The Several Forms of Calcium, Carbonate. 505 



upon the concentration, rate and mode of mixing of the con- 

 stituent solutions, as well as upon the solubility of the substance 

 in the medium in which it is being precipitated. This consti- 

 tutes the reason for using diffusion processes (as a means of 

 securing very slow mixing and thus keeping down the degree 

 of supersaturation) for the preparation of good crystals of spar- 

 ingly soluble substances,* and likewise for the occurrence in 

 nature of large crystals of substances which in the laboratory 

 appear only as very small crystals or even as " amorphous " 

 material. 



Now the precipitation of a substance, such as calcium car- 

 bonate, which appears in unstable forms, is a more complicated 

 question, though, as we believe, the above principles still hold. 

 When the product [Ca ++ ] [CO'j] has attained the value K Q 

 characteristic of calcite, the latter will begin to precipitate if 

 there be nuclei of itself or of some isomorphous substance ; 

 but if nuclei are absent, the product [Ca ++ ] [CO 3] may 

 increase until it has exceeded the value characteristic of one or 

 other, or of all, of the unstable forms and consequently the 

 precipitation of any of the three forms is now possible. The 

 form which actually precipitates is, in the absence of nuclei 

 isomorphous with any of these forms, determined presumably 

 by whichever nucleus first separates ; but as to which this is 

 likely to be under given conditions, nothing definite can be 

 stated at the present time. It may be remarked merely that, if 

 we suppose that all of the types of p re-nuclei are present simul- 

 taneously in the liquid, the question as to which appears is a 

 question of the relative probability of the configurations of the 

 several nuclei, and hence is not likely to be elucidated until 

 more is known about the real structure, and mode of growth, 

 of crystals. It is obvious, therefore, that the appearance of 

 any form is a matter of chance, in so far as we can tell, since 

 it depends upon factors which we cannot even specify. Never- 

 theless there are some indications of proximate causes which 

 we shall point out, though they cannot yet be interpreted 

 unambiguously. 



One essential condition is that the solution must be virtually 

 supersaturated with respect to the more stable forms before 

 the less stable forms can possibly separate out ; this implies 

 moreover, that unstable forms are likely to appear only if the 

 ratio of solubilities is not far removed from unity.f Now in 



* See Johnston, J. Am. Chera. Soc, xxxvi, 16, 1914. It maybe mentioned 

 that soluble substances, such as NaCl, which usually crystallize well, may be 

 obtained as very fine-grained "amorphous'' material by precipitating it 

 rapidly in a medium in which it is very sparingly soluble. 



f At a given temperature this ratio of solubilities of any two forms of the 

 same substance is affected little, if any, by change of solvent ; consequently 

 one may plausibly infer that in an endeavor to precipitate unstable forms of 

 any substance, one should choose a medium in which the substance is not 



Am. Jour. Soi. — Fourth Series, Vol. XLI, No. 246. — June, 1916. 

 35 



