Mr Hood on some Peculiar Changes y ^c. 287 



ordinary and most important changes occur, and shall point out some at 

 least of the modes by which we can demonstrate the truth of this asser- 

 tion by actual experiment. 



The principal causes which produce this change arc percussion, heat, 

 and magnetism ; and it is doubtful whether either of these means per se 

 will produce this effect ; and there appear strong reasons for supposing 

 that, generally, they are all in some degree concerned in the production 

 of the observed results. 



The most common exemplification of the effect of heat in crystallizing 

 fibrous iron, is by breaking a wrought iron furnace bar, which, whatever 

 quality it was of in the first instance, will, in a short time, invariably be 

 converted into crystallized iron ; and by heating, and rapidly cooling by 

 quenching with water a few times, any piece of wrought iron, the same 

 effect may be far more speedily produced. 



In these cases we have at least two of the above causes in operation — 

 heat and magnetism. In every instance of heating iron to a very high 

 temperature, it undergoes a change in its electric or magnetic condition ; 

 for, at very high temperatures, iron entirely loses its magnetic powers, which 

 return, as it gradually cools to a lower temperature. In the case of 

 quenching the heated iron with water, we have a still more decisive as- 

 sistance from the electric and magnetic forces ; for Sir Humphry Davy 

 long since pointed out* that all cases of vaporization produced negative 

 electricity in the bodies in contact with the vapour j a fact which has 

 lately excited a good deal of attention, in consequence of the discovery 

 of large quantities of negative electricity in effluent steam. 



These results, however, are practically of but little consequence ; but 

 the effects of percussion are at once various, extensive, and of high im- 

 portance. We shall trace these effects under several different circum- 

 stances. 



In the manufacture of some descriptions of hammered iron, the bar is 

 first rolled into shape, and then one-half the length of the bar is heated 

 in a furnace, and immediately taken to the tilt-hammer and hammered ; 

 and the other end of the bar is then heated and hammered in the same 

 manner. In order to avoid any unevcnness in the bar, or any difference 

 in its colour, where the two distinct operations have terminated, the 

 workman frequently gives the bar a few blows with the hammer on that 

 part which he first operated upon. That part of the bar has, however, 

 by this time become comparatively cold j and if this cooling process has 

 proceeded too far when it receives this additional hammering, that part of 

 the bar immediately becomes crystallized, and so extremely brittle that it 

 will break to pieces by merely throwing it on the ground, though all the 

 rest of the bar will exhibit the best and toughest quality imaginable. 



* Davy's Chemical Philosophy, p. 138. 



