_Effects of Water._—Since but few substances, particularly among those forming rocks, are totally insoluble even in pure water,[9] 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 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 universally present and on account of its great ultimate efficacy, is _Carbonic dioxid_, 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 formation, and of its greater density as compared with air (1½ as against 1). 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. [9] See Chapter 18. _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 ordinary kinds are attacked by it; only the purest white crystalline quartzite may be considered as sensibly proof against it. Granite and the rocks related to it are rather quickly acted upon, because of the presence of the feldspar minerals containing potash, soda and lime as bases[10] together with alumina. [10] The increase of solvent power on feldspar when carbonated instead of distilled water is used, was well exemplified in an experiment made by Headden (Bull. 65, Color. Exp’t Sta., p. 29), who allowed pure distilled and carbonated water respectively to act on fresh but finely pulverized feldspar, with frequent shaking, for five days. The distilled water dissolved .0081 gram, the carbonated water, .0723 gram of solids, or nearly nine times as much as the distilled water. Both residues gave strong reactions for potash with platinic chlorid.