288 Mr Hood on some Peculiar Changes 



This change, therefore, has been produced by percussion (as the primary 

 anient), when the bar is at a lower temperature than a welding heat. 



We here see the effects of percussion in a very instructive form. And 

 it must be observed that it is not the excess of hammering which produces 

 the effect, but the absence of a sufficient degree of heat at the time the 

 hammering takes place ; and the evil may probably be all produced by 

 four or five blows of the hammer, if the bar happens to be of a small size. 

 In this case we witness the combined effects of percussion, heat, and mag- 

 netism. When the bar is hammered at the proper temperature, no such 

 crystallization takes place, because the bar is insensible to magnetism. 

 But as soon as the bar becomes of that lower degree of temperature at 

 which it can be affected by magnetism, the effect of the blows it receives 

 is to produce magnetic induction, and that magnetic induction and conse- 

 quent polarity of its particles, when assisted by further vibrations from 

 additional percussion, produces a crystallized texture. For it is perfectly 

 well known that in soft iron, magnetism can be almost instantaneously 

 produced by percussion, and it is probable that the higher the tempera- 

 ture of the bar at the time it receives the magnetism, the more likely will 

 it be to allow of that re-arrangement of its molecules which would con- 

 stitute the crystallization of the iron. 



It is not difficult to produce the same effects by repeated blows from a 

 hand-hammer on small bars of iron ; but it appears to depend upon some- 

 thing peculiar in the blow, which to produce the effect must occasion a 

 complete vibration among the particles in the neighbourhood of the part 

 which is struck. And it is remarkable that the effects of the blows in all 

 cases seem to be confined within certain limited distances of the spot which 

 receives the strokes. Mr Charles Manby has mentioned to me a circum- 

 stance which fully bears out this statement. In the machine used for 

 blowing air at the Beaufort Iron Works, the piston-rod of the blowing 

 cylinder, for a considerable time, had a very disagreeable jar in its motion, 

 the cause of which could not be discovered. At last the piston-rod broke 

 off quite short, and close to the piston; and it was then discovered that 

 the key had not properly fastened the piston and the rod together. The 

 rod at the fracture presented a very crj^stallizcd texture ; and as it was 

 known to have been made from the very best iron, it excited considerable 

 surprise. The rod was then cut at a short distance from the fracture, 

 and it was found to be tough and fibrous in a very high degree ; shewing 

 what I have already pointed out, that the effects of percussion generally 

 extend only a very short distance. In fact, we might naturally expect, 

 that, as the effect of vibration diminishes in proportion to the distance from 

 the stroke which produces it, so the crystallization, if produced by this 

 means, would also diminish in the same proportion. The eflect of mag- 

 netism alone may also be estimated from this circumstance. The rod 

 would of course be magnetic throughout its whole length ; this being a 

 necessary consequence of its position, independent of other circum- 



