] XTEBKSTIXG EXPEBUHSHTS. 
10 :? 
form of hillocks, like many of the porphyries in Auvergne, m 
the Euganean mountains, and in the Cordilleras of the Andes. 
The objections against the volcanic origin of obsidians, 
founded on their speedy loss of colour, and their swelling 
by a slow tire, have been shaken by the ingenious experi- 
ments of Sir James Hall. These experiments prove, that a 
stone which is fusible only at thirty-eight degrees of Wedg- 
wood’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 degree of the 
same pyrometer. I applied the blowpipe to some black 
pumice-stone from the volcano of the isle of Bourbon, which, 
?n the slightest contact with the flame, whitened and melted 
into an enamel. 
But whether obsidians be primitive rocks which have un- 
dergone 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 Tenerifl'e is not loss pro- 
blematic. This subject is the more worthy of being inves- 
tigated, since it is generally interesting to the geology of 
volcanoes ; and since that excellent mineralogist, Jl . Fleuriau 
do Bellevue, after having examined Italy and the adjacent 
islands with great attention, affirms, that it is highly im- 
probable 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 stone3 
adherent to the obsidians of the Peak of Tenerifl'e do not 
unite to them accidentally, but arc produced by the expan- 
sion ol an elastic fluid, which is disengaged from the compact 
' 'ri'eous matter. This idea had for a long time occupied the 
mind of a person highly distinguished for his talents and re- 
putation at Quito, who, unacquainted with the labours of the 
mineralogists ot Europe, had devoted himself to researches 
°n the volcanoes ot his country. Don Juan de Larea, 
mie oi those men lately sacrificed to the fury of faction, 
lad been struck with the phenomena exhibited by obsidians 
exposed to a white heat. He had thought, that, wherever 
loieanoes act in the centre of a country covered with por- 
r it °k s idi au , the elastic fluids must cause a 
w clung ot the liquified mass, and perform an important part in 
