r larger shares of their actions. If it is a question of heat, then
the exterior must lose it or gain it faster than the interior; and in a
medium which is now warmer and now colder, the two must habitually
differ in temperature to some extent--at least where the size is
considerable. If it is a question of light, then in all but absolutely
transparent masses, the outer parts must undergo more of any change
producible by it than the inner parts--supposing other things equal; by
which I mean, supposing the case is not complicated by any such
convexities of the outer surface as produce internal concentrations of
rays. Hence then, speaking generally, the necessity is that the primary
and almost universal effect of the converse between the body and its
medium, is to differentiate its outside from its inside. I say almost
universal, because where the body is both mechanically and chemically
stable, like, for instance, a quartz crystal, the medium may fail to
work either inner or outer change.

Of illustrations among inorganic bodies, a convenient one is supplied by
an old cannon-ball that has been long lying exposed. A coating of rust,
formed of flakes within flakes, incloses it; and this thickens year by
year, until, perhaps, it reaches a stage at which its exterior loses as
much by rain and wind as its interior gains by further oxidation of the
iron. Most mineral masses--pebbles, boulders, rocks--if they show any
effect of the environment at all, show it only by that disintegration of
surface which follows the freezing of absorbed water: an effect which,
though mechanical rather than chemical, equally illustrates the general
truth. Occasionally a "rocking-stone" is thus produced. There are formed
successive layers relatively friable in texture, each of which, thickest
at the most exposed parts, and being presently lost by weathering,
leaves the contained mass in a shape more rounded than before; until,
resting on its convex under-surface, it is easily moved. But of all
instances perhaps the most remarkable is one to be seen on the west bank
of the Nile at Philae, where a ridge of granite 100 feet high, has had
its outer parts reduced in course of time to a collection of
boulder-shaped masses, varying from say a yard in diameter to six or
eight feet, each one of which shows in progress an exfoliation of
successively-formed shells of decomposed granite: most of the masses
having portions of such shells partially detached.