_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; and though the productions of the animal and vegetable kingdom are much more submitted to the power of atmospheric agents than those of the mineral kingdom, yet, as in the instances which have just been mentioned, oxygen gradually destroys the equilibrium of the elements of stones, and tends to reduce into powder, to render fit for soils, even the hardest aggregates belonging to our globe. Electricity, as a chemical agent, may be considered not only as directly producing an infinite variety of changes, but likewise as influencing almost all which take place. There are not two substances on the surface of the globe that are not in different electrical relations to each other; and chemical attraction itself seems to be a peculiar form of the exhibition of electrical attraction; and wherever the atmosphere, or water, or any part of the surface of the earth gains accumulated electricity of a different kind from the contiguous surfaces, the tendency of this electricity is to produce new arrangements of the parts of these