when a degradation from chemical or
mechanical causes takes place in their inferior parts. The forms upon
the surface of the globe are preserved from the influence of gravitation
by the attraction of cohesion, or by chemical attraction; but if their
parts had freedom of motion, they would all be levelled by this power,
gravitation, and the globe would appear as a plane and smooth oblate
spheroid, flattened at the poles. The attraction of cohesion or chemical
attraction, in its most energetic state, is not liable to be destroyed by
gravitation; this power only assists the agencies of other causes of
degradation. Attraction, of whatever kind, tends, as it were, to produce
rest--a sort of eternal sleep in Nature. The great antagonist power is
heat. By the influence of the sun the globe is exposed to great
varieties of temperature; an addition of heat expands bodies, and an
abstraction of heat causes them to contract; by variation of heat,
certain kinds of matter are rendered fluid, or elastic, and changes from
fluids into solids, or from solids or fluids into elastic substances, and
_vice versa_, are produced; and all these phenomena are connected with
alterations tending to the decay or destruction of bodies. It is not
probable that the mere contraction or expansion of a solid, from the
subtraction or addition of heat, tends to loosen its parts; but if water
exists in these parts, then its expansion, either in becoming vapour or
ice, tends not only to diminish their cohesion, but to break them into
fragments. There is, you know, a very remarkable property of water--its
expansion by cooling, and at the time of becoming ice--and this is a
great cause of destruction in the northern climates; for where ice forms
in the crevices or cavities of stones, or when water which has penetrated
into cement freezes, its expansion acts with the force of the lever or
the screw in destroying or separating the parts of bodies. The
mechanical powers of water, as rain, hail, or snow, in descending from
the atmosphere, are not entirely without effect; for in acting upon the
projections of solids, drops of water or particles of snow, and still
more of hail, have a power of abrasion, and a very soft substance, from
its mass assisting gravitation, may break a much harder one. The
glacier, by its motion, grinds into powder the surface of the granite
rock; and the Alpine torrents, that have their origin under glaciers, are
always turbid, fr