ty, though excessively slow and gradual, yet
are much more efficient in the great work of destruction. It is to the
general chemical doctrines of the changes produced by this powerful agent
that I must now direct your especial attention.

_Eub_.--Would not the consideration of the subject have been more
distinct, and your explanations of the phenomena more simple, had you
commenced by dividing the causes of change into mechanical and chemical;
if you had first considered them separately, and then their joint
effects?

_The Unknown_.--The order I have adopted is not very remote from this.
But I was perhaps wrong in treating first of the agency of gravitation,
which owes almost all its powers to the operation of other causes. In
consequence of your hint, I shall alter my plan a little, and consider
first the chemical agency of water, then that of air, and lastly that of
electricity. In every species of chemical change, temperature is
concerned. But unless the results of volcanoes and earthquakes be
directly referred to this power, it has no chemical effect in relation to
the changes ascribed to time simply considered as heat, but its
operations, which are the most important belonging to the terrestrial
cycle of changes, are blended with, or bring into activity, those of
other agents. One of the most distinct and destructive agencies of water
depends upon its solvent powers, which are usually greatest when its
temperature is highest. Water is capable of dissolving, in larger or
smaller proportions, most compound bodies, and the calcareous and
alkaline elements of stones are particularly liable to this kind of
operation. When water holds in solution carbonic acid, which is always
the case when it is precipitated from the atmosphere, its power of
dissolving carbonate of lime is very much increased, and in the
neighbourhood of great cities, where the atmosphere contains a large
proportion of this principle, the solvent powers of rain upon the marble
exposed to it must be greatest. Whoever examines the marble statues in
the British Museum, which have been removed from the exterior of the
Parthenon, will be convinced that they have suffered from this agency;
and an effect distinct in the pure atmosphere and temperate climate of
Athens, must be upon a higher scale in the vicinity of other great
European cities, where the consumption of fuel produces carbonic acid in
large quantities. Metallic substances, such as iron,