PHYSICAL PROPERTIES OF MINERALS II 



unchanged ; all three forms may be assumed by the same mineral, 

 and they thus properly belong in the same system. Similar relations 

 may be observed between the crystalline forms of the other systems. 



It might be supposed that the crystalline systems and the rela- 

 tions of their imaginary axes were merely mathematical devices to 

 reach a convenient classification of forms. Such a conclusion 

 would, however, be a very erroneous one. Crystalline form is the 

 expression of molecular structure, and many of the physical proper- 

 ties of minerals are determined by their mathematical figure. It 

 is clear that the physical properties which depend upon form are 

 not inherent in the molecules of the mineral, but are conditioned 

 by the way in which the molecules are built up into the crystal. 

 Amorphous substances refract light equally in all directions, and 

 are thus called isotropic ; but when an amorphous substance crystal- 

 lizes, it assumes the qualities proper to its crystalline form. Thus 

 water is isotropic, while the hexagonal crystals of ice are singly 

 refractive in only one direction, doubly refractive in two. The 

 same substance may, under different circumstances, crystallize in 

 different systems, and will then display the properties appropriate 

 to each system. 



Not only the refractive powers of a crystal, but also its mode of 

 expansion when heated, and its conductivity of electricity and heat 

 depend upon its form. 



The crystals of the isometric system, which have their three axes 

 of equal length, are singly refractive in all directions, expand 

 equally when heated, and conduct heat and electricity equally in 

 all directions. Those of- the tetragonal and hexagonal systems, 

 which have one axis longer or shorter than the others, are doubly 

 refractive along the lateral axes, expand equally when heated, and 

 show equal conductivity along these axes. Along the principal 

 axis they are singly refractive, expand to a different degree when 

 heated, and display a different conductivity along this axis than 

 along the others. In the orthorhombic, monoclinic, and triclinic 

 systems, which have all the axes of unequal lengths, the crystals 

 are singly refractive in two directions ; they expand unequally and 

 conduct differently along all their axes. 



