copper, bronze,
brass, tin, and lead, whether they exist in stones, or are used for
support or connection in buildings, are liable to be corroded by water
holding in solution the principles of the atmosphere; and the rust and
corrosion, which are made, poetically, qualities of time, depend upon the
oxidating powers of water, which by supplying oxygen in a dissolved or
condensed state enables the metals to form new combinations. All the
vegetable substances, exposed to water and air, are liable to decay, and
even the vapour in the air, attracted by wood, gradually reacts upon its
fibres and assists decomposition, or enables its elements to take new
arrangements. Hence it is that none of the roofs of ancient buildings
more than a thousand years old remain, unless it be such as are
constructed of stone, as those of the Pantheon of Rome and the tomb of
Theodoric at Ravenna, the cupola of which is composed of a single block
of marble. The pictures of the Greek masters, which were painted on the
wood of the abies, or pine of the Mediterranean, likewise, as we are
informed by Pliny, owed their destruction not to a change in the colours,
not to the alteration of the calcareous ground on which they were
painted, but to the decay of the tablets of wood on which the intonaco or
stucco was laid. Amongst the substances employed in building, wood,
iron, tin, and lead, are most liable to decay from the operation of
water, then marble, when exposed to its influence in the fluid form;
brass, copper, granite, sienite, and porphyry are more durable. But in
stones, much depends upon the peculiar nature of their constituent parts;
when the feldspar of the granite rocks contains little alkali or
calcareous earth, it is a very permanent stone; but, when in granite,
porphyry, or sienite, either the feldspar contains much alkaline matter,
or the mica, schorl, or hornblende much protoxide of iron, the action of
water containing oxygen and carbonic acid on the ferruginous elements
tends to produce the disintegration of the stone. The red granite, black
sienite, and red porphyry of Egypt, which are seen at Rome in obelisks,
columns, and sarcophagi, are amongst the most durable compound stones;
but the grey granites of Corsica and Elba are extremely liable to undergo
alteration: the feldspar contains much alkaline matter; and the mica and
schorl, much protoxide of iron. A remarkable instance of the decay of
granite may be seen in the Hanging T