34 ANNUAL OF SCIENTIFIC DISCOVEEY. 



especially. I refer to the supposed and popularly so-called granula- 

 tion of fibrous wrought iron. 



Although this subject has engaged my attention for a series of 

 years, and I have taken pains to obtain correct information, I yet 

 hesitate to express any decided opinions, that would cover the whole 

 field of investigation. The question at large I consider open yet. 

 This much only I believe to be settled, that good iron will undergo 

 no change in course of time, unless it is acted on by great heat, or 

 is under the influence of strong continuous vibrations under tension. 



As an exception to this last proposition, may be cited the case of 

 old anchors and chains, which, after being exposed on the ground or 

 in the ground a great length of time, had become considerably 

 rusted and reduced in strength. Aside from rusting, magnetic influ- 

 ences were supposed to have been at work in destroying the strength 

 of these irons. But it should be remarked, that none of these have 

 been sufficiently well examined to warrant sound conclusions. It 

 is true that the -earth forms a great magnet, whose magnetism is 

 maintained by the sun ; and that the magnetic condition of all 

 metals is more or less depending upon the great parent magnet. A 

 steel magnet that has lost its power or tension, when buried in the 

 earth, will be restored by its magnetic currents. But how far the 

 cohesion and elasticity of wrought iron may be affected by these 

 currents, we are yet ignorant of. When a bar of iron is drawn 

 apart by a tensile strain, the fractured ends are magnetically excited 

 and will attract iron filings, at the same time that they become 

 heated. Both phenomena, magnetism as well as heat, will always 

 accompany the forcible rupture of iron, as can be readily ascertained 

 by experiment. The same phenomena are also exhibited when iron 

 is hammered cold, the heat in this case being more apparent than 

 the magnetism. 



The cohesion and elasticity of wrought iron, although different 

 properties, appear to be closely related. In speaking of elasticity, 

 I mean the natural elasticity, and not what is produced by the forced 

 process of tempering. And here may be pointed out a marked phys- 

 ical difference between steel and iron. While the hardening or 

 tempering of steel can be carried to almost any degree, that of the 

 latter cannot. 



Whatever destroys or impairs the elasticity of iron or steel will also 

 affect its cohesion. And this fact has also a significant magnetic 

 bearing. Tempered or hardened steel possesses more tensile strength 

 than soft steel. Now, when tempered steel loses its hardness by an- 

 nealing, it assimilates nearer to soft iron in its relation to magnetism. 

 Red-hot iron is not attracted by a magnet, while a steel magnet en- 

 tirely loses its magnetic properties on being heated red-hot. Another 

 remarkable fact is, that artificial as well as natural magnets, when 

 overloaded, become weakened. And so does the cohesion and elas- 

 ticity of an iron or steel bar become weakened by overloading. 



The limit of elasticity, or of the recuperating force, as it might be 

 termed, of iron and steel is generally stated at one-third of their 

 ultimate strength. I am of the opinion that this is much oyer-esti- 

 mated for soft puddled irons, and under-estimated for good hammered 

 charcoal irons, and still more for steel. 



