474 Johnston, Merwin) and Williamson — 



legitimately be drawn from them ; this the authors have 

 endeavored to do. In regard to some of the doubtful points 

 considerable experimental work has been done in order to 

 verify or amplify the statements found in the literature ;* there 

 are yet some points which remain unsettled and must continue 

 to remain so until more is known generally about the relations 

 between unstable and stable forms and in particular of the 

 factors which determine whether one form or another shall 

 appear. This article, it is hoped, will help to dispel some of 

 the errors and misconceptions which have been current, and to 

 indicate some of the points upon which present knowledge is 

 unsatisfactory. 



At ordinary temperature calcium carbonate occurs in at least 

 three anhydrous crystalline forms, viz., the two well-known forms 

 calcite and aragonite, and another which we have designated 

 /i-CaCO s . At atmospheric pressure and at all temperatures 

 from 970° down to 0° (or lower), calcite is the stable form ; 

 under these conditions aragonite is relatively unstable, though 

 its transformation to calcite may under certain circumstances 

 be so slow as to be inappreciable, while the third form is still 

 less stable and transforms more readily. Four other reputed 

 forms, the so-called vaterite, conchite, ktypeite, and lublinite, 

 have also been described ; there seems now to be little doubt 

 that vaterite is a porous calcite, that conchite and ktypeite are 

 porous aragonite, and that lublinite is merely a special variety 

 of calcite. In the literature, moreover, there is occasional refer- 

 ence to " amorphous" calcium carbonate as if it were a distinct 

 form ; this question also we discuss later. A number of 

 hydrates of calcium carbonate have also been described ; but 

 with the exception of one, the hexahydrate, CaC0 3 .6H 2 0, their 

 definite existence is uncertain, since the experimental evidence 

 leaves very much to be desired. 



The authors' own experimental work has been. concerned 

 with the preparation and properties of the three indubitably 

 distinct anhydrous forms and with the hexahydrate. Of these 

 calcite alone is stable, at atmospheric pressure and at temper- 

 atures above 0° ; consequently the others persist only under 

 circumstances such that their rate of transformation to the more 

 stable form is small. Correlated with this, no doubt, is the 

 fact that all methods for the preparation of the unstable forms 

 are occasionally unsuccessful, and for no apparent reason ; con- 

 sequently it is difficult to make any categorical statement as to 

 the factors which favor the production of any one form. The 



* The authors have not attempted to give a reference for every statement 

 cited, as it did not seem worth while, in view - of the vast amount of litera- 

 ture pertaining to calcium carbonate, to trace each statement to its source ; 

 they realize moreover the probability that most of the ideas on which the 

 present treatment is based have, in some form, been discussed previously. 



