THE STRUCTURE OF METALS 209 



Prof. Tammann has recently applied this hypothesis in 

 detail to the explanation of the properties of hardened metals. 

 He rejects the assumption of an unstable amorphous state 

 on several grounds, of which the principal are the absence of 

 any permanent alteration in a metal when the pressure applied 

 is equal in all directions and the fact that cold-working generally 

 produces a slight diminution of density, whilst the application 

 of an increased pressure might be expected to lead to the 

 formation of a denser rather than of a lighter modification. 

 The diminution of density is attributed to the formation of 

 minute gaps between different lamellae when the amount of 

 slipping and shearing becomes large. It is supposed that the 

 energy expended in causing slipping is stored in the crystal 

 and that thin lamellae are constantly tending to reunite to form 

 larger crystals ; the greater energy-content of the hardened 

 metal and its tendency to return to the normal condition of 

 coarse crystallisation are due to the same cause. 



The diminished electrical conductivity of a metal which has 

 been hardened by drawing into wire is readily explained on 

 the hypothesis of an amorphous modification. If this be 

 rejected, the diminution must be attributed partly to internal 

 rupture of the material and partly to the effect of orientation 

 by sliding, it being assumed that the conductivity of a metal 

 is greater in the direction perpendicular to the principal 

 cleavage than parallel to it. There are obvious difficulties in 

 the way of such an explanation but it is supported by experi- 

 ments which show that drawn wires recover their conductivity 

 on annealing though severely twisted wires are rendered 

 permanently worse conductors. In the first case the effect 

 is due to reorientation of the crystals and a rise of temperature, 

 by allowing freer play to the capillary forces, brings about 

 recrystallisation, whilst in the twisted wire reorientation can- 

 not account for the lessened conductivity, which must be due 

 to cracks and therefore does not disappear on annealing. The 

 argument does not appear to be conclusive and the writer 

 prefers the hypothesis of an amorphous material, especially 

 in consideration of the microscopical evidence from polished 

 and unpolished metals which is not discussed by Prof. 

 Tammann. 



The theory of thin lamellae in metals, the number of which 

 1 Zeitsch. ElektrocJmn. 1912, 18, 584. 



