498 Johnston, Merwin,' and Williamson — 



under a wide variety of conditions may be calculated with con- 

 fidence.* 



The relative solubility of calcite and aragonite has been the 

 subject of several investigations, which finally have yielded 

 reasonably satisfactory results. Kohlrausch and Rose-r deter- 

 mined the specific conductance of conductivity water saturated 

 with respect to aragonite (rj A ) and to calcite (?7 C ), and Footed 

 made similar measurements in water previously saturated with 

 CO„ at 1 atm. From the ratio of these two specific conduct- 

 ances Va/Vc the ratio of the respective solubility -product con- 

 stants of aragonite (K A ) and calcite (Jl c ) is readily derived, 

 subject only to the assumption that the presence of CO, above 

 both solutions was identical ; for, from the equations discussed 

 in the previous paper§ 



K^_ [Ca++] A [CQ3] A _ [Ca-"-] A [HCQ 3 ] A ' _ / V A 3 

 K c - [Ca++] c [CO-Jc - [Ca++] c [HCC^y _ \ V J • 



Foote also determined the ratio K A /K C by means of compara- 

 tive experiments on the equilibrium CaC0 3 + T\-C 2 Q 4 ^ — > 

 CaC 2 4 + K 2 C0 3 ; but the results at the higher temperatures 

 are uncertain because, as Foote observed, a partial transforma- 

 tion of the aragonite to calcite occurred. || Kendall*" made 

 very careful direct determinations of the solubility of calcite 

 and aragonite in " freshly distilled water" which contained a 

 " trace of C0 2 " ; consequently the results can not be used as 

 absolute values of the solubility, though the ratio derived from 

 them is again significant provided that the partial pressure of 

 C0 2 was the same in both series of measurements. In this 

 case, if S A and S c are the respective solubilities (expressed in 

 moles per liter) of aragonite and calcite 



S 2 

 Kk [Ca++] A [HCOtU" 8 »-Jt rs 



&c [Ca- + ] c [HCO^Jc 2 s ^cl \ #c 



Seyler and Lloyd** made a couple of experiments " at the 

 temperature of the laboratory " in which they determined all 



* For the methods of calculation see the papers already cited. 



f Kohlrausch and Rose, Z. physik. Chern., xii, 239, 1893; ibid., xliv, 237, 

 1903. 



% Foote, ibid., xxxiii, 751, 1900. 



§ Johnston, J. Am. Chem. Soc, xxxyii, 2011, 1915. This particular rela- 

 tion was also derived by Foote (loc. cit.). 



I This implies that some grains of calcite were present in the material, for 

 we have found that pure aragonite is stable in presence of water at 100° for 

 some weeks and that this transformation is not appreciably accelerated in 

 presence of fine-grained calcium oxalate. 



If Kendall, Phil. Mag., xxiii, 958, 1912. 



** Seyler and Lloyd, J. Chem. Soc, xcv, 1347, 1909. 



