12 Lange, Some Remarkable Sled Crystals. 



crystallised in the regular system. They reduced ferric 

 chloride in a current of hydrogen at temperatures corre- 

 sponding to the different allotropic forms, and then 

 examined under the microscope the crystals thus obtained. 

 The results showed that " gamma "-iron occurs in all 

 combinations of the cube with the octahedron, and that 

 "beta"- and " alpha "-iron crj^stallise in cubes, and are 

 isomorphous. From what has already been said as to the 

 formation of the crystalline structure in solidifying steel 

 under mutual interference of the crystal growths, we shall 

 be prepared to find, on examining a polished and etched 

 section of normally cooled mild steel under the microscope, 

 granules possessing definite boundaries and without definite 

 geometrical form. We do, in fact, find that the structure of 

 such steel reveals a pattern of irregular polyhedra. These 

 polyhedrons are, however, differently affected by the acid 

 and reflect the Hght differently, thus showing that they each 

 possess a different orientation, and by carrying the etching 

 further it is frequently possible to see the perfect uni- 

 formity of orientation of the secondary crystals within 

 each of these polyhedrons. As the boundary lines of 

 these granules bear no relation to the internal structure of 

 the same, they are called allotrimorphic crystals as com- 

 pared with idiomorphic crystals, in which the external 

 form corresponds to the internal symmetry, as is the case 

 in the " pine-tree" crystals which I have just described. 



It is not within the scope of this paper for me to refer 

 to the crystalline structure of cast iron, or to the small 

 but perfectly formed crjstal structures frequently found 

 in cast iron cavities. The crystalline structures that 

 occur in iron rich in silicon or manganese have also 

 little direct bearing on our subject. 



I may, however, refer to the so-called " furnace- 

 crystals" or " bears " of carbonless, or almost carbonless 



