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, 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