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 surfaces; thus a positively electrified cloud, acting even at a great distance on a moistened stone, tends to attract its oxygenous, or acidiform or acid, ingredients, and a negatively electrified cloud has the same effect upon its earthy, alkaline, or metallic matter.  And the silent and slow operation of electricity is much more important in the economy of Nature than its grand and impressive operation in lightning and thunder.  The chemical agencies of water and air are assisted by those of electricity; and their joint effects combined with those of gravitation and the mechanical ones I first described are sufficient to account for the results of time.  But the physical powers of Nature in producing decay are assisted likewise by certain agencies or energies of organised beings.  A polished surface of a building or a statue is no sooner made rough from the causes that have been mentioned than the seeds of lichens and mosses, which are constantly floating in our atmosphere, make it a place of repose, grow, and increase, and from their death, their decay, and decomposition carbonaceous