WEATHERING 155 



same persistent activity that she showed in its making. 

 If we examine the flints built into a wall or a church, we 

 shall find that their black surface is now veiled with a 

 white film ; similarly, flint implements, which we may 

 be sure were quite black when freshly chipped into shape, 

 are now found covered with a white patina, which may 

 be of considerable thickness ; and, again, flint pebbles 

 may be met with in all stages of destruction, some when 

 broken open proving to be white all through. This 

 change of appearance is accompanied by a loss of sub- 

 stance ; the black flint is impervious, the white extremely 

 porous, as may be proved by applying it to the tongue, 

 when it will be found to adhere, or more elegantly by 

 immersing it in some staining fluid, when it will become 

 coloured by absorption. Thus the silica which under 

 certain conditions passes out of solution into the solid 

 form, under others is again dissolved away. The change 

 in conditions is possibly brought about by removal from 

 the enclosing chalk ; evidently it is a somewhat nicely- 

 balanced play of forces which determines whether solution 

 or crystallisation shall prevail. 



Having traced the silica of a flint to its source in 

 siliceous sponges, we may next inquire, Whence does the 

 sponge obtain its silica ? The only possible source would 

 appear to be the sea. The sponge lives by maintaining 

 an almost constant flow of sea-water through the walls 

 of its body. It thus obtains food and oxygen, and 

 probably the silica which forms its skeleton. Of course 

 there is a chance that some of this may be derived from 

 its food, but this consists of minute organisms, which in 

 their turn must have obtained any silica they contain 

 from sea-water. Still, in what follows we shall assume 

 that the silica is extracted directly from the sea. If now 

 we turn to chemical analyses of sea- water we shall find, 

 as we might expect, that they agree in indicating the 



