142 tECTURE Xirf. 



regard to its new form, as it possessed in its original state; and when the 

 alteration of form has once commenced, those powers are neither increased 

 nor diminished by continuing the operation : the degree of flexure or torsion, 

 required for producing a further alteration, appears also to be little varied: 

 thus if the spider's web could at first be twisted only one half round, so as to 

 retain the jjower of returning to its original state, without any permanent 

 alteration of form, it would never acquire the power of returning more than 

 half a revolution, however it might be twisted. From a want of attention to 

 this consideration, a late respectable author has called in question, without 

 sufficient reason, tlie accuracy of Mr. Bennet's experiments. 



A variation of ductility, in any substance, does not appear to depend on 

 any change in the magnitude of the ultimate powers of cohesion and repul- 

 sion. Steel, whether perfectly hard, or of the softest tem])er, resists flexure 

 with equal force, when the deviations from the natural state are small: but 

 at a eeitain point the steel, if soft, begins to undergo an alteration of form ; 

 at another point it breaks if much hardened; but when the hardness is mo- 

 derate, it is capable of a much greater curvature without either permanent 

 alteration or fracture ; and this quality, which is valuable for the purposes of 

 springs, is called toughness, and is opposed to rigidity and brittleness on the 

 one side, and to ductility on the other. There may, however, be an ap- 

 parent difference in the stiffness of some substances in different states, arising 

 from the greater facility with which their dimensions are extended in one di- 

 rection while they are contracted in another: thus elastic gum appears to 

 possess a much greater degree of stiffness when its hardness is increased by 

 cold than when it is at a more elevated temperature ; but the change pro- 

 duced in this case by heat is not an increase of that ductility which facilitates 

 a permanent alteration of form, but rather of the toughness which allows a 

 temporary change of figure, continuing only while the force is applied. The 

 effect of forging and of wiredrawing tends to lessen the ductility of metals, 

 and to render them tough, and even rigid: so that in hammering copper and 

 brass, and in drawing wire, it is necessary to anneal the metals more than 

 once by fire, in order to restore their ductility, Avhich is lessened by the opera- 

 tion. The corrosion of the surface of a metal by an acid is also said to 

 render it brittle ; but it is not impossible that this apparent brittleness may 

 be occasioned by some irregularity in the action of. the acid. 



