AGENCIES IN DEPOSITION OF CALCIUM CARBONATE 733 



The change of solubility with temperature, the proportion of 

 C0 2 being constant, is evident from Table II, which contains values 

 interpolated from the curve expressing the observations by Wells, 1 

 as well as the molar absorption-coefficient (c) of C0 2 and the calcu- 

 lated value (K f c ) at each temperature. There is a slight error 

 involved in identifying K' c with the solubility-product constant 

 [Ca ++ ] [CO 7] except at temperatures close to 18 , because in calcu- 

 lating K' c we have — -for lack of better knowledge — assumed that the 

 ratio (nr) of the first to the second ionization-constant of H 2 C0 3 is 

 independent of the temperature; nevertheless, since these values of 

 K' c were obtained from actual measurements of solubility, they 

 enable one to calculate 2 for any temperature up to 30 the solubility 

 of calcite under any conditions of C0 2 pressure or salt-concentration. 3 



TABLE II 



The Solubility of Calcite under Atmospheric Conditions 



(P = 0.0003 2), AND THE Solubility-Product Constant 



at Several Temperatures 



From the foregoing it follows that in order to decide definitely 

 if a natural water is saturated with respect to calcite one must know: 

 (a) the concentration of free C0 2 in the water, (b) the temperature, 

 (c) the concentrations of the other constituents present. Of these 

 the third is the only one which has in general been satisfactorily 

 ascertained, but it is only of subsidiary importance; experimental 

 data on the two important factors are commonly either lacking or 



1 R. C. Wells, Jour. Wash. Acad. Sri., V (1915), 617. 



2 For the mode of calculation see Johnston, op. rit., p. 201 1. 



3 This holds only so long as calcite is the stable phase. If the salt-concentration 

 is such that some other carbonate (e.g., a double carbonate) is the stable phase, the 

 appropriate constant must be employed in place of that characteristic of calcite. 



