MINERALOGY. 



[Sect, VIIL 





tiient substances ; it is sufficient that there is an exact 

 relation between the bases and the acids they contain , or 

 between their ison^orphous substances."* 



The external geometric forms of minerals were, as far 

 back as 1784, discovered by Bergman and 



Bergman and Haiiy not 

 always to be those which might be considered funda- 

 mental or primary, since many were found capable of 

 bein^ split or divided into other forms, represeiitin 



the solid arising from the free adjustment of the compo- 

 nent particles, the body of the crystal and external form 

 being made up of an aggregation of many primary crys- 

 tals, or of some modification of a primary crystal. Of 

 the aggregation of primary crystals the common mineral, 

 calcareous spar, affords a familiar example. The funda- 

 mental crystal of carbonate of lime is a given rhombohe- 

 dron, yet the external forms of this mineral are very 



varied, so mucn so 



that 



the Comte de Bourn on was 



enabled to describe nearlv 800 modifications of them. 

 The primary form alone, as external, is much more rare. 

 The crystallization of carbonate of lime also well illus- 

 trates dimorphism. The common kind, as we have seen, 

 has a fundamental crystal of a given rhombohedral form, 

 but there is another kind, named arragonite, wherein, 



* Dafrenoy, 'Traite de Mineralogie/ tome i. p. 19- This is a most 

 excellent workj and should be in the hands of those who desire an ex- 

 tended knowledge of mineralogy. We may also mention Phillips's 

 * Elementary Treatise on Mineralogy/ an English edition by Allan, and 

 an American edition by Alger: Dana's ^System of Mineralogy' (New 

 York and London) ; Beudanfs ' Traite Elcmentaire de Mineralogie / 

 Rammelsberg's - Handworterbuch des Cheniischen Theils des Mineral- 

 ogie f Blum's * Die Pseudomorphen des Mineralreichs f Rose's * Elements 

 der Krystallographie;' Dr, Karsten's * System der Mineralogie ' (Ber- 

 lin ^ and others. 





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