Is fusible only at
thirty-eight degrees of Wedgwood's pyrometer, yields a glass that
softens at fourteen degrees; and that this glass, melted again and
unvitrified (glastenized), is fusible again only at thirty-five
degrees of the same pyrometer. I applied the blowpipe to some black
pumice-stone from the volcano of the isle of Bourbon, which, on the
slightest contact with the flame, whitened and melted into an
enamel.
But whether obsidians be primitive rocks which have undergone the
action of volcanic fire, or lavas repeatedly melted within the
crater, the origin of the pumice-stones contained in the obsidian
of the Peak of Teneriffe is not less problematic. This subject is
the more worthy of being investigated, since it is generally
interesting to the geology of volcanoes; and since that excellent
mineralogist, M. Fleuriau de Bellevue, after having examined Italy
and the adjacent islands with great attention, affirms, that it is
highly improbable that pumice-stone owes its origin to the swelling
of obsidian.
The experiments of M. da Camara, and those I made in 1802, tend to
support the opinion, that the pumice-stones adherent to the
obsidians of the Peak of Teneriffe do not unite to them
accidentally, but are produced by the expansion of an elastic
fluid, which is disengaged from the compact vitreous matter.
