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PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the searching tests of optical analysis. For the Mineralogist, 

 indeed, the polariscope with its accessories has supplemented the 

 goniometer, in the same way that the spectroscope has the balance 

 of the Chemist. 



What has been stated concerning the optical characters of 

 minerals is equally true of their other physical properties ; for 

 the researches of recent years have shown all these properties 

 to be intimately related to the symmetry of the crystal in 

 which they are displayed. In every crystal, the faces of each group 

 bearing the same relations to its axes exhibit characteristic pecu- 

 liarities in their lustre, in their hardness, and in the manner in which 

 they are acted upon by solvents ; and these serve to distinguish such 

 groups of faces from others in the same crystal having different 

 relations to its axes. The elasticity of crystals, their power of 

 conducting heat and electricity, and their phosphorescent, electric, or 

 magnetic properties, whether natural or induced, are all manifested 

 in varying degrees along certain directions which can be shown to 

 be related to the particular symmetry of the crystal. 



The more carefully we study both the forms and the physical 

 properties of minerals, the more are we impressed by the conviction 

 that the most intimate relations exist between these characters and 

 the chemical composition of the minerals. The phenomenon of 

 " plesiomorphism," as Miller proposed to call it, that is the slight 

 variation in the angular measurements of crystals in the same 

 species or group, when any of the constituents are replaced by 

 vicarious or isomorphous representatives, very strikingly illustrates 

 this conclusion. And the exact study of the optical properties of 

 minerals shows that the slightest variation in the relative proportions 

 of these vicarious constituents makes its influence felt by changes 

 in their colour, in their pleochroism, in the nature and amount of 

 their double refraction, in the position of their optic axes, and, 

 indeed, in the whole assemblage of the properties of the crystal. 



To the admirable investigations of Tschermak on the felspars, the 

 amphiboles and pyroxenes, the micas, and other groups of minerals 

 we are largely indebted for the establishment of this conclusion ; 

 while Doelter, Max Schuster, and other mineralogists have contri- 

 buted many striking observations which serve to extend and fortify it. 



The application of the microscope to the study of the internal 

 structure of minerals — their Histology — has led to the recognition of 

 many beautiful and unsuspected phenomena. Studied in this way, 

 the seemingly homogeneous masses exhibit many interesting inter- 



