64 PHYSICAL PROPERTIES OF MINERALS 



the hardness is less on the basal plane of the prism, that is, on 

 the cleavage surface, than it is on the sides of the prism. On 

 the contrary, the terniinatkn of a crystal of cyanite is harder 

 than the lateral planes The degree of hardness in different 

 directions may be obtained with great accuracy by means of an 

 instrument called a sclerometer. 



2. TENACITY. 



The following rather indefinite terms are used with reference 

 to the qualities of tenacity, malleability, and flexibility in min- 

 erals : 



1. Brittle. — "When a mineral breaks easily, or when parts of 

 the mineral separate in powder on attempting to cut it. 



2. Malleable. — When slices may be cut off, and these slices 

 will flatten out under the hammer, as in native gold, silver, 

 copper. 



3. Sectile. — When thin slices maybe cut off with a knife. 

 All malleable minerals are sectile. Argentite and cerargyrite 

 are examples of sectile ores of silver. The former cuts nearly 

 like lead and the latter nearly like wax, which it resembles. 

 Minerals are imperfectly sectile when the pieces cut off pul- 

 verize easily under a hammer, or barely hold together, as sele- 

 nite. 



4. Flexible. — When the mineral will bend, and remain bent 

 after the bending force is removed. Example, talc. 



5. Elastic. — When, after being bent, it will spring back to 

 its original position. Example, mica. 



A liquid is said to be viscous when on pouring it the drops 

 lengthen and appear ropy. Example, petroleum. 



3. SPECIFIC GRAVITY. 



The specific gravity of a mineral is its weight compared with 

 that of some substance taken as a standard. For solids ar.d 

 liquids distilled water, at 60° F., is the standard ordinarily 

 used ; and if a mineral weighs twice as much as water, its spe- 

 cific gravity is 2 ; if three times it is three. It is then necessary 

 to compare the weight of the mineral with the weight of an 

 equal bulk of water. The process is as follows : 



First weigh a fragment of the mineral in the ordinary way, 



