The Several Forms of Calcium Carbonate. 487 



the solution. By using the larger particles from one experi- 

 ment while still moist as nuclei in a second, one can increase 

 somewhat the size of the aggregates. When magnesium salts 

 were present, spherical aggregates* were obtained also ; this is 

 contrary to the statements of some previous investigators.f 

 who may, however, have erred by reason of faulty methods of 

 characterizing their products. 



A test supposed to differentiate "vaterite" from calcite and 

 aragonite was proposed by Spangenberg;}; and later " modified " 

 and used by Peine ;§ it consists in boiling the material with 

 distilled water for a period of from 15 minutes to 2 days and 

 observing whether new rhombs of calcite have appeared, on the 

 basis that under these conditions " vaterite " would dissolve and 

 recrystallize. But Peine himself states that this test fails 

 sometimes. Many of the precipitates, however, contained 

 material which from his own description we identify as the 

 //.-form ; and doubtless this material was transforming. 



The fibrous aggregates, prepared as described above, are not 

 all spherical, but appear in many shapes some of which alone 

 are indistinguishable from single crystals. Many of the 

 spherulites show a zonal structure, while practically all are not 

 completely homogeneous throughout their mass ; a few of them 

 are perfect, and nearly all of them show in parallel polar- 

 ized light the typical dark cross with a series of colored 

 rings. The maximum path difference was found to be at 

 a distance from the center of about 062 of the radius of 

 the spherulite ; this maximum was 3*4 wave-lengths at 

 650/a/x in one sample of spherules O03 mm in diameter. 

 Prom the length of the path through the sphere where 

 the path difference is greatest, the " double refraction of 

 the spherulite" considered as a unit was calculated to be 

 O092. It proved impossible to separate these aggregates into 

 fragments of single crystals ; so that it was a matter of con- 

 siderable difficulty to come to a definite decision as to whether 

 these aggregates constitute a distinct crystalline modification of 

 calcium carbonate or are merely very fine-grained aggregates 

 of fibres of one of the other forms. We shall now discuss the 

 evidence bearing on this question. 



In a few preparations spherulites of sufficient size — abqut 

 0'03 mm diameter — were found from which, by crushing, we 

 obtained radial splinters having parallel extinction, negative 



* The character of this product was such that no useful information could 

 have been derived from a chemical analysis ; so none was attempted. 

 \ Cf . p. 480 ante. 



JK. Spangenberg, Z. Kryst., Hi, 529, 1913. 

 § J Peine, Inaug. Diss., Jena, 1918, p. 13. 



