J. IJ. HAEFORD — CnARACTERISTICS OP MINERALS. 105 



Between these extremes all minerals may be classed in reference to 

 their hardness.* 



Weight. — This is a constant distinctive characteristic of minerals, 

 and is reckoned by comparison with the weight of a similar bulk of 

 distilled water. It is termed specific gravity. 



While some minerals are lighter than water, and will con- 

 sequently swim on its surface, the ordinary earthy minerals, such 

 as stones, weigh about 2-6 times the weight of the same bulk of 

 water. The metallic substances or ores are heavier, and culminate 

 in gold, which weighs 19-3 times heavier than water. An inter- 

 mediate place is occupied by the earthy mineral called baryta, the 

 sulphate of which has a specific gravity of about 4-5, whereas that 

 of sulphuret of iron (pyrites) is only 4 '3. Baryta derives its name 

 from its weight. In general the greater specific weight of a body 

 indicates the presence of metal, and it is said that the sulphate of 

 baryta has in this way been mistaken for white lead ore. Mallea- 

 bility, ductility, brittleness (distinct from hardness), flexibility, 

 toughness, elasticity, are other characteristics of minerals, constant 

 and uniform in their occurrence, and all serving to distinguish 

 various mineral bodies. 



Appearance. — The appeal of minerals to the sight is in various 

 ways, their colour, lustre, transparency, — their metallic or non- 

 metallic aspect, — their resinous, vitreous, pearly, chatoyant, silky, 

 iridescent appearances. All these are worthy of remark and present 

 characteristics of distinct minerals. The streak is important to be 

 noticed, that is, the colour of the powder of the mineral produced 

 by scratcliing. As an instance of this point may be mentioned the 

 oxide of copper, which being grey in the natural state, becomes red 

 when in powder. This causes the substance to be familiarly 

 termed red oxide of copper. 



Electricity and Maynetism afford characteristics that are im- 

 portant. As instances the diamond and amber may be mentioned. 

 They become electric when rubbed. The magnetic ore of iron 

 possesses the magnetic property in its natural state, and most ores 

 of iron influence the magnet after being heated. 



Crystallisation. — The tendency of divers minerals to assume 

 regular geometric shapes is a conspicuous test of their nature. 

 The dimensions of their angles of form may be reckoned with 

 the utmost nicety. The simplest form, the cube, or perhaps the 

 octahedron, or still more simply the tetrahedron, and the various 

 forms related to them, form the class from which the phenomenon 

 of double refraction is absent. When a ray of light passes obliquely 

 through a transparent substance, it suffers refraction. If the trans- 

 parent medium is a crystal of the cubic class of form, the refraction 

 is single, — that is, the ray of light is undivided. When the crystal 



* The scale is as follows : — 



1. Talc, common foliated variety. 6. Felspar, cleavable variety. 



2. Mica. 7. Quartz, transparent variety. 



3. Calc-spar, transparent variety. S. Tupaz, tr;iusparent crystal. 



4. Fluor-spar, crystallised variety. 9. Sapphire, or corundum. 



5. Apatite, transparent crystal. 10. Diamond. 



VOL. II. — PT. III. 8 



