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Sect. VII L] 



MINERALOGY. 



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with this difference, however, in the case of alumina, that 

 it IS not one of the simple substances which chemists con- 

 sider carbon to be, but a compound of a metal (aluminium) 

 and of oxygen. The ruby and sapphire are well-knowji 

 transparent minerals, but it is not necessary that the par- 

 ticles of even a simple substance should be arranged in 



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what* in common lanL?;uap;e, mav be termed a r»erfect 

 manner to make the mineral transparent. We may take 

 as familiar examples of the contrary crvBtals of gold, 

 silver, and copper. 



As the knowledge of mineralogy advanced, it was dis- 

 covered that there existed an intimate connexion between 

 the chemical composition and physical structure of min 

 rals when their constituent substances could form those 



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arrangements of their particles known to us as crystals* 

 This led to the view that vvhen minerals possess the same 



chemical composition, tlicj also always present the same 

 crystalline system. 



This is now known not to be strictly true. Tlie same 

 bodies have been found to occur under two different and 

 incompatible forms, and to this the term dimorphism has 

 been applied. Certain substances have also been dis- 

 covered to replace others, without altering the form of a 

 mineral, and to this the name isomorphism has been given. 

 The known dimorphous bodies are very few, not more 

 than about. 10 in 350 crystallized minerals. The sub- 

 stances which are isomorphous being ascertained, no very 

 great difficulty is experienced on this head. M. Dufrcnoy 

 has well remarked that " it is not necessary, in order to 

 present the same composition, that minerals should 

 exactly contain the same weight of their simple consti- 



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