PHYSIOGRAPHY. 21 



Magnetic Iron. 



Cleavage, parallel with the primary form : in some vari- 

 eties perfect, and easily obtained ; in others, entirely oblit- 

 erated by conchoidal fracture. Fracture conchoidal, un- 

 even. Surface, the dodecahedrons commonly streaked 

 parallel to their edges of combination with the octahedron, 

 faces of the octahedral trigonal-icositetrahedron (fig. 282.) 

 smooth, though curved ; the surface of all the other forms 

 is smooth. 



Lustre metallic : in some varieties, imperfectly. Color 

 iron-black. Streak black. Opake. 



Brittle. Hardness =5'5 . . . 6-5. Sp. gr. =5*094, oc- 

 tahedrons imbedded in chlorite. 



Compound Varieties. Twin-crystals : axis of revolu- 

 tion perpendicular, face of composition parallel to a face of 

 the octahedron. Massive : composition granular, of vari- 

 ous sizes of individuals, and different degrees of cohesion. 

 If the composition be almost impalpable, fracture becomes 

 flat conchoidal, even or uneven. 



1. Before the blow-pipe, it is infusible; but assumes a brown color, 

 and loses its attractive influence over the magnetic needle, after having 

 been exposed to a great heat. It is soluble in heated muriatic acid, but 

 not in nitric acid. It may be obtained crystallized by fusing it; and crys- 

 tals are likewise often produced in the process of roasting the ore which 

 contains this mineral. 



2. Analysis. 



By H 



Protoxide of iron - ..... 94-38 



Magnesia ....... 0-16 



The loss is oxygen, as the mineral contains both protoxide and perox- 

 ide of iron, according to BERZELITJS, in the proportion of 30 98 to 69-02, 

 (the whole content of oxygen being 28-215,) or, according to KOBELL, 

 protoxide 24-48 .to 25-92, and peroxide 74-QS to 75-52. 



3. Magnetic Iron occurs in beds in primitive rocks, more commonly 

 in gneiss, occasionally in clay-slate, hornblende-slate, chlorite slate, 



