GUIDE TO THE MINBRAI/OGIC COLLECTIONS 135 



Stellated structure. Consisting of radiating individuals producing star- 

 like forms. 



Striated. Grooved or fun-owed in parallel linos. 



Stiiations. Parallel grooves or furrows on the surfaces of crystals. 



Symmetry. The regularity in the recurrence of faces and angles of the 

 same kind. 



System, one of the six divisions based on symmetry into which :ill 

 crystals are divided. 



Tetragonal. Fourfold. 



Tetrahedron. An isometric form bounded by four equilateral triangles; 

 identical with the regular polyhedron of solid geometry. 



Telia hexahedrons. Isometric forms bounded by 24 isosceles triangles, the 

 faces are grouped in six groups of four, each group corresponding to 

 one face of a cube or hexahedron. 



Third order. A term applied, in the tetragonal and hexagonal systems, to 

 pyramids and prisms the faces of which are related to those of the cor- 

 responding first order forms as shown on p. 27. fig. 102. 



Trapezohedrons. Crystal forms of the isometric, tetragonal and hexagonal 

 systems hounded by trapezohedral faces. 



Triclinic. Presenting the lowest degree of symmetry and referable to 

 three inclined axes. 



Trigonal. Threefold or triangular. 



Trisoctahedrons. Isometric forms bounded by 24 triangular faces ar- 

 ranged in eight groups of three, each group corresponding to one of the 

 faces of an octahedron. 



Twin crystals. Intergrowtbs of like crystals of the same substance sym- 

 metrically disposed with respect to certain lines but not in parallel 

 position. 



Twinning plane. In twin crystals, an imaginary plane common to both 

 individuals. 



Uniaxial. A term used to include in an optical division crystals of the 

 tetragonal and hexagonal systems. 



I nit form. A prominent crystal form which is arbitrarily chosen from 

 among those of a given species to determine the axial ratio. 



Vicinal planes. Low prominences produced on some crystal faces by dis- 

 turbances during the growth of the crystal or by other causes. 



Widmanstatten lines. Lines of crystalline structure developed on the 

 polished surfaces of meteorites by the action of corrosive agents. 



LIST (>F ELEMENTS 



Aluminium 



Antimony 



Argon 



Arsenic 



Barium 



Beryllium 



Bismuth 



Boron 



^ Ml 



OLS AN 



> ATOMIC Wi- 



[GHTS 







.\1 



27 



lt ro mill 





Br 



79.8 



SI. 



120 



( 'admitim 





Cd 



111.7 



A 



40 



Calcium 





(a 



39.9 



As 



71.'.' 



Carbon 





c 



12 



P.a 



137 



( 'elilllll 





Oe 



1 to 



Be 



'j. l 



( 'esilllil 





Cs 



132.7 



Bl 



2< >7 . r> 



Chlorin 





CI 



.' 15 . 5 



B 



ii 



( [hromlum 





<T 



52 . ■ 



