STRUCTURE OF METALS 57 



detailed knowledge of the changes and transforma- 

 tions which these materials undergo as the result 

 of heating and cooling or of mechanical treatment. 

 While we cannot, obviously, follow out the details 

 of the great mass of new knowledge thus obtained, 

 there are a few of the wider generalisations which 

 can, perhaps, be sufficiently explained to afford an 

 insight into the inter-action of the new knowledge 

 with technical practice. 



When a piece of a pure metal, such as copper, 

 gold or silver, has been properly prepared by 

 polishing and etching and is then examined under 

 the microscope, a very characteristic "pattern" or 

 structure is seen a structure which is extremely 

 similar in all pure metals. The surface of the metal 

 is seen to be mapped out into a number of roughly 

 polygonal areas which appear to be divided from 

 one another by fine black lines; the surface has 

 the appearance of a fine mosaic pavement consisting 

 of somewhat irregular pieces very beautifully fitted 

 together. The sizes of the individual grains of this 

 mosaic vary widely according to the rate at which 

 the particular piece of metal has been cooled from 

 fusion; thus in lead, when slowly cooled, they 

 may be large enough to be readily seen with the 

 unaided eye, while in rapidly-cooled iron they 

 may be so small that two hundred of them placed 

 side by side would scarcely measure an inch. It 

 has been shown, however, that each of these grains 

 for the polygonal areas seen on a prepared surface 



