SAMPLE LESSON ON COAL. 173 



Form. — The exterior form is irregular, therefore it is 

 massive. (Had it had plane surfaces set at definite angles it 

 would have been a crystal.) 



Structure. — The broken surface is irregular, hence amorphous. 

 (Rock salt shows crystalline and marble crystallized structure. ) 



Cleavage. — Lacking, although some specimens cleave along 

 the natural lines of formation and therefore simulate crystalline 

 structure and basal cleavage. 



Fracture. — It breaks something like glass with shell-like 

 surfaces, therefore conchoidal. 



Tenacity. — It cannot be hammered into thin plates like 

 copper, therefore it is not malleable ; it cannot be shaved into 

 plates, hence it is not sectile ; such operations would break it 

 into little bits, therefore it is brittle. (Why is it not elastic or 

 flexible ?) 



Color. — Black. 



Streak. — The powder when obtained by scratching or filing 

 shows black on a white surface, therefore the streak is black. 



Lustre. — Metallic^ shining. 



Diaphaneity. — Light does not penetrate it, hence it is 

 opaque. 



Touch. — It feels smooth. 



Magnetism. — A magnet has no effect on either the fragment 

 of coal or on its powder even when the latter is heated. 



Electricity. — When a piece of coal is rubbed on a woollen 

 cloth it does not move a bit of paper or pith near which it is 

 held. 



Weight. — Light when compared with quartz as medium. 



Specific Gravity. — A lump of hard coal weighing 8 J ounces 

 weighed in water 3| ounces; hence its s.g. is 1*79, found 



