300 TERMINOLOGY, . 208. 



tides of the mineral may change their relative situation, 

 .without absolutely losing their connexion. Ex. Several 

 metals, as hexahedral Gold and hexahedral Silver. 



5, Flexible, if the particles, whose relative situation has 

 been changed, do not resume their former situation. There 

 are flexible minerals, which are neither ductile nor malleable. 

 Ex. Several metals, hexahedral Silver-glance, and several 

 varieties of prismatic Talc-mica. 



6, Elastic, if the particles, whose relative situation has 

 been changed, resume their former situation. Ex. Several 

 varieties of rhombohedral Talc-mica, and of black Mineral- 

 resin. 



A fluid mineral is more particularly said to be 



1, Liquid, if in pouring it out from a vessel, perfect 

 round drops are formed. Ex. Water, several Acids, fluid 

 Mercury, and several varieties of black Mineral -resin. 



2, Viscid, if the drops are not round, but ropy. Ex. Se- 

 veral varieties of black Mineral-resin. 



Expansible minerals do not shew any further differences 

 in this respect. They comprehend the Gases and some 

 Acids. 



It is evident that all these properties are subject to small 

 variations, and that they pass into each other by insensible 

 gradations. 



. 208. HARDNESS. 



Hardness in general may be defined to be the 

 resistance of solid minerals to the displacement of 

 their particles. The magnitude of this resistance is 

 their Degree of Hardness. 



Hardness is a very useful property in the Natural His- 

 tory of the Mineral Kingdom, particularly so in its deter- 

 minative part. 



Nothing is attended with greater difficulties, than the 

 establishment of an accurate scale for the degrees of bard- 



