258 NEW ZEALAND NATURE-STUDY BOOK 
can be touched without leaving a mark ; others readily soil 
whatever they come in contact with. Before making this 
test, care should be taken to brush off any dust adhering to 
the surface. 
Its colour and lustre should next be dealt with. The 
black colour will be recognised in any specimen. The 
lustre, which varies with the kind of Coal, from brilliant or 
shiny to dull, can be seen properly only on a surface 
exposed by a clean fresh fracture. Pieces of Coal when 
taken from the seam at first have this shiny lustre ; but 
exposure to the air roughens the surface and in other ways 
causes the lustre to a great extent to disappear ; hence the 
necessity of examining a freshly-exposed surface. Pupils 
will at once recognise that the mineral is opaque, when 
reference to other opaque substances might be made. The 
necessity for doing this will depend on the intelligence and 
state of progress of the class. By tapping the specimen 
with some hard object or dropping it on the floor its 
brittleness will become apparent. A few other brittle 
substances, ¢g., Glass and Chalk, might then be examined 
so as to pave the way for a contrast between the properties 
brittleness and hardness, which should be made perfectly 
clear. In order to test its hardness, a knife or a file’ 
should be used—reference here might also be made to the 
scale of hardness—if this has already been dealt with. If 
not, the hardness of the mineral might be contrasted with 
that of other substances, e.g., Chalk, Wood, Lead, Iron, Glass. . 
This last substance will furnish a convenient material for 
bringing out the difference between brittleness and hardness. 
Glass and Coal are both brittle ; but the former is much 
harder than the latter ; for it can readily be made to scratch 
Coal. 
A sharp tap with some hard substance—the same test in 
fact as that used for brittleness —will reveal the fracture 
