INSIDES OF METALS — ZAPFFE 261 



However, particular attention is called here to other matters. First 

 is the fact that the outstanding markings are at exactly 90° to one 

 another. This is because iron fractures on a crystal plane that can 

 be described as the face of a cube. Just as the hexagonal-rhombohedral 

 bismuth crystal in plate 1, figure 2, displayed equilateral triangles, so 

 the cubic iron shows squares and rectangles. Here intersecting 

 cleavages provide the cubic symmetry, rather than twins. Even the 

 meandering markings will break down on close observation to show 

 themselves as minute stepwise composites of 90° markings. The 

 whole pattern, and particularly these tiny stepwise markings, give 

 strong expression to an elaborate architecture existing within a single 

 grain ; and they certainly stand as impressive evidence in the favor of 

 a general micellar theory. Little wonder that Haiiy hypothecated his 

 "molecules integrantes." The grain is visually composed of tiny sub- 

 grains, or micelles, and without them it would be difficult to explain 

 the pattern. 



A NEW ERA OF ENGINEERING MATERIALS? 



Many things must be left unsaid in a brief review of so vast a sub- 

 ject ; but one particular issue follows from all this work which holds 

 extravagant promise for future developments in engineering and hence 

 in civilization itself. This is the fact that calculations, using many 

 different approaches, all agree that the atoms of metals actually cohere 

 with strengths of the order of several millions of pounds per square 

 inch. Today the greatest achievement in engineering materials is 

 of the order of two or three hundred thousand pounds per square inch. 

 The reason that the observed strengths of materials are so vastly 

 inferior to the theoretical atomic cohesion is generally agreed to be 

 the subdivisional structure within the grain. The only thing not yet 

 agreed upon is the nature and the origin of that substructure. The 

 type of microscopic study here described greatly increases the infor- 

 mation on this prize problem of solid-state physics. 



For a better understanding of the problem, the micellar theory has 

 been offered. Right or wrong, the solution is certainly nearer; and, 

 when the answer is found, it will bring with it a definite possibility 

 of utilizing a new order of cohesive forces in developing the full 

 theoretical strength of metals and perhaps other engineering materials. 



