60 THE POPULAR SCIENCE MONTHLY. 



Obviously the most general trait is the greater amount of change 

 wrought on the outer surface than in the inner mass. In so far as the 

 matters of which the medium is composed come into play, the unavoid- 

 able implication is that they act more on the parts directly exposed to 

 them than on the parts sheltered from them. And in so far as the 

 forces pervading the medium come into play, it is manifest that, ex- 

 cluding gravity, which affects outer and inner parts indiscriminately, 

 the outer parts have to bear far 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 con- 

 centrations 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 sup- 

 plied by an old cannon-ball that has been long lying exposed. A coat- 

 ing of rust, formed of flakes within flakes, incloses it ; and this thick- 

 ens 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 oxida- 

 tion 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 illus- 

 trates the general truth. Occasionally a " rocking-stone " is thus pro- 

 duced. There are formed successive layers relatively friable in text- 

 ure, 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 eight or ten 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. 



If, now, inorganic masses, relatively so stable in composition, thus 

 have their outer parts differentiated from their inner parts, what must 



