THE CHEMICAL PROCESSES OF SOIL FORMATION. I7 



applied to rocks; while the corresponding but more complex 

 action within the soil itself is usually termed fallowing. 



Effects of Water. Since but few substances, particularly 

 among those forming rocks, are totally insoluble even in pure 

 water, 1 and some (such as gypsum) may be considered easily 

 soluble in the same, the rain water must exert solvent action 

 wherever it penetrates. In nature, however, strictly pure water i 

 does not occur, it being difficult to obtain it even artificially. \ 

 Among the " impurities " almost always contained in natural 

 water, there are several that materially increase its solvent 

 power. Foremost among these, both because almost univer- 

 sally present and on account of its great ultimate efficacy, is 



Carbonic dio.vid, in contact with water forming carbonic 

 acid, the acidulous ingredient of all effervescent waters, the 

 gas which is produced in nature by innumerable processes, 

 such as decay, putrefaction, fermentation, the slow or rapid 

 combustion of vegetable and animal substances, such as wood, 

 charcoal and all other fuels; by the respiration of animals; 

 in the burning of limestone, etc. It is therefore of necessity 

 contained in air, on an average to the extent of about 1-3000 of 

 its bulk in the general atmosphere, but locally in considerably 

 higher proportions because of proximity to sources of forma- 

 tion, and of its greater density as compared with air ( i ]/ 2 as 

 against i). It may thus accumulate in inhabited buildings, in 

 cellars, wells, mines, caves ; and it is contained in considerable 

 proportion in the air of the soil. Moreover, being easily soluble 

 in water (to the extent of an equal volume at the ordinary 

 temperature and barometric pressure) it is contained in all 

 natural water, whether of rains, rivers, springs or wells, and 

 largely of course in that percolating the soil. Such waters 

 may therefore be considered as being acid solvents; and as 

 such, they exercise a far more energetic and far-reaching effect 

 than would pure water. 



Carbonated water a universal solvent. While limestones 

 are the rocks most obviously acted upon by carbonated water, 

 few if any resist it altogether. Even quartz rocks of the ordi- 

 nary kinds are attacked by it ; only the purest white crystalline 

 quartzite may be considered as sensibly proof against it. 



1 See Chapter 18. 



