238 DISCOURSE ON THE STUDY 



then, we have an instance of two kinds of elas- 

 ticity in one substance ; a feebler effort of recovery 

 from distorted figure, and a more violent one from 

 a state of altered dimension. Both, however, ori- 

 ginate in the same causes, and are referable to the 

 same principles ; the former being in fact only a 

 modified case of the latter, as the effort of a steel 

 spring, when bent, to recover its former shape, is 

 referable to the same forces which give to steel its 

 hardness and strength to resist actual compression 

 and fracture. 



(259.) The toughness of a solid, or that quality 

 by which it will endure heavy blows without break- 

 ing, is again distinct from hardness though often 

 confounded with it. It consists in a certain yielding 

 of parts with a powerful general cohesion, and is 

 compatible with various degrees of elasticity. Mal- 

 leability is again another quality of solids, especially 

 metals, quite distinct from toughness, and depends 

 on their capability of being deprived of their figure 

 without an effort to recover it and without fracture. 



(260.) Tenacity, again, is a property of solids more 

 directly depending on the cohesion of their parts 

 than toughness. It consists in their power of resist- 

 ing separation by a strain steadily applied, while 

 the quality of toughness is materially influenced by 

 their disposition to communicate through their sub- 

 stance the jarring effect of a blow. Accordingly, 

 the tenacity of a solid is a direct measure of the 

 cohesive attraction of its parts, and is the best 

 proof of the existence of such a power. 



