FUNDAMENTAL PROPERTIES OF MATTER. 



The crystals have three axes intersecting at right angles, two of which are 

 of equal length and one either longer or shorter than the other two. The fun- 

 damental forms of this system are the quadratic octahedron (also known as 

 square-based double pyramid) and the right square prism (Figs. 3 and 4). 



FIG. 3. FIG. 4. 



Quadratic octahedron. Right-square or quadratic prism. 



Some substances crystallizing in this system are potassium ferrocyanide, calo- 

 mel, nickel sulphate, tin, tin oxide, magnesium sulphate, zinc sulphate. 



(3) Rhombic system, also known as the trimetric or right prismatic system. 



FIG. 5. FIG. 6. 



Rhombic octahedron. Double six-sided pyramid. 



The crystals have three unequal axes intersecting at right angles. The fun- 

 damental form of this system is the rhombic octahedron or right rhombic 

 double pyramid (Fig. 5). Some substances crystallizing in this system are 



