THE STRUCTURE OF METALS 201 



This view is perfectly consistent with a high degree of brittle- 

 ness. Cobbler's wax is a typical example of a substance which 

 flows like a viscous liquid but yet is brittle under a suddenly 

 applied stress ; the combination of these two properties, at 

 first sight contradictory, is not uncommon. The amorphous, 

 isotropic character of the hard modification is fully in accordance 

 with such a view, which is further supported by considerations 

 of the following kind. 



A modification which stands to the ordinary crystallised 

 metal in the relation of an undercooled liquid must be unstable 

 at all temperatures below the melting point. At the ordinary 

 temperature it is related to the crystalline metal as glass is 

 to the mixture of crystallised silicates which is formed from 

 it when it devitrifies or as vitreous silica is to quartz. 

 It may thus be expected to show, relatively to the crystals 

 of the same metal, a lower density and a greater activity 

 towards solvents and to exhibit a tendency to crystallisation 

 whenever the circumstances are favourable. These expecta- 

 tions are fulfilled. A cold-worked metal is actually of some- 

 what lower density than one that is fully annealed, although 

 the difference is small, as is natural in view of the fact that 

 the conversion always remains incomplete. The greater sen- 

 sitiveness of hardened metals to attack by chemical agents 

 has already been mentioned and is confirmed by determinations 

 of electrolytic potential, which show that a highly worked 

 metal always becomes the anode when coupled in an electro- 

 lyte with a piece of the same metal in an annealed condition. 



The tendency to return to the crystalline form is also well 

 marked. The change takes place with extreme slowness at the 

 ordinary temperature but much more rapidly when the tempera- 

 ture is raised. At a certain point, termed the " crystallisation 

 temperature " by Dr. Beilby, the return takes place suddenly ; the 

 progress of annealing may be followed by means of tests of 

 elasticity or still more conveniently by determinations of the 

 thermo-electric difference between the specimen and one of fully 

 annealed metal. 



The tendency to recrystallise must be present in all cold- 

 worked metals even at atmospheric temperatures, although 

 greatly restrained by the internal viscosity. It usually becomes 

 evident, however, even under such unfavourable conditions, to 

 a sufficient extent to constitute a serious difficulty in technical 



