or how many times heavier it is than an equal volume of water. The bulk 
of the sample being unknown, it cannot be compared with an equal bulk of 
water; therefore this has to be determined by displacement, the difference in 
weight between the sample weighed in air and water being the weight of an 
equal volume of water, or the amount of water displaced by the specimen. There¬ 
fore we divide the weight of the specimen by the weight of the water displaced, 
' whicli gives its relative weight to an equal volume of water, the weight of the 
water displaced being obtained by subtracting the weight of the substance weighed 
in water from its weight in air. The specific gravity enables us simply to calcu¬ 
late the weight of a cubic foot of a substance, as we know that a cubic foot of 
water weighs 1,000 ounces; therefore, if we multiply 1,000 by the specific gravity 
of any substance, we have the weight of one cubic foot of it: thus the specific 
gravity of quartz is 2 6 x 1,000=2,600, a cubic foot of pure quartz, therefore, 
weighs 2,600ozs. Specific gravity is taken advantage of in separating substances 
from one another, as in gold workings. 
Of course it is quite impossible to test the specific gravity of a mineral in the 
bush, but a very good idea can be formed of it, as everyone must have noticed the 
difference in weight to the bulk of different substances (termed heaviness) ; 
this is really its specific gravity. 
Tenacity 
Is one of the most useful physical properties of metals; it implies the power 
a substance has of holding together. Thus iron, in the form of steel, is the most 
tenacious metal, as a wire of that metal will stand a greater strain than a wire 
of equal size of any other metal; but in mineralogy it is used on broader lines, 
and may be said to include the following properties: — 
1st. Brittle.—When a mineral breaks easily, or when it powders up on 
attempting to cut it. 
2nd. Malleable.—When it can be beaten out into plates with a hammer. 
3rd. Sectile. — When a slice may be cut off with a knife, like cutting a slice 
off wax, not splitting like mica, 
4th. Flexible.—When a mineral will bend, and remain bent. 
5tli. Elastic.—When a mineral will bend, but will spring back to its original 
position as soon as the bending pressure is released. 
Colour. 
The colour of a mineral may be under two heads: first, the general colour 
as seen in a mass; next, the streak, which is the colour of the mineral when 
observed in a finely divided state, and is most generally observed by making a 
mark with the substance to be tested on a piece of unglazed porcelain. 
Diaphaneity. 
This is the property possessed by substances of transmitting light, or, in other 
words, its transparency. The degree to which minerals possess this property may 
be divided under five heads:— 
1. Transparent: When a mineral offers no obstruction to the passage of 
light, and objects can be seen distinctly through it. 
2. Subtransparent: When light can pass freely through, but objects are 
indistinctly seen through it. 
3. Translucent: When light can pass through it, but objects cannot be 
seen through it. 
4. Subtrauslucent: When light alone is transmitted faintly at the edges. 
5. Opaque: When no light is transmitted. 
