240 Natural History of Volcanos and Harthquakes. 
suvius of the year 1785, melted at 18° of Wedgewood’s pyrome- 
ter, lava from Torre del Greco not till 40°. But their fusibility 
varied very considerably, according as the melted lava had been 
cooled rapidly to a glass, or more slowly to a stony erystalline 
mass. Thus, for example, those two lavas, when in the form of 
a glass, both melted at the same degree, (18°,) whilst the lava of 
1785 was less fusible than that of Torre del Greco, when of a 
stony nature.* From other appearances it may, in general, be 
concluded, that the fusibility of lavas is between that of silver 
and copper. Thus in the lava which destroyed Torre del Greco, | 
some gold and a few copper coins were found unmelted ; but the 
silver coins were melted and baked together with some copper 
coins.t Davy found that a copper-wire of ;'; of an inch in diam- 
eter, and a silver-wire of ,'; of an inch, thrust into the lava neat 
its source, instantly melted.{ A wire of copper $ of an inch in 
diameter, which I held in a stream of fused basalt, flowing out 
from a furnace, melted immediately. But the basalt was doubt- 
less heated far above its fusing point. Now according to Daniel,$ 
silver melts at 2233° F., but copper at 2548° F*.; we may there- 
fore take a mean of 2282° F. (=100U° R.) for the melting point 
of lava. sa 
Now, if we suppose the increase of temperature to sane 
follow the same progression as has been discovered in accessl id 
depths, the lava must be in a state of fusion, according to the 0 é 
servations near Geneva and in Cornwall, at the depth of a 
113505 feet, and from those in the Erzgebirge at about 12682 
feet below the level of the sea near Vesuvius or Etna.|| ~ 
: Par verted, 
* Glass is well known to be acted upon in a similar manner. When te ‘cs 
by being melted and slowly cooled again, into Reaumar’s porcelain, it beco 
usible. 
ns fa 
t Thompson : Notices of an English Traveller, &c. Breislak. (Voyage bag 4 
Camp., vol. i, p. 279,) mentions, that when bell-metal was plunged inte ne 
the zinc melted out, leaving the copper behind. 
_ £ Ann. de Chim. et de Phys. vol. xxxviii, p. 138. 
§ Journ. of Science, xxiii. yen 
|| According to my observations made on a cooling basalt-ball of pebstrr ss 
inches diameter, and which I shall communicate afterwards, the paeaaea om 
perature from the surface towards the centre of the earth, seems to take Fb. 
in an arithmetical, but ina geometrical progression. But the exponent 0 Begs 
gression being very little greater than 1, this progression comes very. - propor 
arithmetical one. The depths, above calculated, being but insignificant supposing 
tion to the diameter of the globe, no great error has been committed in 
