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Sect. VIII.] 



MINERALOGY. 



237 



sketched in most treatises on mineralogy, and works have 



been dedicated solely to it. 



Experiments in the laboratory have pointed out, what 

 the m;)de of occurrence of natural minerals would have 



of 



led us to expect, that the multiplied modifications 

 some primary or fundamental form observed, much depend 

 upon the conditions under which inorganic substances 

 may have crystallized. So long ago as 1788 the experi- 

 ments of Leblanc showed this, and the every-day expe- 

 rience in laboratories and chemical manufactories proves 

 it. If by accident or design a solution of some given 



substance be added to another, the crystals of that which 

 would be otherwise formed from the first solution become 

 modified in shape : all the crystals so produced being 

 generally similar. Thus also in nature, all collector? of 

 minerals know that certain localities, in other words, giTcn 

 conditions arising from the combination of rocks and other 



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circumstances where the minerals arc found, produce 

 crystals of some substance with a marked crystalline 

 exterior, so that not unfrequently it is not difficult for a 

 mineralogist, when differently modified crystals of the 

 same substance are before him, to point out from whence 



each may have been obtained. Again^ 



mineralogists are 



well aware that some mine, or in other words, some mine- 

 ral vein, or part of a mineral vein^ will afford a modifica- 

 tion of a known mineral most abundantly for a time, and 

 no similar modification be afterwards discovered in it. 

 This is but the result of certain conditions, which have 

 obtained in the particular cavity of the crack in the en- 

 closing rocks, and v*hich otherwise variously filled consti- 

 tutes the vein. 



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