484 MINERALOGY. 



any given mineral is ascertained, by trying which of the speci- 

 mens it will scratch. Beginning at the highest number, the se- 

 ries is descended till the member which the given substance will 

 or will not scratch. The hardness of the body is now com- 

 pared with these two bounding numbers, by passing corners of 

 the substances over a fine file ; and the resistance of the sub- 

 stance to the file, allows a pretty accurate comparison of the re- 

 lative hardness. The degree of hardness is then expressed by 

 the number to which the substance most approximates in this 

 respect ; and minute shades of hardness are expressed by deci- 

 mals, supposing ten equal divisions to intervene between each 

 member of the scale. 



4. SPECIFIC GRAVITY. The specific gravity of minerals, as 

 compared with water, is determined by means of the hydrostatic 

 balance, and other instruments. 



Other characters employed in the description of the species, 

 subspecies, and varieties of minerals are, 



1. Colour. The principal colours in the mineral kingdom 

 are, according to Werner, eight, viz. white, gray, black, blue, 

 green, yellow, red, and brown. Each of these colours, however, 

 shades into a great number of varieties, many of which have been 

 accurately defined in suites of coloured patches, to limit the pre- 

 cise tint. Of the use of definite terms relating to colour, the 

 work of Mr P. Syme * is an able illustration. 



Many minerals possess the property of changing colours ac- 

 cording to the light in which they are viewed. This is termed 

 the play of the colour, and is exemplified in the diamond and 

 precious opal. Others possess a changeability of colour, as in 

 Labrador felspar and common opal, the last of which when view- 

 ed on the surface is milk-white, but when held between the eye 

 and the light is lime yellow. Iridescence is another characte- 

 ristic of some minerals ; and a mineral is said to be tarnished, 

 when it shows upon its external surface or on that of the dis- 

 tinct concretions, fixed colours, different from those on its inte- 

 rior or fractured surface. 



2. Form. Common external forms are those in which there 

 are neither a determinate number of planes meeting under deter- 

 minate angles, nor any resemblance to known natural or artificial 



* Werner's Nomenclature of .Colours, adapted to Zoology, Botany, Chemistry, 

 Mineralogy, Anatomy, and the Arts. 8vo. 



