The Several Forms of Calcium, Carbonate. 50 7 



One other point remains to be noticed — namely, the conse- 

 quences of the fact that the solubility of a substance depends 

 upon the fineness of grain, that a solution saturated with respect 

 to fine grains is supersaturated with respect to larger grains 

 aud conversely ; this phenomenon is made use of constantly as 

 a means of coarsening the grain of fine precipitates and so ren- 

 dering them filterable. This difference in solubility is not 

 marked until the grains become very fine ; nevertheless, in the 

 case of a substance capable of existence in more than one form, 

 the solubility of a large crystal of the less stable form may be 

 less than that of very fine particles of the more stable form. 

 Consequently, especially since the difference in solubility of 

 crystals of aragonite and calcite is so small, it may be that large 

 crystals of aragonite are really stable with respect to very 

 small particles of calcite. On this basis one can account for 

 the fact that aragonite free from calcite may be kept for along 

 time in contact with water at 100° without undergoing any 

 change to calcite ; for the minute calcite particles which would 

 tend to appear are unstable with respect to the crystalline 

 aragonite. 



This problem will require further and more extended inves- 

 tigation before more definite statements as to the rationale of 

 the appearance of unstable forms can be made. It is recog- 

 nized that the experimental basis of the foregoing remarks is 

 less secure than one could wish ; yet any hypothesis which aids 

 in the interpretation and correlation of phenomena is better 

 than none at all, and is especially necessary in such a puzzling 

 problem as this, where we are trying to ascertain what factors 

 actually determine the apparently accidental appearance of 

 one or other of the unstable forms. 



Some Ap2^Ucations to the Natural Minerals. 



Up to this point we have considered in the main the prepara- 

 tion and properties of the three distinct crystalline forms of 

 calcium carbonate in the pure state; but a discussion of then- 

 geologic relationships must take into account the effect of possi- 

 ble impurities (held mechanically or in solid solution) and of 

 porosity upon their properties. As to the presence of impuri- 

 ties and their effects upon the crystals, there are few data which 

 can be correlated ; and a determination, no matter how care- 

 fully it is made, of a single property of crystals of unknown 

 purity is of little value as a contribution towards ascertaining 

 the genetic relationships or to determinative mineralogy. If 

 the slight value of even a carefully determined single charac- 

 teristic of a mineral, as compared to the high value of two or 

 more characteristics determined upon the same specimen, were 



