484 Johnston, Merwiri, and Williamson — 



in bis aragonite ; for, as noted above, pure aragonite can be 

 kept for days at 100° without appreciable transformation. 



^u.-CaC0 3 . 



In the earlier experiments, especially those made about 25°, 

 we observed that the precipitate contained a small proportion 

 of scale-like aggregates and hexagonal plates differing from the 

 bulk of the precipitate ; one refractive index was found to be 

 about 1'56, and it was noted that these aggregates gave the 

 same color tests as aragonite. One day, however, in endeavor- 

 ing to prepare good crystals of calcite, we found a considerable 

 proportion of this material which was, moreover, in better 

 formed crystals ; and successive experiments yielded a quantity 

 of it. Subsequently still better crystals of it were obtained, 

 mixed with aragonite, by conducting precipitation experiments 

 at about 60°, and this appears to be the best temperature for 

 its preparation. It is, however, impossible to specify the 

 optimum conditions for its appearance, for the result appears 

 to be, so far as we are now concerned, a matter of chance and 

 will probably remain so until the obscure determining factors 

 shall have been ascertained. 



One obvious prerequisite for the precipitation of an unstable 

 form such as yu-CaC0 3 * is that one must have a solution which 

 is virtually supersaturated with respect to both calcite and 

 aragonite, which are more stable, and hence less soluble than 

 is yti-CaCOg ; so that one must endeavor to minimize the effect 

 of those factors which tend to relieve supersaturationf — 

 particularly to avoid the presence of nuclei of the less solu- 

 ble (more stable) forms, and violent shaking of the solution. 

 Moreover in the endeavor to prepare large crystals of such an 

 unstable form, one is between the devil and the deep sea ; for 

 the slow precipitation necessary to the formation of large crys- 

 tals also favors the transformation to the more stable form. 



This form, which we have designated /*-CaC0 3 , is con- 

 taminated, when prepared as above, by calcite or aragonite ; 

 from either of which, however, it is easily separated by flota- 

 tion in a liquid of density about 2 - 6. Our products, purified 

 in this way, still contained a small proportion of the more 

 stable form, sufficient nevertheless to render illusory any 

 measurements of the solubility of /x-CaC0 3 . The material was 

 dried at 120° — a process which did not affect its appearance in 

 any way — and analyzed, with the following results: 



* A similar remark applies to the precipitation of any unstable form ; it 

 will be discussed more fully postea. 



■(•With respect to these factors see S. W. Young, J. A.m. Chem. Soc, 

 xxxiii, 148, 1911 ; S. W. Young and Ji. J. Cross, ibid., 1375. 



