422 NATURAL HISTORY. 



of hardness has been introduced which serves as a stand- 

 ard of comparison. It is only necessary to draw the file 

 across the specimen, or scratch one with another, and the 

 comparative hardness is easily ascertained. The follow- 

 ing scale consists of ten minerals which gradually in- 

 crease in hardness as the ciphers advance, that is 

 1 to 10. 



1. Talc. 6. Feldspar. 



2. Rock Salt or Gypsum. 7. Quart/. 



3. Calcareous Spar. 8. Topaz. 



4. Fluor Spar. 9. Sapphire. 



5. Apatite. 10. Diamond. 



Rock Salt is usually in the scale of hardness for No. 

 2, and if, on drawing a file across a mineral, it is im- 

 pressed as easily as rock-salt, the hardness is said to be 

 two ; if as easily as Feldspar, the hardness is 6 ; rock- 

 salt, for instance, is written thus : H. = 2,0. - If the 

 file abrades the mineral with the same ease as No. 6, 

 Feldspar, and produces an equal depth of abrasion with 

 the same force, its hardness is said to be 6. If with 

 more facility than 6, but less than 7, that marks the 

 difference and is thus written, 6 = 7. 



The specific gravity of a mineral is its weight, com- 

 pared with that of another substance of equal volume, 

 whose gravity is taken at unity. For solids and liquids, 

 distilled water at 60 Fahrenheit is the standard ordin- 

 arily used ; and if a mineral weighs twice as much as 

 water, its specific gravity is 2 : if three times, 3 ; and is 

 thus written : Quartz, G. = 2, 7, which means that 

 Quartz is 2, 7 , times heavier than a . corresponding 

 bulk of water. Minerals are divided into four classes : 

 1. Earthy Minerals ; 2. Salts : 3. Carboniferous Min- 

 erals ; 4. Metallic Ores or Metals. 



