480 Johnston, Merivin, and Williamson — 



a carbonate — e. g., PbC0 3 or SrC0 3 — isomorphous with aragon- 

 ite. Warth* states that aragonite is precipitated from alkaline 

 solutions at ordinary temperatures, but he was mistaken in this 

 conclusion because he did not use optical methods of identifica- 

 tion, but relied on the chemical tests which, as we have shown, 

 would not enable him to distinguish aragonite in such material 

 with any degree of certainty from the other forms. f Many 

 attempts have been made to find some relation between the 

 precipitation of aragonite and the substances present in the solu- 

 tion, apart from those which yield carbonates isomorphous 

 with the other forms of calcium carbonate. But to the authors 

 the results seem inconclusive ; for one thing, too few experi- 

 ments were performed, and for another, precipitations carried 

 out under apparently similar conditions do not always yield the 

 same result, as we have found. This is perhaps not surprising 

 if we consider that we are dealing with the appearance of an 

 unstable form, which however differs but little in solubility 

 from the stable form, and that the factors which really determine 

 crystal structure and habit are far from being clearly under- 

 stood yet. It has been considered that the presence of salts of 

 magnesium;}; or of ammonium§ favors the formation of aragon- 

 ite, but the main criteria used in these identifications were the 

 chemical tests which, as we have shown, are ambiguous; we 

 have performed a large number of experiments designed to 

 test this question but we have been unable to discover any 

 definite effect of the presence of magnesium or ammonium 

 salts. Indeed we tried out the effect of each of the other major 

 constituents of sea-water, but were unable to note any specific 

 effect except in the case of sulphate;! to this point we shall 

 return later. Moreover the statement, frequently met with, 

 that 30° is a limiting temperature above which aragonite, and 

 below which calcite, is precipitated, has no foundation in fact. 

 Our best crystallized aragonite was made by slow precipitation 

 in hot water at a temperature of 85-90°, a method which is suc- 

 cessful nearly always. The procedure is as follows : Two 

 burettes are fastened above a 800 cc beaker containing water (or a 

 solution of the salt whose influence is to be investigated) main- 

 tained at the proper temperature and provided with an efficient 



* Warth, Centralblatt Min. Geol., 1902, 492. 



f By reason of this same circumstance the conclusions of Meigen (Neues 

 Jahrb. Min., 1903, ii, 19), of Linck (Neues Jahrbuch, Beil. Bd., xvi, 495, 

 1903, and of others, are open to question. 



%G. Linck, Neues Jahrbuch, Beil. Bd., xvi, 495, 1903; F. Cornu, Oesterr. 

 Z. Berg. Hiittenwesen, lv, 1907; F. Vetter, Z. Kryst., xlviii, 45, 1911. 



§J. Peine, Inaug. Diss., Jena, 1913; abstract in Neues Jahrbuch, 1915, i, 

 309. 



|| Peine varied his sulphate concentration concurrently with the others, 

 but omitted to tfike this variation into account in interpreting his results. 

 Moreover he identified certain six-sided crystals or aggregates with aragonite, 

 whereas they were undoubtedly the form designated by us the /z-form. 



