substance called pumice, so much used for polishing. It may be
regarded as the solidified froth of lava, and is so light that it
floats on the surface of water.

The lavas of different mountains, when cooled and hardened, differ
much in their appearance and composition. Among those of Iceland is
found the beautiful black volcanic glass named obsidian. It is a
good deal used for ornamental purposes; for it possesses the
peculiar property of presenting a different appearance according to
the manner in which it is cut. When cut in one direction it is of a
beautiful jetty black; when cut across that direction it is
glistering gray. The lavas of Vesuvius are generally of a brown
colour, and are also used in the arts. In them are found the
beautiful olive-green crystals of the mineral called olivine,
sometimes used by jewellers. But the most useful of all volcanic
productions is native sulphur, in which Mount Etna has been very
prolific. It is to this mountain chiefly, therefore, that we are
indebted for our beautiful fire-works--our squibs, crackers, Roman
candles, serpents, Catherine-wheels, and sky-rockets. Would it had
produced nothing more harmful than these! But it has also supplied
one of the ingredients of that villainous gunpowder, which has been
the means of thrusting so many of our fellow-creatures prematurely
out of the world. Etna, however, can hardly be held responsible for
this sad misuse of the valuable substance which it affords; while
even gunpowder itself has, on the whole, been of vast benefit to
mankind. Could we only refrain from shooting each other with it, we
might regard it as an almost unmixed good; for it has helped us
greatly in forming our roads, railways, and tunnels, and in working
our quarries and mines.