THE CHEMISTRY OF THE SOlL. 65 



(a) A large proportion of the minerals which form rocks are hydrated 

 silicates, containing a certain proportion of combined water. The hydration 

 of these minerals is accompanied by considerable increase in bulk, which 

 assists in the disintegration of the rock-mass, and also produces a substance 

 which is more readily attacked than the original silicate. A good example 

 of the expansion of rock on hydration is met with in tiie well-known opera- 

 tion of taking a cast in plaster of Paris, which is burnt gypsum, a substance 

 which frequently occurs in rock-masses. When plaster of Paris is moistened 

 with water it absorbs the water and becomes a hydrate, and in so doing 

 expands so that if a mould is made round a coin or medal and filled with 

 moistened plaster, this will expand in drying out and setting, and force itself 

 into the irregularities of the surface, forming a reproduction of the original 

 medal. 



(&) It is, however, the solvent action of water whioh plays the most 

 important part in the breaking up of rocks. Even pure water exercises a 

 slight solvent action on the minerals of which rocks are composed. If a 

 little finely-powdered felspar be shaken up with water, the water will show 

 an alkaline reaction, and turn red litmus paper blue — showing that some- 

 thing has been dissolved from the water. The same thing happens with 

 powdered glass. Absolutely pure water is, however, not met with in nature. 

 Natural water contains appreciable quantities of dissolved materials, some of 

 which, such as carbonic acid, ammonia, nitric acid, exert a very considerable 

 solvent action upon rock particles. 



Carbonic acid is present in all natural waters to a greater or less extent. 

 Even in rain-water the quantity, though small and very variable, is appreci- 

 able, and amounts to as much as 1 per cent. The solvent action of waters 

 holding carbonic acid in solution is very considerable. Some rocks, such as 

 limestone, are completely soluble in such a solvent. On this account waters 

 flowing over limestone rocks are invariably hard owing to their containing 

 carbonate of lime in solution. 



Ordinary spring and river waters do not, indeed, contain very much car- 

 bonic acid, but some springs contain a very considerable quantity of the gas 

 dissolved under pressure, and the effervescence of many mineral waters is 

 due to the escape of this gas when the pressure is released on reaching the 

 surface. 



Other rocks and minerals are not so readily dissolved as is carbonate of 

 lime, but all are very much more soluble in water containing this gas than 

 in pure water. Felspar, for example, is very readily attacked, the carbonic 

 acid removing the potash (or other base) and leaving a residue consisting 

 of pure clay. Even silica is attacked by it, and the minerals composed of 

 hydrated silica — such as flint, jasper, and even quartz — are sensibly attacked 

 by it. Quartzite is the only rock that may be regarded as practically 

 unattacked by carbonic acid. 



The ammonia contained in rain-water has also a very powerful solvent 

 action upon silicates. Finely-powdered glass is quite readily soluble in 

 ammonia; and the windows of stables, where organic matter is undergoing 

 decay and ammonia is evolved, are frequently frosted by the action of this 

 latter substance. 



Nitric acid in the air also aids in the solvent action of rain-water. 



The dissolved saline matter in rain-water also helps in the disintegration 

 of rocks, both by its chemical action and by its mechanical. In the neigh- 

 bourhood of the sea especially the air contains particles of salt which are 

 dissolved by the rain and carried into the minute abrasions and crevices 



t 54797-C 



