506 Johnston, Merwin\ and Williamson — 



our experiments, carried out as described previously (p. 480), 

 considerable supersaturation occurred invariably ; in general no 

 precipitate was observed until from 20 to 50 cc of each solution 

 bad been added, the actual amount depending mainly on the 

 snbstance initially present in the beaker. This amount of 

 supersaturation was unquestionably greatest in those experi- 

 ments in which the less stable products appeared, particularly 

 so when the solutions were alkaline, whence we infer that the 

 presence of alkali favors supersaturation in the case of CaC0 3 . 

 This action of the alkali* may be plausibly interpreted in two 

 ways : 



(1) that it serves as a peptising agent, just as it does with 

 many colloids, keeping down the size of the particles ; 



(2) there is ground for the belief — though it cannot be 

 regarded as established — that calcite appears more readily 

 when the concentration of bicarbonate ([HC0 3 ~]) is higher ; on 

 which basis any factor such as free alkali (i. e. [OH - ]) which 

 diminishes [HC0 3 ~] will be prejudicial to the formation of 

 calcite, and consequently promote the appearance of the less 

 stable forms, f Incidentally it may be remarked that, if this 

 belief be correct, the point at which the solution is absolutely 

 neutral will possess no special significance in connection with 

 the appearance of one or the other formj because the acidity 

 or alkalinity of the solution is not a direct measure of the con- 

 centration of HC0 3 ~ ; in other words, in this case (and probably 

 also in analogous cases, sulphides for instance), the point of 

 absolute neutrality is not a definite limit beyond which either 

 form may not appear. 



However this may ultimately prove to be, our work leads us 

 to believe that, in the absence of nuclei ensuring the precipita- 

 tion of calcite or aragonite, the initial precipitate at tempera- 

 tures ranging from about 15° up to 70°, or even higher, is the 

 /u,-form, alone or mixed with the other forms. On this basis 

 one may account for the precipitation of aragonite at tempera- 

 tures upwards of 70° on the plausible supposition that at such 

 temperatures the rate of transformation of the initially-formed 

 very fine j3articles of these unstable forms has become so great 

 that they cease to appear, the result being that the next in 

 order of stability precipitates. 



too soluble. This may be con-elated with the fact that soluble substances 

 are seldom, if ever, encountered as unstable forms, except when the solu- 

 bilities of these two forms are exceedingly close together (as in the immedi- 

 ate neighborhood of the transition point). 



* In other cases, on the other hand, alkalinity seems to favor the precipita- 

 tion of the stable form ; see Allen, Crenshaw, and Merwin, this Journal, 

 xxxiv, 341, 1912; ibid., xxxviii, 393. 1914, on the forms of ZnS, HgS and 

 FeS 2 . 



f Nevertheless " vaterite," which we believe to be a special habit of cal- 

 cite, appears most readily in alkaline solution. 



