1S4 Mr. C. Hood on Changes in the Structure of Iron 



some of the most refractory substances ; but by themselves 

 they are unable to produce these effects on iron» or at least 

 the operation must be extremely slow. 



Another circumstance which occurred under Mr. Manby's 

 observation, confirms generally the preceding opinions, A 

 small bar of good tough iron was suspended and struck con- 

 tinuously with small hand-hammers, to keep up a constant 

 vibration. The bar, after the experiment had been continued 

 for some considerable time, became so extremely brittle, that 

 it entirely fell to pieces under the light blows of the hand- 

 hammers, presenting throughout its structure a highly cry- 

 stallized appearance. 



The fracture of the axles of road vehicles of all kinds is 

 another instance of the same kind. I have at different times 

 examined many broken axles of common road vehicles, and 

 I never met with one which did not present a crystallized 

 fracture, while it is almost certain that this could not have 

 been the original character of the iron, as they have fre- 

 quently been used for years with much heavier loads, and at 

 last have broken without any apparent cause, with lighter 

 burdens and less strain than they have formerly borne. The 

 effects, however, on the axles of road vehicles are generally 

 extremely slow, arising, I apprehend, from the fact that, al- 

 though they receive a great amount of vibration, they possess 

 a very small amount of magnetism, and are not subject to a 

 high temperature. The degree of magnetism they receive 

 must be extremely small, from their position and their con- 

 stant change with regard to the magnetic meridian the abs- 

 ence of rotation, and their insulation by the wood spokes of 

 the wheels. Whether the effects are equally slow with iron 

 wheels used on common roads, may perhaps admit of some 

 question. 



With railway axles, however, the case is very different. 

 In every instance of a fractured railway axle, the iron has 

 presented the same crystallized appearance ; but this effect, 

 I think, we shall find is likely to be produced far more ra- 

 pidly than we might at first expect, as these axles are subject 

 to other influences, which, if the theory here stated be correct, 

 must greatly diminish the time required to produce the change 

 in some other cases. Unlike other axles, those used on rail- 

 ways rotate with the wheels, and consequently must become 

 during their rotation highly magnetic. Messrs. Barlow and 

 Christie were the first to demonstrate the magnetism by ro- 

 tation produced in iron, which was afterwards extended by 

 Messrs. Herschel and Babbage to other metals generally, in 

 verifying some experiments by M. Arago. It cannot, I think, 



