200 SCIENCE PROGRESS 



whilst the inner core is broken into cylindrical fragments with 

 conical ends separated by distinct cavities. 



It might be thought that this result would only be attained 

 when the whole of the metal had been converted into the hard 

 material but this is not the case. When a wire that is hard- 

 drawn as far as possible is examined, it is apparent that the 

 greater part is still composed of the original crystalline metal 

 but that the crystal grains have been reduced in size by 

 crushing and that each small grain is enclosed in a hard shell 

 of the modified material. Further slipping along cleavage 

 planes is hindered or prevented by this comparatively unyield- 

 ing, brittle casing. When a section of such a hardened rod 

 or wire is etched, the shell or casing is dissolved more readily 

 than the crystalline core, so that the structure becomes visible. 



One of the most characteristic properties of the hard modifica- 

 tion produced by strain is its power of flowing. Thus in the 

 hard-drawn wire it envelops the unchanged cores, filling the 

 intercrystalline spaces without a break. This property is 

 most conveniently studied in the surface films produced by 

 polishing. Whilst the grinding of a metal surface with emery 

 or similar abrasives is simply a process of cutting, innumerable 

 fine grooves being produced, the subsequent process of polishing 

 with alumina or rouge is of a totally different character. Dr. 

 Beilby has shown that even in the case of such brittle metals 

 as bismuth or antimony the surface layer flows like a viscous 

 liquid under such treatment. The grooves are partly smoothed 

 out by removal of the intervening matter and partly filled up 

 or bridged over. Etching removes the altered film ; scratches 

 which had merely been bridged over during polishing reappear 

 on etching. This reappearance of "latent" scratches has long 

 been familiar to those who have examined etched sections. 

 Measurements made on polished crystals of calcite by an in- 

 genious chemical method show that the thickness of the surface 

 layer of modified material is of the order of 500-1000 /x/i. A 

 pattern once developed in an alloy by etching maybe obliterated 

 by polishing, in which case the gradual disappearance of the 

 structure as the altered material flows into the hollows may 

 be followed with great ease. 



It is observations of this kind that have led to the con- 

 ception of the hardened modification of cold-worked and 

 polished metals as an undercooled liquid of high viscosity. 



