﻿25G Dr. J. A. Ewing on the 



in 



placing its first brickbat at random, and then piling the others 

 side by side with the first in geometrical regularity of orien- 

 tation, until the pile, or the branches it shoots out, meets the 

 advancing pile of a neighbour ; and so the structure goes on, 

 until the whole space is entirely filled by a solid mass con- 

 taining as many grains as there have been nuclei from which 

 the growth began. 



We now know that this process of crystal growth occurs 

 not only in the solidification of a metal from the liquid state, 

 but in many cases during cooling through a " critical " tempe- 

 rature when the metal is already solid. We know also that 

 the process may in certain conditions go on slowly at very 

 moderate temperatures. We know also that the process of 

 annealing is essentially the raising of the metal to a tempe- 

 rature at which recrystallization may take place, though the 

 metal remains solid while this internal rearrangement of its 

 particles goes on. Whether crystallization occurs in solidifying 

 from the liquid or during the cooling of an already solid 

 piece, it results in the formation of an aggregate of grains, 

 each one of which is a true crystal. Their size may be large 

 or small — in general, quick cooling means that crystallization 

 starts from many nuclei, and the resulting grains are conse- 

 quently small • with very slow cooling you get a gross 

 structure made up of grains of a much larger size. 



For simplicity of statement I shall ask you in what follows 

 to confine your attention to simple metals, omitting any re- 

 ference to alloys. Alloys present many complexities, into 

 which we need not at present enter. With simple metals 

 every crystalline grain is made of the same substance : the 

 elementary brickbats are all exactly alike, though there may 

 be the widest variation from grain to grain as regards the 

 form of the grain, and also as regards the direction in which 

 the elementary brickbats are piled. In any one grain they 

 are piled with perfect regularity, all facing one way, like a 

 regiment of perfectly similar soldiers formed up in rows, 

 where each man is equidistant from his neighbours, before 

 and behind, as well as to right and to left. Or perhaps I 

 might compare them to the well-drilled flowers of an early 

 Victorian wall-paper. 



It was shown by Mr. Rosenhain and myself * that when a 

 piece of metal is strained beyond its limit of elasticity, so that 

 permanent set is produced, the yielding takes place by means 

 of slips between one and another portion of each crystal grain. 

 A part of each crystal slides over another part of the same 



* Ewing and .Rosenhain, " The Crystalline Structure of Metals," 

 Bakerian Lecture, Phil. Trans. Hoy. Soc. vol. cxciii. A, 1899. 



