HABDNESS — TENACITY — SPECIFIC GEAVITT. <J3 



2. PHYSICAL PROPERTIES OF 



MINERALS. 



The physical properties referred co in the description and 

 determination of minerals are here treated under the following 

 heads: (1) Hardness; (2) Tenacity; (3) Specific Gravity; 

 (4) Refraction, Polarization ; (o) Diaphaneity, Color, Lustre; 

 (6) Electricity and Magnetism ; (7) Taste and Odor. 



1. HARDNESS. 



The comparative hardness of minerals is easily ascertained, 

 and should be the first character attended to by the student in 

 examining a specimen. It is only necessary to draw a file 

 across the specimen, or to make trials of scratching one with 

 another. As standards of comparison the following minerals 

 have been selected, increasing gradually in hardness from talc, 

 which is very soft and easily cut with a knife, to the diamond. 

 This table, called the scale of hardness, is as follows : 



1, talc, common foliated variety ; 2, rock salt • 3, calcite, 

 transparent variety ; 4, Jiuorite, crystallized variety ; 5, apatite, 

 transparent crystal ; 6, orthoclase, cleavable variety ; 7, quartz, 

 transparent variety ; 8, topaz, transparent crystal ; 9, sapphire, 

 cleavable variety ; 10, diamond. 



If, on drawing a file across a mineral, it is impressed as easily 

 as Jiuorite, the hardness is said to be 4 ; if as easily as ortho- 

 clase, the hardness is said to be 6 ; if more easily than ortho- 

 clase, but with more difficulty than apatite, its hardness is de- 

 scribed as 5|- or 5*5. 



The file should be run across the mineral three or four times, 

 and care should be taken to make the trial on angles equally 

 blunt, and on parts of the specimen not altered by exposure. 

 Trials should also be made by scratching the specimen under 

 examination with the minerals in the above scale, since some- 

 times, owing to a loose aggregation of particles, the file wears 

 down the specimen rapidly, although the particles are very 

 hara. 



In crystals the hardness is sometimes appreciably different in 

 degree in the direction of different axes. In crystals of micft 



