Water charged with carbonic anhydride brings about the destruction of many rocky formations by removing the lime, alkali, &c., from them. This process has been going on and continues on an enormous scale. Rocks contain silica and the oxides of various metals; amongst others, the oxides of aluminium, calcium, and sodium. Water charged with carbonic acid dissolves both the latter, transforming them into carbonates. The waters of the ocean ought, as the evolution of the carbonic anhydride proceeds, to precipitate salts of lime; these are actually found everywhere on the surface of the ground in those places which previously formed the bed of the ocean. The presence of carbonic anhydride in solution in water is essential to the nourishment and growth of water plants.
Although carbonic anhydride is soluble in water, yet no definite hydrate is formed; nevertheless an idea of the composition of this hydrate may be formed from that of the salts of carbonic acid, because a hydrate is nothing but a salt in which the metal is replaced by hydrogen. As carbonic anhydride forms salts of the composition K 2 CO 3 , Na 2 CO 3 , HNaCO 3 , &c., therefore carbonic acid ought to have the composition H 2 CO 3 —that is, it ought to contain CO 2 + H 2 O. Whenever this substance is formed, it decomposes into its component parts—that is, into water and carbonic anhydride. The acid properties of carbonic anhydride are demonstrated by its being directly absorbed by alkaline solutions and forming salts with them. In distinction from nitric, HNO 3 , and similar monobasic acids which with univalent metals (exchanging one atom for one atom of hydrogen) give salts such as those of potassium, sodium, and silver containing only one atom of the metal (NaNO 3 , AgNO 3 ), and with bivalent metals (such as calcium, barium, lead) salts containing two acid groups—for example, Ca(NO 3 ) 2 , Pb(NO 3 ) 2 —carbonic acid, H 2 CO 3 , is bibasic , that is contains two atoms of hydrogen in the hydrate or two atoms of univalent metals in their salts: for example, Na 2 CO 3 is washing soda, a normal salt; NaHCO 3 is the bicarbonate, an acid salt. Therefore, if M′ be a univalent metal, its carbonates in general are the normal carbonate M′ 2 CO 3 and the acid carbonate, M′HCO 3 ; or if M″ be a bivalent metal (replacing H 2 ) its normal carbonate will be M″CO 3 ; these metals do not usually form acid salts, as we shall see further on. The bibasic character of carbonic acid is akin to that of sulphuric acid, H 2 SO 4 , but the latter, in distinction from the former, is an example of the energetic or strong acids (such as nitric or hydrochloric), whilst in carbonic acid we observe but feeble development of the acid properties; hence carbonic acid must be considered a weak acid . This conception must, however, be taken as only comparative, as up to this time there is no definitely established rule for measuring the energy of acids. The feeble acid properties of carbonic acid may, however, be judged from the joint evidence of many properties. With such energetic alkalis as soda and potash, carbonic acid forms normal salts, soluble in water, but having an alkaline reaction and in many cases themselves acting as alkalis. The acid salts of these alkalis, NaHCO 3 and KHCO 3 , have a neutral reaction on litmus, although they, like acids, contain hydrogen, which may be exchanged for metals. The acid salts of such acids—as, for instance, of sulphuric acid, NaHSO 4 —have a clearly defined acid reaction, and therefore carbonic acid is unable to neutralise the powerful basic properties of such alkalis as potash or soda. Carbonic acid does not even combine at all with feeble bases, such as alumina, Al 2 O 3 , and therefore if a strong solution of sodium carbonate, Na 2 CO 3 , be added to a strong solution of aluminium sulphate, Al 2 (SO 4 ) 3 , although according to double saline decompositions aluminium carbonate, Al 2 (CO 3 ) 3 , ought to be formed, the carbonic acid separates, for this salt splits up in the presence of water into aluminium hydroxide and carbonic anhydride: Al 2 (CO 3 ) 3 + 3H 2 O = Al 2 (OH) 6 + 3CO 2 . Thus feeble bases are unable to retain carbonic acid even at ordinary temperatures. For the same reason, in the case of bases of medium energy, although they form carbonates, the latter are comparatively easily decomposed by heating, as is shown by the decomposition of copper carbonate, CuCO 3 ( see Introduction), and even of calcium carbonate, CaCO 3 . Only the normal (not the acid) salts of such powerful bases as