892 IDIOPHYSICS. 



tion, or by the aid of some simple mechanical experiment. Such 

 are the color, lustre, hardness, and specific gravity; which are 

 easily noted, and are, perhaps, the most important external characters 

 to which we can refer. 



The color, of earthy minerals, may vary from a very slight ad- 

 mixture of foreign ingredients ; but in ores, it is a better test of the 

 composition. The colors selected by Werner, as standards for the 

 comparison of minerals, were the eight following : snow white ; ash 

 gray ; velvet black ; Prussian blue ; emerald green ; lemon yellow ; 

 carmine red; and chesnut brown. Besides the colors of minerals in 

 mass, that of their streaky when abraded by a file, is often a useful 

 characteristic. The lustre of minerals, is distinguished as metallic, 

 vitreous, resinous, pearly, silky, or adamantine ; either of which may 

 vary in intensity. As regards the transmission of light, minerals are 

 either transparent, translucent, or opaque : and some minerals are 

 characterized by the degree of refraction, or by the double refraction 

 of light; as others are by the property of phosphorescence, or assum- 

 ing a luminous appearance by friction or by heat. 



The hardness of minerals, is often an important characteristic ; 

 and is expressed, in the manner which Mohs proposed, by a scale 

 of numbers, referring to a series of minerals, each of which will 

 scratch any one of the preceding. The scale, thus constructed, is 

 as follows: 1. talc; 2. rock salt; 3. calcareous spar; 4. fluor spar; 

 5. apatite ; 6. feldspar ; 7. quartz ; 8. topaz ; 9. sapphire ; and 10. 

 diamond ; which is the hardest substance known. To these, Breit- 

 haupthas added two intermediate degrees; 2|, foliated mica ; and 5, 

 scapolite ; subdividing the largest intervals of the scale. As regards 

 their aggregation, minerals are either brittle, sectile, malleable, flex- 

 ible, or elastic : and their fracture is either conchoidal, even, uneven, 

 or hackly, as in broken iron. The specific gravity of minerals, is 

 found, on the principles of Hydrics, by dividing the weight of the 

 mineral by the weight of an equal bulk of water ; which is the loss 

 of weight when it is suspended in water. Minerals are also charac- 

 terized, in some few cases, by their electric, or magnetic properties, 

 or by their taste, or odor. 



Among the chemical properties most frequently employed by the 

 mineralogist, are the action of acids, and the effects of the blowpipe. 

 The acids used for this purpose, are chiefly sulphuric, nitric, and 

 muriatic, in a diluted state. Most of the carbonates, when exposed 

 to either of these acids, are decomposed, with effervescence of carbo- 

 nic acid gas ; and most of the sulphurets, when acted upon by either 

 sulphuric or muriatic acid, especially with the aid of heat, are recog- 

 nized by the fetid odor of the sulphuretted hydrogen, otherwise called 

 hydrosulphuric acid, which they evolve. The blowpipe, is a tube, one 

 end of which is applied to the mouth, and the other end terminates in 

 a small orifice, through which a jet of air is thus forced into the flame 

 of a lamp or candle, causing a conical flame, of intense heat, to pro- 

 trude in the direction of the jet. By this flame, the fusibility, or 

 combustilify, of minerals is tested ; and many refractory minerals 

 are rendered fusible by adding some salt, as a flux ; while the color 

 and appearance of the melted drop, or bead, thus obtained, often indi- 



