244 Mr. Mallet on the Physical Conditions 



22. — Molecular Constitution of Wr ought-Iron^ and the Law of Direction of its 



Crystals or Fibre. 



193. When wrought-iron in any of the usual forms of its manufacture is frac- 

 tured, its molecular structiu'e presents itself, more or less distinctly pronounced, 

 in one or other of three forms : — 



1°. Its mass consists of minute crystals of nearlyuniform size, whose facets 

 present themselves at all possible angles, like that of refined 

 sugar. 



This " saccharoid" structure usually belongs to the most highly 

 refined iron, and often to hard steely irons, such as those of Swe- 

 den. 



The larger bars of Low Moor iron present, perhaps, the finest 

 examples of this structure. 

 2°. The surface of fracture consists of large, sometimes very large, lamel- 

 lar spangles or plates, the facets of crystalline cleavage, whose 

 directions tend to general coincidence with the surface of frac- 

 ture. The number, size, and direction of these facets vary in the 

 same mass with the direction of fracture. This is the structure 

 of all large and heavy forgings, or very large rolled bars, in which 

 , the planes of crystallization tend towards a general perpendicu- 



larity to the surfaces of external contour. This and the former 

 structure are often found irregularly imited in the same surface 

 of fracture in ill-manufactured iron, and, united with the suc- 

 ceeding, it is the usual one presented by small common bar-iron. 

 3'. The fracture (hard to produce, owing to the greater flexibility of the 

 iron than in either of the preceding cases) when efiected, pre- 

 sents long, parallel fibre, or bacillary crystals, running in the 

 direction of the longest dimension of the bar. This is the structure 

 of the best and toughest iron, such as that for making chains and 

 rivets, good boiler-plates, &c. It is found partially combined 

 with the 1st and 2nd in some inferior irons. 

 194. We found in cast-iron that the law of arrangement of its crystals is to 

 place themselves perpendicularly to the surfaces of the mass. In wrought-iron 



