﻿Molecular Structure of Metals. - 265 



showed that this action begins with nothing more or less than 

 slight slip on surfaces where the strain is locally sufficient to 

 exceed the limit of elasticity. An alternating stress, which 

 makes the surfaces slip backwards and forwards alter- 

 nately many thousands, or it may be millions of times, pro- 

 duces an effect which is seen on the polished surface as a 

 development of the slip-lines into actual cracks, and this soon 

 leads to rupture. 



We have, therefore, to look for an effect equivalent to an in- 

 terruption of continuity across part or the whole of a surface of 

 slip, an effect progressive in its character, becoming important 

 after a few rubbings to and fro if the movement is violent, 

 but only after very many rubbings if the movement is slight. 



That there is a progressive action which spreads more or 

 less into the substance of the grain on each side of the ori- 

 ginal surface of slip was clearly seen in the experiments re- 

 ferred to. It was found that a slip-band visible on the polished 

 surface of the piece broadened out from a sharply defined line 

 into a comparatively wide band with hazy edges, and this 

 w r as traced to an actual heaping up of material on each side 

 of the step which constituted the original line. 



I think this suggests that under alternating stresses which 

 cause repeated backward and forward slips, these do not occur 

 strictly on the same surface in the successive repetitions, and 

 hence the disturbance spreads to some extent laterally. It 

 may be conjectured that slip on any surface leaves a more 

 or less defective alignment of the molecular centres; that is 

 to say, the rows on one side of the plane of slip cease to lie 

 strictly in line with those on the other side. If this occurs 

 over neighbouring surfaces, as a result of slips on a number 

 of parallel planes very close together, the metal throughout 

 the affected region loses its strictly crystalline character, and 

 with it loses the cohesion which is due to strict alignment. 



Mr. G. T. Beilby, in a very suggestive paper *, has advanced 

 grounds for believing that portions of a metal may pass from 

 a crystalline to an amorphous formation under the mechanical 

 influence of severe strain, as in the hammering of gold-leaf or 

 the drawing of wire, and that this occurs in the polishing of a 

 metallic surface, and also in the internal rubbing which takes 

 place at a surface of slip within the grain. In both cases he 

 suggests the formation of an altered layer. When a polished 

 metal surface is etched, the altered layer is dissolved away, 

 and the normal structure below it is revealed. 



AVithout accepting all Mr. Beilby's conclusions. I think the 

 idea of an altered and more or less amorphous layer is sup- 

 ported by the considerations I am now putting forward. We 

 have assumed that in normal crystallization the intermolecular 

 forces lead to a normal piling, in which each molecule touches 

 * Beilby, " The Hard and Soft States in Metals," Phil. Mag. August 1904. 



