4 
These bodies having been tested in the form of crystals, 
the results given have been arrived at by the harder body 
cutting the softer. The degrees of hardness, 1, 2, and 3, 
may be proved by the finger nail, 1 to 6 by a knife; 6 to 
10 give out fire when struck by a steel, and produce from 
the file a sound more or less harsh. 
SPECIFIC GRAVITY. 
Specific gravity means the weight of a body in relation 
to its own volume. It is, however, generally applied to the 
weight of any body as compared with that of an equal 
bulk of distilled water. It is ascertained by first weighing 
the body in the atmosphere, and then under water, and 
by dividing the first found absolute weight by the difference, 
which is really the weight of the displaced water. For 
this purpose minerals are used which have been reduced 
to small fragments, and if they are porous, in the form of 
pow'der, and the experiment is made at a mean temperature 
of 60° Fahr. It is well also to repeat the weighing several 
times, in order to secure a delicate balancing and an exact 
weight. Experience shows that the metalloids, and some 
combustible minerals derived from the vegetable kingdom, 
present the lowest specific gravity, from 05 to 4‘9; the 
ordinary earthy minerals vary from 2 to 3, the precious 
stones from 2’8 to 4 - 6; the ores and heavy metals from 4 
to 24. 
CHARACTERS DEPENDING ON LIGHT AND 
COLOUR. 
Optical Deportment of Minerals. 
Many minerals, and especially such as are crystallised, 
are perfectly transparent, some are only translucent, while 
others, such as the metals, are quite opaque. Transparency 
is generally considered as a mark of purity. 
If a ray of light falls on a transparent body, it is more 
or less refracted, that is to say, diverted from its course ; in 
many bodies, however, such as calcite, gypsum, etc., the ray 
divides into two parts, so that the image of an object placed 
under it appears double, as is seen in Plate B, Fig. 17. 
This is a prism of Icelandic double-refracting spar, whose 
fissured edges have been polished to smoothness. Under it 
is placed a representation of two rings, which, when 
observed from before, appears doubled and somewhat dis¬ 
placed. If the prism be moved over the picture to the 
right or left, the rings are displaced differently, and it is 
observed that _ the two entering rays of light present a 
different divergence. 
When thinly-cut plates of transparent crystals are seen 
in polarised light, for example between crossed plates of 
transparent tourmaline, which have been cut parallel to 
their axes, splendid rings of colour are seen, which, in 
each mineral, present something peculiar, so that all bodies 
having a single optical axis, as for instance, calcite, or, in 
a wider sense, all monometric and dimetric figures present 
similar images to that shown in Fig. 18, Plate B, which 
is a representation of calcite cut perpendicularly to the 
axis. If the tourmaline plates are laid parallel, there 
appears in such minerals, instead of a dark cross, a white 
one, as seen in Fig 19, and the rings present the comple¬ 
mentary colours of those of the former image ; in place of 
red there is green, blue is replaced by yellow, and so forth. 
If two such mineral plates are crossed at right angles, 
there appears, instead of the dark cross in the middle, an 
S-shaped fourfold figure. If the mineral, rock crystal for 
instance, is compressed by a screw, the circles of colour 
are distorted into parabolic figures, while, at the same 
time, the elasticity of the rock crystal is resisted. Crystals 
having two axes, such as trimetric, monoclinic, and triclinic 
bodies, under this arrangement present other appearances 
and colours, as in Figs. 20 and 21, which are taken from 
crystals of saltpetre. 
The colours which minerals present in ordinary light 
are fixed or essential to every mineral if they depend on 
its chemical composition; they are accidental if they only 
arise from known impurities, or from non-essential mixtures. 
Thus, for instance, rock crystal is colourless; in the ame¬ 
thyst, however, it is combined with oxide of manganese. 
In the same manner most coloured precious stones derive 
their colour from the metallic oxides ; the red from the 
oxides of manganese and iron, the blue and green from 
the oxides of copper and chrome. A glance at the plates 
will show that in others all the colours in the mineral 
kingdom are found, simple as well as mixed; and that they 
occur in the most varied shades and shapes may be seen 
in Plates III. and IY. 
The metals, and many ores, have particularly striking 
colours, and possess the property known as metallic lustre. 
We make use of the expressions gold-yellow, brass-yellow, 
silver-white, tin-white, lead and iron-grey, all derived from 
the colours of the corresponding metals. 
The streak is proved by scratching with a knife, or 
by rubbing off colour on a rough fragment of porcelain, 
and often affords a very good means of distinction between 
many allied substances, as, for instance, between the oxide 
of iron or magnetic ironstone and brown iron ore, the 
different ores of manganese, etc. 
There are other peculiarities of colour, which are 
sometimes accidental and sometimes characteristic, such as 
play of colour, iridescence, change of colour (see Plate 
VI., Figs. 9 and 10), dichroism, or the property of 
having different colours when viewed in two different direc¬ 
tions (Plate X., Fig. 17), and variegated or dove-necked 
tarnished surfaces (Plate XVII., Fig. 2). 
The lustre may differ in such degree and quality, from 
strongly to slightly lustrous, splendent to dull, as is ex¬ 
pressed by terms like the following: adamantine, vitreous, 
resinous, pearly, silky metallic lustre, and the like. 
RELATION OF MINERALS TO ELECTRICITY, 
MAGNETISM, AND HEAT. 
Many minerals become electric on being rubbed, such 
as tourmaline, sulphur, anthracite, and numerous others, 
some exhibiting positive, and others negative electricity. 
Some, especially the native metals, and many varieties of 
quartz, conduct electricity; others, such as felspar, and 
similar oxidised combinations, are non-conductors; while 
others, again, on being heated, exhibit polar electricity, so 
that one end of the crystal is positive and the other 
negative. The examination is effected by means of the 
electrometer, or by an animal hair fastened to a piece of 
