445 
For the irregular twenty-four-faced trapezohedrons the 
following are the values of p and v. 
B i[l | f] p=68° 54' v=19° 57'. 
E$[l i 2] fx — Ql° 16' V = 26°17'. 
^=67° 13' v = 28° 32'. 
Hill f 2] ju = 64° 37' v = 38° 13', 
KHl | 8] p = 63° 37' v = 53° 55'. 
i{[l f 5] ^=60° 56' v = 48°55'. 
Mi[l f 10] /x=61° 41' v = 56° 18'. 
JV^-[1 2 4] ju = 51° 45' v=48° 11'. 
BUI 5 10] ju = 22° 46' v=72°17 / . 
147. Some crystals have a tendency to split in directions 
parallel to a certain form. This is called a cleavage-plane. If 
they split readily, the cleavage is called a perfect one. Sub- 
stances which crystallize in the cubical system have only been 
observed to split or cleave parallel to the planes of the cube, 
octahedron, and rhombic dodecahedron. 
Minerals whose crystals cleave parallel to the faces of the cube } 
those printed in italics indicating that the cleavage is easy 
and perfect : — 
Alabandine. 
Altaite. 
Analcine. 
Argentite. 
Chromite. 
Clausthalite. 
Cobaltine. 
Cubane . 
Embolite. 
Franklinite. 
Gahnite. 
Galena. 
Gersdorffite. 
Hauerite. 
Iridium. 
Iron. 
Lerbachite. 
Linneite. 
Magnetite. 
JSJ aumannite. 
Periclase. 
Perowskite. 
Pyrite. 
Pyrochlore. 
Salt. 
SJcutterudite. 
Smaltine. 
Spinelle. 
Stannine. 
Stein mannite. 
Sylvine. 
Ullmanite. 
Minerals whose crystals cleave parallel 
octahedron : — 
to the faces of the 
Alum. 
Arsenite. 
Boracite. 
Bornite. 
Chromite. 
Cuprite. 
Diamond. 
Eisennickelkies. 
Fahlerz. 
Fluor. 
Franklinite. 
Gahnite. 
Griinauite. 
Magnetite. 
Sal ammoniac. 
Senarmontite. 
Smaltine. 
Spinelle. 
Minerals whose crystals cleave parallel 
rhombic dodecahedron : — - 
to the faces of the 
Alabandine. 
Amalgam. 
Argentite. 
Blende. 
Eulytine. 
Garnet. 
Hauyne. 
Ittnerite. 
Leucite. 
Skutterudite. 
2 i 
Smaltine. 
Sodalite. 
Stannine. 
Tennantite. 
VOL. IX. 
