272 
Reviews and Extro.cts .— J\^atural History. 
that there exist many sub-marine Volcanoes, of which tlie island formed in 1707, 
near Santorin, those in 1638, 1720, and 1811, near St. Michael, in the Azores, 
that in 1783, on the south-west coast of Iceland, and other phenomena of a like 
jiature, afford ample proof. 
Volcanoes do not always continue in a state of activity, but experience periods 
of cessation, and even remain quiet for centuries, and some have now the appear¬ 
ance (of being' finally exhausted. The first symptoms of volcanic action, arc 
subterraneair noises, sliakings of the ground, and the emission of smoke by the 
crater. When the air is ag’itated, the smoke rolls along in immense clouds, which 
cover the surrounding country with'darkness j jets of burning sand and ashes from 
the Volcano traverse these columns, and clouds which sometimes emit lightnings. 
Then follow eruptions of incandescent masses of stone, which are projected into the 
air, and fall in all directions around the Volcano, in the form of sand, dust, cin¬ 
ders, or stones; the shakings of the earth continue ; the melted matter conlain(;d 
in the Volcano 'ascends into the crater, iind overflowing the lowest part of its edge, 
descends along the sides of the mountains, sometimes with great Velocity, but 
more usually in a stream flowing with slow and majestic motion. Frequently an 
opening takes place in the side of the mountain, through which the lava makes its 
escape. Enormous currents of mud and water burst forth, and torrents of rain 
fall from the atmosphere, while the noxious exhalations often spread over the sur¬ 
rounding country, and destroy animal and vegetable life. 
The smoke of Volcanoes generally consists of aqueous vapour, impregnated with 
hydrogen and carbonic acid gtises, or sulphuric acid. The ashes consist of pow¬ 
der, of a grey colour, in a state of extreme tenuity, and are usually mixed with 
sand. In the eruption of Hecla, in 1766, complete darkness was produced by the 
cloud of ashes to a great distance; and in 1812, a cloud of volcanic ashes and 
sand, from a Volcano in the island of St. Vincent’s, covered the whole of Bar!)a- 
does, producing such a degree of darkness, that at noon the nearest oliject could 
not be distinctly perceived. Volcanic sand consists of small cinders, usually mixed 
with crystals of augite and felspar, and forms the greater portion of the substances 
projected by Volcanoes. Scoriae, or cinders, consist of portions of the melted lava 
projected by the escape of elastic fluids. One of the most remarkable substances 
produced by Volcanoes is sulphur, which, Humboldt remarks, is very rare in cra¬ 
ters which are still burning, while all the old Volcanoes ultimate'y become solfa- 
terras. * 
Lava is the ma.ss of incandescent and melted matter, which carried upwards to 
the mouth of the crater by the force of the elastic fluirls, escapes at tlie lowest 
part of the brim, or through an aperture formed in the side of the mountain. By 
the eruption of the Skaptar Jokid, in Iceland, in 1783, two vast streams of lava 
were produced, one of which was fifty, the other forty miles in length, with a 
breadth in some places extending fifteen miles, and an average depth of one hun¬ 
dred feet. Von Buch, who witnessed the eruption of Vesuvius in 1805, saw a 
torrent descend from the summit to near the sea, over a space of more than seven 
thousand metres, (sibout four miles and a Iralf,) in three hours. In general, how¬ 
ever, currents of lava move with extreme slowness ; and Dolomieu mentions one 
which took two years to traverse a space of three hundred and eighty metres. 
The tenacity of lavas is usually so great that it is difficult to thrust a stick into 
them while they are flowing; and .‘Spallanzani hardly produced any Impression 
upon a current by throwing very l-urge stones upon it. Lavas take also an extra¬ 
ordinary time in cooling. Currents are mentioned which weie still progressing 
ten years after their eruption. Spalhunzani, while j)assing over a ciu'rent which 
had ceased to flow for eleven months saw throimh the cracks in its surface that it 
- o 
still was red. 
