The Several Forms of Calcium Carbonate. 475 



nature of the precipitate, in this as in other cases, appears to be 

 determined by the character of the first nuclei, but the factors 

 which determine the character of these nuclei are still obscure. 

 But, before taking up the methods of preparation, we shall 

 discuss the tests by means of which the several forms may be 

 distinguished from each other. 



Methods for Identification of the Several Forms. 



Measurements, with the aid of the petrographic microscope, 

 of optical andcrystallographic properties were relied on chiefly 

 as a means of determining the forms of calcium carbonate 

 present in precipitates, and of recognizing the presence of solid 

 solution or of impurities. In most cases such methods are 

 rapid and reliable, yet chemists have been slow to adopt them, 

 in 6pite of the fact that in many cases they yield in a few 

 minutes definite information which would be difficult — -or 

 impossible — to ascertain from purely chemical tests. A peculiar 

 difficulty appears in the application of these methods to fine- 

 grained aggregates of calcium carbonate, because of the 

 extremely strong birefringence of the crystalline forms ; never- 

 theless microscopical examination constitutes by far the surest 

 criterion for the identification of separate crystals, and is much 

 less uncertain than the others when applied to aggregates such 

 as frequently occur in precipitates of calcium carbonate. In 

 the latter case definite information may be obtained from a 

 microscopical examination of the fine-grained aggregates before 

 and after they have been heated for half a minute to 500-600°, 

 i. e., from a test of the stability of the material, for under such 

 treatment aggregates of calcite are not broken down, whereas 

 aggregates (or single crystals) of aragonite or /x-CaCO s change 

 to calcite, the aggregates changing always to a much simpler 

 structure and often to a single grain of calcite. 



Hitherto most writers have made use of other tests, which, 

 however, have not always been free from ambiguity. For in- 

 stance, density determinations have often been used ; but if 

 the results are to be capable of a definite interpretation, one 

 must make certain that the sample used is homogeneous and 

 free from bubbles, inclusions, or enmeshed material, and this 

 can usually (though not always, e. g., in samples of " vaterite ") 

 be ascertained by microscopical examination. In general, the 

 density alone is not a safe criterion of identification,* though 

 in particular cases it may be. Thus if the density of a sample 

 of calcium carbonate is as high as 2'85, one is safe in consider- 

 ing that it is mainly aragonite ; but if the density is much 



* See, for instance, Johnston and Adams, J. Am. Chem. Soc, xxxiv, 563, 

 1912. 



