as I have already remarked, enter the sand
nearly in a vertical direction. One, however, which was less
regular than the others, deviated from a right line, at the
most considerable bend, to the amount of thirty-three degrees.
From this same tube, two small branches, about a
foot apart, were sent off; one pointed downwards, and the
other upwards.
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