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 crystallisation. 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 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.
Pages:
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149