COAL 259 
of Coal, which in some cases is very characteristic. The 
specimen, when operated on, should be held in the hand ; if 
placed on a firm object, when struck, the whole specimen 
may fly to pieces. 
The cleavage of the mineral may be examined at the 
same time, though cleavage and fracture must not be 
confused, Cleavage is the tendency to split along definite 
planes and leaves smooth flat surfaces—fracture, on the 
other hand, may take place in any direction in the mineral, 
and generally leaves irregular surfaces exposed. Some 
varieties of Coal break with a distinctly conchoidal 
fracture, others when struck split up into cubical pieces. 
Chip off a piece of the specimen under examination and 
note carefully the exposed surface. If this shows con- 
centric ridges gradually diminishing towards the point 
where the specimen was struck, something like the lines of 
growth of a bivalve shell, the fracture is said to be con- 
chordal or shelllike. If, on the other hand, fragments 
cannot be chipped off the specimen, but the whole tends to 
break up into pieces which are roughly cubical, the mineral. 
does not show fracture but is said to possess cubical 
cleavage. Pupils will gradually come to see that, in the 
case of a fracturing mineral, a chip can be taken off practi- 
cally from any convenient projection ; but that cleavage is 
possible only along definite planes. A piece of Coal, for 
instance, that breaks into cubical fragments, splits along 
definite planes approximately at right angles to each other 
and can only with difficulty be made to break up in any 
other way. 
Taste and odour might or might not be tested. 
Neither is important in the case of Coal, but it is as well 
that pupils should accustom themselves to using these tests 
so as not to overlook their application in cases where this 
becomes necessary. 
