﻿Molecular Structure of Metals. 261 



block, or take such a form as a T or L. A sufficient disturb- 

 ance tends to resolve it into agreement with the normal tactics 

 of the molecules which build up the rest of the grain. 



++ 



Fisr. 3. 



«f+ 



t 



It is conjecturally possible that small groups of this kind, 

 possessing little stability, may be formed during the process 

 of crystallization, so that here and there in the grain we may 

 have a tiny patch of dissenters keeping one another in counte- 

 nance, but out of complete harmony with their environment. 



If this happens at all during crystallization, it would seem 

 less likely to happen in free crystallization from a liquid state 

 than in the more constrained process that occurs when a 

 metal already in the solid state crystallizes at a temperature 

 far below its melting-point. Though rare or absent in the 

 "first case, it might occur frequently in the second. There are 

 differences in the appearance of crystal grains under the 

 microscope in metal as cast and in metal as recrystallized in 

 the solid state, of which this may be the explanation. Tt may 

 also explain a difference pointed out by Rosenhain *, that the 

 slip lines in cast metal are straight and regular, whereas in 

 wrought iron and other metals which have recrystallized in the 

 solid state they rarely take a straight course across the crystal, 

 but proceed in jagged, irregular steps. These may be due to the 

 presence here and there of small planes of weakness, resulting 

 from the existence of what I have called dissenting groups. 

 Again, these groups, possessing, as they do, less stability than 

 their normal neighbours, may be conjectured to differ from 

 the normal parts of the grain in respect of electrolytic quality, 

 and to be more readily attacked by an etching reagent. 

 Hence, perhaps, the conspicuous isolated geometrical pits that 

 appear on etching a polished surface of wrought iron. 



It will help in making clear these points, and others that 



* W. Rosenhain, " The Plastic Yielding of Iron and Steel," Journal of 

 Iron and Steel Institute, Xo. 1 for 1904, p. 335. 



