248 Mr. Mallet on the Physical Conditions 



susceptible of development in size. In the case of very highly refined iron (in 

 the language of the iron-master, " over- wrought" iron, in which there has been 

 no "cinder" left), with all its carbon perfectly combined, and thus approaching 

 to steel, the crystals are so minute, often so perfectly iflicroscopic, that in large 

 bars no other than the uniform " saccharoid" structure is discernible, though 

 the "fibrous" becomes perfectly developed in very small ones. This is the 

 case with the fine Low Moor ii'on, which, in rolled bars of 2^ inches in dia- 

 meter and upwards, presents a fracture almost identical with that of cast-steel, 

 but in rivet rods, a fine fibrous one. 



202. I have used the term " fibre" as being already long in use, and con- 

 veying well the character of this particular form of crystallization to the eye ; 

 but it should be clearly understood that the " fibre" of the toughest and best 

 iron is nothing more than the crystalline arrangement of inorganic matter, and 

 that the false analogies continually used, in which such fibre is spoken of and 

 reasoned upon, as if identical with that of organic bodies, such as wood, hemp, 

 &c., have no reality or basis in nature, and only tend to mislead (Note E). 



The principles upon which the development in size of individual crystal 

 depends, however, will be best understood when we have considered the — 



23. — Effects on Wroughi-Iron of Forging into great Masses. 



203. In rolled bars, which we have hitherto treated of, the pressure of the 

 rolls unaccompanied by impact, though conveyed only to the one point of the bar 

 at a time, is in succession, and with great uniformity, applied to every part of 

 its length. Moreover, the intensity of the pressure upon the unit of siu'face, 

 or in relation to the section of the bar, steadily increases as the latter diminishes 

 in size or cross section at each successive passage through the rolls. 



204. A very different set of conditions occur, however, in a forged bar or 

 mass. The whole of the pressures now are due to impacts, suddenly applied to local 

 points of the surface, and thence unequally transmitted through the ductile, or par- 

 tially ductile heated metal to its interior. The pressure at the siu'face, due to any 

 blow measured for the time of the hammer's descent through the sjjace through 

 which the surface before the blow has descended, is rapidly lost in transmission 

 within the mass, by inertia, and by the corpuscular forces of whatever sort that 

 the substance of the heated iron opposes to change of form. Blow follows blow 



