and the Nature of the Liquidity of Lavas. 137 
such a bubble from the surface of the liquid mass of lava in the 
vent. In moderately tranquil eruptions these succeed each other 
at considerable intervals. In the ease of Stromboli, I noted that 
about five minutes usually occurred between every two explo- 
sions. When the eruption assumes a violent character, as in 
the Vesuvian one of 1822, the eructations, for such they are, 
succeed each other so rapidly as to produce an almost continuous 
roar, like the blowing-off of a thousand steam-boilers. And each 
explosion gives birth to one of those great globular volumes of 
white vapour, which, rolling over and over each other as they 
rise in the air in avast column, occasion one of the most remark- 
able and magnificent appearances of a paroxysmal volcanic erup- 
tion. In the midst of these clouds of snowy vapour, a black 
column of stones, scoria, and ashes may be seen to shoot up to 
a vast height, generally attended with copious discharges of elec- 
tricity generated by the friction of the ejected fragments, and 
forming a singular contrast to the jet of aériform matters. 
In some rare cases it is possible to witness the actual rise and 
bursting of these great bubbles of vapour. Spallanzani on his 
visit to Stromboli in 1780 saw the liquid surface of lava at a 
white heat within the orifice of the volcano surge alternately 
upwards, and after bursting like a great bubble, fall back again 
out of sight. In 1819 I was myself able to witness the same 
interesting phenomenon probably from the same position, a high 
point of the external crater-rim which overlooks the vent. At 
each belch, a shower of tattered fragments of lava, torn from the 
surface of the bubble as it broke, rose into the air with a cloud 
of yapour and a fierce roar, while steam seemed to be at inter- 
vals blowing off from another neighbouring vent. Hoffman, 
who visited the same volcano a few years later, describes in 
minute detail precisely the same phenomena. 
The vast size of some craters, already noticed, may afford a 
notion of the enormous volumes of gaseiform matter that must 
have been discharged through them at the time of their forma- 
tion by continuous explosions lasting for weeks and even months ; 
since each individual bubble of vapour must have been of a 
magnitude to fill the entire horizontal section of the crater ; and 
even for some time to aid in enlarging the area of this aperture 
by violent pressure against its rocky sides. The prodigious 
force with which they ascend, and therefore the great depth at 
which they are generated, may be judged from the vast vertical 
height, measured in miles, to which they have been seen to 
shoot up a continuous columnar fountain of ejections, consisting 
not merely of scoria and ashes, but often of rocky fragments of 
great size. 
These, by their mutual friction, as they alternately fall back 
