involved in the Construction of Artillery. 257 



220. The subject is one requiring, for its being completely understood, a 

 very cautious and difficult research, but one worthy of being at once made. 

 The following conclusions from existing knowledge may, however, be provi- 

 sionally offered as probably not far from correct : — 



r. There seems no reason to believe that any moderate extension or 

 compression, and, therefore, no moderate flexures, however long 

 continued or often repeated, produce any molecular change what- 

 ever in wrought-iron, provided that — 



a. The range of extension or compression be far within the elastic 



limits. 



b. That the velocity with which the extension or compression is made 



be not extremely great, i. e. not approaching to or beyond the 

 " pulse period," due to the elasticity of the material (134). 

 2". Nor any reason to suppose that jarring or vibration, unless accom- 

 panied by some permanent change of form in the mass, is capable 

 of affecting any molecular change whatever, provided that the 

 material shall have been previously in a state of molecular repose, 

 i. e. free from internal strains, due to form, contraction in coolins, 

 &c. 



Nor is it probable that abrasion alone, such as the grinding 

 away of the bearings of railway axles, or the scoring and rifling 

 of the chase of a wrought-iron gun by the passage of the shot, 

 produces any molecular change, but — 

 3'. It does appear certain from many well-observed phenomena, that in- 

 stantaneous changes of molecular structure, and reversals or trans- 

 position of the crystalline axes, can be produced in wrought-iron 

 at ordinary temperature, by the violent application of mechanical . 

 force, producing suddenly change of form at one or more points 

 of the surface of the mass, provided the directions of the force 

 and the extent of change of form, be such as to produce internal 

 strains and inequalities of pressure, and that the extent of these 

 latter is greater in any one direction than the resistances due to the 

 elasticity and its range in the material. 



