40 Master Minds of Modern Science 



arrangement of the molecules of which it is composed, and 

 examination by means of X-ray spectroscopy discloses 

 the arrangement of the molecules. When a steel-founder 

 produces a steel ingot he has changed the structure of the 

 iron as originally smelted by adding a certain proportion 

 of carbon atoms to the atoms of iron. Now the micro- 

 scope can show the existence of separate crystals in a 

 metal, but not the arrangement of atoms in a crystal. 

 That is where the X-ray comes in, and already it has 

 thrown a flood of light upon the inner meaning and 

 purpose of all the complex properties of metals. 



All through the centuries metal-workers have worked 

 by rule of thumb, experimenting more or less blindly, 

 occasionally with profit, but more often failing. Now with 

 the aid of the X-ray they are beginning to work with 

 some degree of certainty, and have already discovered 

 many interesting secrets. For instance, it is known that 

 the properties of metals depend on the variety of crystal- 

 line structure. Under pressure some sets of atoms in 

 crystals tend to slip over other sets. A fairly thick sheet 

 of aluminium, if composed of a single crystal, can be bent 

 in a man's hand, yet an ordinary piece of the same metal 

 is quite stiff. In this latter piece the crystals are pointing 

 in all directions, so that some are always ready to take the 

 strain. 



A metal usually becomes harder when beaten. This 

 has been known to metal-workers for thousands of years. 

 A bronze sword dug up in Shropshire had edges almost as 

 hard as steel, and it was found that the sword had been 

 hardened by beating while it was cold, being tempered 

 afterward to remove the brittleness. The man who made 

 that sword had no idea, of course, why beating hardened 

 his metal. It is the X-ray that has shown us how the 

 beating rearranges the crystals. 



Bronze was the first of the alloys. It is made of copper 

 and tin ; both are soft metals, yet a compound of the two 



