ON THE CRYSTALLINE STRUCTURE OF MI-TAI.s. 



361 



use in electrical transformers. The following account of what we have observed may 

 be read as applying not only to iron but to other metals. Within the limit of 

 elasticity no effects of strain are detected, but when the yield point is reached H 

 remarkable change is seen on the surface of the crystalline grains. As soon as plastic 

 <li -formation logins the faces of the grains show fine black lines, and as the strain 

 increases these lines increase in number; they are more or less straight and parallel 

 in each grain, but are differently directed in different grains. The first lines to appear 

 are those approximately transverse to the pull, but as the strain increases systems of 



Fi. 11. 



inclined lines appear on other grains. With further straining some of the grains 

 begin to show more than one system of such lines, and eventually two, three, and 

 even four systems of intersecting lines on a single grain may l>e seen. 



A characteristic example of these lines as exhibited by iron is shown in fig. 12, 

 which is a photograph of a piece of Swedish iron strained by pull. Figs. 13 and 14 

 are two views of the same group of crystalline grains in another piece of Swedish 

 iron, fig. 13 being taken before straining, and fig. 14 after a considerable amount of 

 straining. When the piece is much strained the surface becomes so rough as to make 

 it difnViilt to M-riiK- n satisfactory photograph. 



\ol.. f\ III. A. 3 A 



