264 Mr. Mallet on the Physical Conditions 



solids, in contact with them at the moment of explosion, almost witli the facility 

 of rigid solids. Thus, a few grains of fulminating silver strike a hole through 

 a thick iron plate, or indent the face of a steel anvil on which they may be 

 exploded. Indeed, pursuing this consideration, we might calculate the velocity 

 of evolution of the gases of decomposition of such bodies, which has not yet 

 been done. Except, therefore, under tlie stroke of such formidable compounds, 

 we need never dread the fracture of any of the metals applied for ordnance, by 

 the velocity of impulse from the mass of an elastic gas only. The striking mass 

 requires to have the rigidity and weight of a solid body to produce fracture 

 at lower velocities. 



231. But in all these cases the character of the fracture is the same, whatever 

 be the velocity with which it is produced. The sealing-wax and the pitch, alike, 

 present a vitreous fracture, whether broken slowly, or shattered suddenly 

 against a rigid mass. The glass and the marble present their characteristic 

 fractures, whether broken by the most gradually applied push, or the sharpest 

 blow ; and so also for every class of unorganized bodies we are acquainted 

 with, except one, namely, that which embraces all bodies possessed of a 

 certain amount of rigidity and ductility united, in connexion with a crystalline 

 arrangement, or the power to assume it. 



This class is chieily confined to the metals, and amongst many of these we 

 find that the character of the fracture varies with the velocity of the blow. 



232. This alteration of fracture is due, then, to either of two causes, or to 

 both conjointly sometimes, viz., either to condensation and hardening, produced 

 by compression, or to crystallization, induced or altered at the moment by 

 compression ; and it is not improbable that every case of metallic hardening by 

 flexure, or by compression, or change of form, is only one of inceptive or incom- 

 plete crystallization, for the metals that crystallize least perfectly and readily, 

 and whose annealing temperature is extremely low (subsequent sections), such 

 as lead and tin, are those that are scarcely hardened at all by flexure, compres- 

 sion, or change of form. Thus, it is scarcely possible to break a piece of pure 

 lead by bending it backwards and forwards any number of times. 



233. To limit ourselves, however, to the case of wrought-iron, the rigidity 

 of which is readily and powerfully affected by compression and change of form. 

 When a thick plate is struck by a shot, its plane surface, for the instant 



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