INTRODUCTION. x^ii 



Frangihility^ or Tenacity, 



The following terms are employed to denote the relative degrees of tena- 

 city in minerals. 



1. Brittle : when the parts of a mineral separate In grains or powder on 

 attempting to cut it with a knife, as in Cale Spar. 



2. Sectile. This character is intermediate between brittle and malleable, 

 and Is used to denote when pieces may be cut ofi" with a knife without falling 

 to powder, although the mineral, nevertheless, admits of being pulverized 

 under the hammer. 



3. Malleable : when slices may be cut off and then flattened out under the 

 blows of a hammer, as is the case with native Gold and Native Silver. 



4. Flexible : when the mineral admits of being bent without breaking, and 

 retains the position given to it, as in Talc. 



5. Elastic : when, after being bent, the mineral flies back to its original 

 position on the removal of the force, as in Mica. 



Fracture. 



Minerals are said to possess three kinds of fracture, viz. : — 



1. Conchoidal, or Shelly: when the fractured surface displays curved 

 concavities bearing more or less resemblance to those in the inside of a 

 bivalve shell. Flint and glass are good examples of this kind of fracture. 



2. Even : when the fractured surtace is not rendered rough by the pre- 

 sence of any minute elevations or depressions. 



3. HacJily : when the elevations are sharp or jagged, as In broken iron. 

 The Cleavage of a mineral is altogether distinct from the Fracture^ with 



which it must, in no manner, be confounded. Cleavage denotes that a 

 mineral can be cleaved or divided mechanically in certain directions, yield- 

 ing smooth surfaces of fracture (called the cleavage-planes)^ parallel with the 

 faces or planes of the primary crystal. 



This may be effected by placing a knife or other sharp edge in a direction 

 parallel with the natural joints, and then giving it a smart blow with a hammer. 

 The minerals which yield to cleavage in one direction only, are said to have 

 a lamellar structure. 



Hardness. 



The manner of testing the hardness of a mineral is by scratching it with 

 one of those named in the following list ; or (which is preferable) by trying 

 each with a file, passing It three or four times, with a rather heavy pressures 

 over the mineral. 



The following scale of hardness, by Mohs, is that generally adopted :— 



1. Talc : the common laminated green variety. 



2. Gypsum: a crystallized variety. 



3. Cdlc Spar : a transparent variety. 



4. Fluor Spar : a crystalline variety. 



5. Apatite : a transparent variety. 



6. Felsvar ( Orihoclase) : white cleavable variety. 



