ower of Pisa; whilst the marble
pillars in the basement remain scarcely altered, the granite ones have
lost a considerable portion of their surface, which falls off continually
in scales, and exhibits everywhere stains from the formation of peroxide
of iron. The kaolin, or clay, used in most countries for the manufacture
of fine porcelain or china, is generally produced from the feldspar of
decomposing granite, in which the cause of decay is the dissolution and
separation of the alkaline ingredients.

_Eub_.--I have seen serpentines, basalts, and lavas which internally were
dark, and which from their weight, I should suppose, must contain oxide
of iron, superficially brown or red, and decomposing. Undoubtedly this
was from the action of water impregnated with air upon their ferruginous
elements.

_The Unknown_.--You are perfectly right. There are few compound stones,
possessing a considerable specific gravity, which are not liable to
change from this cause; and oxide of iron amongst the metallic substances
anciently known, is the most generally diffused in nature, and most
concerned in the changes which take place on the surface of the globe.
The chemical action of carbonic acid is so much connected with that of
water, that it is scarcely possible to speak of them separately, as must
be evident from what I have before said; but the same action which is
exerted by the acid dissolved in water is likewise exerted by it in its
elastic state, and in this case the facility with which the quantity is
changed makes up for the difference of the degree of condensation. There
is no reason to believe that the azote of the atmosphere has any
considerable action in producing changes of the nature we are studying on
the surface; the aqueous vapour, the oxygen and the carbonic acid gas,
are, however, constantly in combined activity, and above all the oxygen.
And, whilst water, uniting its effects with those of carbonic acid, tends
to disintegrate the parts of stones, the oxygen acts upon vegetable
matter. And this great chemical agent is at once necessary, in all the
processes of life and in all those of decay, in which Nature, as it were,
takes again to herself those instruments, organs, and powers, which had
for a while been borrowed and employed for the purpose or the wants of
the living principle. Almost everything effected by rapid combinations
in combustion may also be effected gradually by the slow absorption of
oxygen; a