152 RIGIDITY. HARDNESS. 



force acts upon the body ; the resistance offered by the 



same body to forces which tend to elongate it until 



its particles separate, differs from that presented to 



forces which tend to bend it until it breaks : the former 



is the absolute rigidity, the latter the flexural rigidity of 



a body. Another kind of rigidity is termed the hardness 



of a body. Hardness signifies the resistance offered by 



a body when we attempt to cut or scratch it by another 



body. The finger easily enters a lump of moist clay; 



hence the clay is less hard, or softer than the finger. 



If the clay is dry, the finger can no longer penetrate into 



its substance: the clay is now harder than the finger. 



The relative hardness of bodies is usually compared by 



means of the following scale, in which the softest body 



is the first in the scale, while each succeeding substance 



is harder than the preceding one : 1. Talc (laminated 



variety); 2. Gypsum, or Kocksalt; 3. Calcspar; 4. 



Fluorspar; 5. Apatite; 6. Felspar, white crystalline 



(Orthoclase); 7. Quartz (Rock-crystal); 8. Topaz; 9. 



Sapphire (Corundum); 10. Diamond. 1 



Bodies alter their form more or less under the 

 action of an external force before they finally break 

 a fresh twig differs in this respect from a thin bar of dr} 

 wood; a piece of india-rubber or leather differs fron 

 hardened steel or a piece of slate, a hot bar of wrough 

 iron from a bar of glass: bodies are called flexible 

 tenacious, ductile, etc., to indicate that they permit grea 

 changes of form without breaking; while others whicl 

 are more apt to break are called brittle. 



1 In the above scale each mineral is scratched ly the one that follows \ 

 and scratches the one before it. The hardness of any mineral may be de 

 termined by reference to these types ; thus a piece of galena is scratched b 

 calcspar and scratches gypsum, hence its hardness is 2-5. 



