512 Johnston, Memvin; and Williamson. 



chance, — in other words, it depends upon factors which can- 

 not be controlled at the present time. In so far as we have 

 been able to ascertain, natural aragonite is formed (a) through 

 organic agencies, (b) by deposition from hot springs, (c) when 

 an isomorphous carbonate is present to serve as nucleus ; (d) in 

 salt waters containing sulphate even at ordinary temperatures. 

 Pure aragonite can persist as such only when dry ; but ara- 

 gonite containing other substances in solid solution may thereby 

 be enabled to persist in presence of certain solutions. There 

 are indications that the /A-form often occurs as an intermediate 

 step in the precipitation of the other forms ; but it soon trans- 

 forms in presence of water, a fact which, combined with the 

 fact that it cannot be differentiated from aragonite by means 

 of the usual color tests, is responsible for the circumstance 

 that it has not been recognized as occurring in nature. 



The properties of these three forms, and of the hexahydrate 

 CaC0 3 .6H 2 (which, moreover, is also unstable with respect 

 to calcite even at 0°) are described ; some of the characteristic 

 properties have been collated in Table -i. The several forms 

 can be differentiated from one another by optical, crystallo- 

 graphic, and chemical tests, by observation of density and of 

 the stability of the material when heated ; but it is not safe to 

 trust to a single one of these tests, especially with fine-grained 

 material, as the result may be ambiguous. Much of the work 

 in the past is vitiated by the fact that the criteria employed 

 were insufficient to differentiate one form from another ; and 

 thus many of the statements to be found in the literature require 

 revision. In conclusion, we might point out that a determina- 

 tion, no matter how carefully carried out, of a single property 

 of a substance is of much less value than two or more 

 characteristics of the same specimen : and make a plea for the 

 determination on an analyzed specimen of as many diverse 

 properties as possible, for it is only in this way that it will be 

 possible to coordinate the various observations in a satisfactory 

 way. As an illustration of this point, one cannot state the 

 manner in which the properties of so well-defined and common 

 a mineral as calcite vary with the presence of other substances 

 in it, in spite of the enormous number of isolated observations 

 which have been made ; and it is obvious that such a coordina- 

 tion, once established, would save an immense amount of labor 

 thereafter. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, March 27, 1916. 



