252 NEW ZEALAND NATURE-STUDY BOOK 
This definition would exclude Coal, which by common 
usage is regarded as a mineral, and would include such 
substances as gases, water, and ice. For our purposes - 
we shall certainly consider Coal as a mineral substance, 
though derived from the decomposition of organic matter, 
and shall, with the exception perhaps of Mercury, confine 
ourselves to the investigation of minerals which, under 
atmospheric conditions are solids. 
It is sometimes stated that a mineral substance differs 
from animal and vegetable forms in that it does not grow. 
A young rabbit or a potato plant increases in size as it 
becomes older—whereas a mass of Salt or Limestone does not 
add to its volume as time passes. But in point of fact this 
is not the case. Under certain conditions minerals steadily 
increase in size ; a fact which is capable of easy demonstration 
by allowing crystals of various substances to separate out 
from saturated solutions. Between the animal and vegetable 
kingdoms on the one hand and the mineral kingdom on the 
other, there are other differences besides those contained in 
the definition given above. A mineral increases by addi- 
tions made to its exterior or outer surfaces. An animal or 
a vegetable grows by assimilation of matter which is taken 
into its interior. Again, animals and vegetables are 
bounded by curved or irregular surfaces, whereas minerals 
in their crystallised state, with one or two exceptions, are 
bounded by plane or flat surfaces. 
Examination of Minerals. There are usually three 
methods employed in examining minerals—we may inves- 
tigate their physical characters, their chemical composition, 
and their optical properties. The first two are the only 
ones capable of being undertaken in our primary schools, 
and of these the second can be employed to a very 
limited extent only. 
