GOLD IN SCIENCE AND IN INDUSTRY. 223 



tility Avhich occurs when the overstrain has reached a point at which 

 the crystalline grains are still, to all appearance, only slightly de- 

 formed. At this stage there is no obvious reason why the slipping 

 of the crystalline lanielUv should not continue under the stresses 

 which have initiated it. But far from this being the case, a rela- 

 tively great increase of stress produces little or no further yielding 

 till the breaking point is reached and rupture takes j^lace. 



The study of the surface etfects of polishing, already referred to, 

 had shown that the thin surface film -retained no trace of crystalline 

 structure, while it also gave the clearest indications that the metal 

 had passed through a liquid condition before settling into the forms 

 prescribed by surface tension. From this it was argued that the 

 conditions which prevail at the outer surface might equally prevail 

 at all inner surfaces where movement had occurred, so that every 

 slip of one crystalline lamella over another would cause a thin film of 

 the metal to pass through the liquid phase to a new and noncrystal- 

 line condition. By observations on the elTects of beating pure gold 

 foil it was found that the metal reached its hardest and least plas- 

 tic condition only when all outward traces of crystalline structure 

 had disappeared. It was also ascertained that this complete destruc- 

 tion of the crystalline lamella^ and units could only be accomplished 

 in the layers near the surface, for the hardened substance produced 

 by the flowing under the hammer appears to incase and protect the 

 crystalline units after they become broken down to a certain size. 

 By carefully etching the surface in stages by means of chlorine 

 water or cold a(|ua regia the successive layers below the surface 

 were disclosed. The surface itself was vitreous. Beneath this was 

 a layer of minute granules, and lower still the distorted and broken- 

 up remains of cr^'stalline lamella? and grains were embedded in a 

 vitreous and granular matrix. The vitreous-looking surface layer 

 represents the final stage in the passage from soft to hard, from 

 crystalline to amorphous. By heating the beaten foil its softness 

 was restored, and on etching the annealed metal it was found that 

 ihe crystalline structure also was fully restored. Photomicrographs 

 showing these appearances are exhibited. These microscopic ob- 

 servations w^ere fully confirmed by finding well-marked thermo- 

 electrical and electro-chemical distinctions between the two forms 

 of metal, the hard and soft or the amorphous and the crystalline. 

 The determination of a definite transition temperature at wdiich the 

 amorphous metal passes into the crystalline metal further confirms 

 the phase view of hardening by overstrain and softening by an- 

 nealing. 



It was subsequently proved that tlie property of passing from crys- 

 fallhie to (imorj>]u>iis hy mechanical fow and from, amorphous to 

 crystaUhw hy heat at a dcfnite transition temperatuTe is a general 

 one which is possessed I>y all crystalline solids ivhich do not decom- 



