MANUAL OF NATURAL HISTORY. 591 



quiry, a correct knowledge of the laws of crystal- 

 lography is requisite, as minerals were formerly 

 principally classified according to their outward 

 shape; and crystalline form is still of much im- 

 portance, though not of such primary consequence. 

 Other qualities to be particularly attended to, 

 are lustre, colour, transparency, refraction, irides- 

 cence, polarization, phosphorescence, also specific 

 gravity, hardness, state of aggregation, fracture, 

 taste, odour, and especially chemical composi- 

 tion. Those more immediately suitable for our 

 purpose, as generally and more easily applicable, 

 are, 1. Lustre, which may be metallic, vitreous, re- 

 sinous, pearly, silky, or adamantine, while the inten- 

 sity may be splendent, shining, glistening, glim- 

 mering, or dull. 2. Colours, of which eight are 

 accounted as fundamental in mineralogy, namely, 

 white, gray, black, blue, green, yellow, red, and 

 brown, which are termed "non-metallic" colours; 

 others are spoken of as " metallic/' which are cop- 

 per-red, bronze -yellow, brass -yellow, gold-yellow, 

 silver-white, tin-white, lead-gray, steel-gray, and 

 iron-black : these are subdivided into shades, for 

 each of which a natural standard has been esta- 

 blished, which may be found in Syme's "Nomen- 

 clature of Colours/' or in the more recent work of 

 Hay. 3. Transparency ; minerals may be trans- 

 parent, sub-transparent, translucent, sub-translucent, 

 or opaque. 4. Hardness; this is ascertained by 

 comparing the resistance offered to the action of a 

 file or knife by the mineral in question, with certain 



