1900.] Professor J. A. Ewing on The Structure of Metals. 419 



WEEKLY EVENING MEETING, 



Friday, May 18, 1900. 



Ludwig Mond, Esq., Ph.D. F.R.S., Vice-President, 

 in the Chair. 



Professor J. A. Ewing, M.A. F.R.S. M.Inst.C.E. 



Tlie Structure of Metals. 



Much information has been obtained regarding the structure of 

 metals by the methods of microscopic examination initiated by Sorby, 

 and successfully pursued by Andrews, Arnold, Martens, Osmond, 

 Eoberts-Austen, Stead and others. When a highly polished surface 

 of metal is lightly etched and examined under the microscope, 

 it reveals a structure which shows that the metal is made up, in 

 general, of irregularly shaped grains with well denned bounding 

 surfaces. The exposed face of each grain has been found to consist 

 of a multitude of crystal facets with a definite orientation. Seen 

 under oblique illumination, these facets exhibit themselves by 

 reflecting light in a uniform manner over each single grain, but in 

 very various manners over different grains, and when the angle of 

 incidence of the light is changed one or another grain is seen to flash 

 out with a uniform brightness over its whole surface, while other 

 grains which were bright before become dark. 



These grains are deformed when the metal is severely strained in 

 any way, such as by stretching in the testing machine, or cold-rolling, 

 or wire-drawing. On polishing and etching a strained piece, the 

 grains are found to be on the whole longer in the direction in which 

 the metal has been extended. But when the piece is sufficiently 

 heated, a reformation of structure occurs, and the grains are found to 

 have assumed forms in which there is no direction of predominating 

 length. This process of recrystallisation is what gives rise to the 

 change in mechanical quality associated with annealing. 



The grains are apparently produced in the first instance, while 

 the metal is solidifying from a liquid state, by crystallisation pro- 

 ceeding more or less simultaneously from many different centres or 

 nuclei. The irregular boundaries between one grain and another 

 are due to the casual meeting of these various crystal growths. Each 

 grain is, in fact, a crystal, with all its elementary parts oriented in 

 one way, but its boundaries give no indication of its crystalline 

 character. When polished and etched the true crystalline character 

 of each grain is demonstrated in several ways, notably by the 



