260 NEW ZEALAND NATURE-STUDY BOOK 
That this mineral is non-crystalline can easily be seen 
by contrasting it with other mineral substances which 
occur in well-defined crystal form, ¢.g., Quartz, Calcite, Galena, 
Iron-pyrites. Pupils will see that no approach to a cerystal- 
line structure is apparent in the case of Coal. 
With the upper classes it might be worth while to work 
out the specific gravity of the minerals examined. 
Whether or not this is done circumstances must decide. If 
time permits and the class is well advanced it might, with 
advantage, be undertaken; if, on the other hand, as will 
probably be the case in most country schools, the time 
available is all too short, and two or more Standards are 
grouped for instruction, it had better be omitted. To find 
the specific gravity a moderately sensitive balance is re- 
quired. The more sensitive the balance, the more accurate 
will be the result obtained. Still there is no reason why 
the investigation should be omitted, because of the absence 
of a delicately adjusted and highly sensitive pair of scales. 
The results will be utilised mainly for comparing the specific 
gravities of various substances, so that a fairly sensitive 
balance will find out the necessary information. Work of 
this kind is extremely useful if properly undertaken, 7.¢. if 
the pupils themselves be required to make the investigation, 
for it furnishes a training in accuracy of observation, in 
careful calculation and in the importance attached to 
detail—which must be of infinite value to pupils in after 
years. 
The specific gravity balance here shown is easily used. 
The substance, being examined, is first placed in the scale- 
pan A, and carefully weighed. It is then hung on to the 
hook below the pan A, suspended in water, as shown, and 
again weighed—both results being carefully noted, By 
subtracting the weight in water from the weight in air, the 
loss of weight sustained by the substance when immersed is 
