132 Mr. C. Hood on Changes in the Structure of Iron 



ing iron to a very high temperature, it undergoes a change in 

 its electric or magnetic condition ; for at very high tempe- 

 ratures 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 assistance 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 ; 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 conse- 

 quence ; but the effects of percussion are at once various, ex- 

 tensive, and of high importance. We shall trace these effects 

 under several different circumstances. 



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 unevenness 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 ham- 

 mer on that part which he first operated upon. That part of 

 the bar has, however, by this time become comparatively 

 cold ; and if this cooling process has proceeded too far when 

 it receives this additional hammering, that part of the bar im- 

 mediately 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. This change, therefore, has been pro- 

 duced by percussion (as the primary agent), 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 pi'oduces 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 percus- 

 sion, heat, and magnetism. 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 re- 

 * Davy's Chemical Philosophy, p. 138. 



