The chemical decomposition of rocks is primarily due to the action of the atmosphere, the average composition of which may be stated as follows: ================+==============================+================== | VOLUME PER CENT. | WEIGHT PER CENT. ----------------+------------------------------+------------------ Nitrogen | 78.00 | 75.55 Oxygen | 21.00 | 23.22 Carbonic dioxid | .03-.04 | .045-.060 Ammonia | 1 to 4 millionths | Water vapor | Variable; 48 to 83 grams per | | cubic meter, when saturated | | between O° and 50°C. | ----------------+------------------------------+------------------ In addition to the above, air contains minute amounts of the very indifferent and therefore practically negligible elements, argon, krypton, neon, xenon and helium, the aggregate amount of which in air is somewhat less than one per cent, of which the greater part is argon. So far as known these elements take no part whatever in vegetable or animal life, and possess no known chemical action or affinity. The primary active agents in effecting chemical changes in rocks by which soils are formed, are water, carbonic acid,[8] and oxygen; all therefore ingredients of the atmosphere. Hence the chemical changes so brought about are in the most general sense comprehended within the term weathering, as applied to rocks; while the corresponding but more complex action within the soil itself is usually termed fallowing. [8] Owing to the universal presence of water (H₂O) in air as well as in soils, it is usual and convenient to speak of carbonic dioxid (CO₂) gas when so occurring as carbonic acid (H₂CO₃), of which it produces the effects (CO₂ + H₂O = H₂CO₃).