AGENCIES IN DEPOSITION OF CALCIUM CARBONATE 731 



calcium is ever associated with carbonate. The fact of the con- 

 stancy of this solubility-product in presence of solid calcite 1 enables 

 us to calculate, with all the accuracy required for the purposes of 

 this paper, the solubility of calcite under any specified conditions, 

 e.g., in presence of calcium-ion or carbonate-ion from whatever 

 source derived, provided only that we can ascertain what these 

 ionic concentrations actually are. 



2. The concentration of H 2 C0 3 ("free" C0 2 )in solution is regu- 

 lated by the partial pressure (P) or proportion of C0 2 in the layer 

 of atmosphere in contact with the solution, and conversely; and, 

 for a given value of P, it diminishes with rising temperature, since 

 the absorption coefficient (solubility) of C0 2 diminishes. 



3. At a given temperature the total solubility as usually measured 

 — i.e., the total concentration of calcium in the solution — varies with 

 the concentration of H 2 C0 3 (hence with P), owing to the fact that 

 the latter determines the proportion of carbonate-ion CO 7, 

 hydrocarbonate-ion HCO7, and hydroxide-ion OH - in accordance 

 with definite mathematical expressions; and since the product 

 [Ca ++ ] [CO7] remains constant [Ca ++ ] must vary inversely as [CO7]. 

 The presence of other salts also affects this total solubility; so long 

 as pure calcite is the stable solid phase in equilibrium with the 

 solution, the magnitude of this effect is readily calculable, since the 

 several concentrations always adjust themselves until the solubility- 

 product [Ca ++ ] [CO 7] attains its characteristic value. 



4. The solubility-product constant of calcite diminishes with 

 rising temperature; it is not affected to an appreciable extent by 

 change of hydrostatic pressure. 



The mathematical expressions are given below: 



[H 2 C0 3 ] = cP 



[Ca ++ ] [CO 7] = K c (in presence of solid calcite) 

 [HC07MC07] = /[H 2 C0 3 ] =lcP 

 [OH-MCO7] = m/[H 2 C0 3 ] = m/cP 



where c, K c , I, and m are constants at any given temperature. 2 We 

 may note, moreover, that the free C0 2 and the total C0 2 (i.e., 



1 Similar remarks apply, mutatis mutandis, to impure calcite or to aragonite; 

 to this point we revert later. 



2 For their values and significance, see Johnston, op. cit., p. 201 1. 



