290 Mr Hood on some Peculiar Changes 



engines, we have yet another cause which may tend to increase the ef- 

 fect. The vaporization of water and the ejBfluence of steam have already 

 been stated to produce large quantities of negative electricity in the 

 bodies in contact with the vapour ; and Dr Ure has shewn^* that nega- 

 tive electricity, in all ordinary cases of crystallization, instantly deter- 

 mines the crystalline arrangement. This, of course, must affect a body 

 of iron in a different degree to that of ordinary cases of crystallization ; 

 but still we see that the effects of these various causes all tend in one di- 

 rection, producing a more rapid change in the internal structure of the 

 iron of the axle of a locomotive engine, than occurs in almost any other 

 case. 



Dr "VVollaston first pointed out that the forms in which native iron is 

 disposed to break, are those of the regular octahedron and tetrahedron, 

 or rhomboid, consisting of these forms combined. The tough and fibrous 

 character of wrought iron is entirely produced by art; and we see in 

 these changes that have been described, an effort at returning to the na- 

 tural and primal form ; the crystalline structure, in fact, being the natural 

 state of a large number of the metals ; and Sir Humphrey Davy has shevrn 

 that all those which are fusible by ordinary means assume the form of 

 regular crystals by slow cooling. 



The general conclusion to which these remarks lead us, appears, I 

 think, to leave no doubt that there is a constant tendency in wrought 

 iron, under certain circumstances, to return to the crystallized state ; but 

 that this crystallization is not Hccessarily dependent upon time for its de- 

 velopment, but is determined solely by other circumstances, of which the 

 principal is undoubtedly vibration. Heat, within certain limits, though 

 greatly assisting the rapidity of the change, is certainly not essential to 

 it ; but magnetism, induced either by percussion or otherwise, is an essen- 

 tial accompaniment of the phenomena attending the change. 



At a recent sitting of the Academy of Sciences at Paris, M. Bosquillon 

 made some remarks relative to the causes of the breaking of the axle 

 on the Veisailles railroad ; and he appears to consider that this crystal- 

 lization was the joint effect of time and vibration, or rather, that this 

 change only occurs after a certain period of time. From what has here 

 been said, it will be apparent that a fixed duration of time is not an es- 

 sential element in the operation ; that the change, under certain circum- 

 stances, may take place instantaneously; and that an axle may become 

 crystallized in an extremely short period of time, provided that vibrations 

 of sufficient force and magnitude be communicated to it. This circum- 

 stance would point out the necessity for j^reventing as much as possible 

 all jar and percussion on railway-axles. No doubt one of the great faults 

 of both engines and carriages of every description — but particularly the 

 latter — is their possessing far too much rigidity ; thus increasing the 



• Journal of Science, vol. r. p. 106. 



