1919] on The Hardening of Steel 497 



resulting product was always a complex of the two varieties. In 

 using the expression "amorphous," Sir George Beilby was careful 

 to explain that amorphous meant non-crystalline, in the sense that 

 the molecules are not marshalled in crystalline order and orientation, 

 while the addition of the qualifying term vitreous narrowed the field 

 to substances which in some degree resemble the glass-like form 

 assumed by the silicates on cooling from the molten state. Glass is 

 a typical amorphous substance, the useful qualities of which depend 

 entirely on the fact that it is in the non -crystalline state. As is well 

 known, it is a very hard substance. 



By careful experiments Sir George Beilby was able to show that 

 the amorphous constituent formed by cold work is not thermally 

 stable, but that on annealing it reverts at a given temperature to the 

 crystalline state. Well-marked differences of properties, according to 

 whether the metal was wholly crystalhne or a complex of vitreous- 

 amorphous and crystalline units, were established. Thus, the crystal- 

 line phase of a given metal was shown to be thermally stable but 

 mechanically unstable, whereas the amorphous phase was mechani- 

 cally stable and thermally unstable. 



Subsequently, in the discussion on a paper by Professor Edwards 

 and myself," Sir George Beilby explained why in his view the 

 vitreous-amorphous was harder than the crystalline phase. By hard- 

 ness was meant that the metal was more or less rigid, and that it had 

 the power to resist larger or smaller deforming stresses without 

 suffering permanent deformation. He proceeded to consider how 

 this rigidity might be affected by the molecular structure of the 

 metal. " One fundamental property of individual molecules was 

 that they were space-filling entities which resisted the encroachment 

 into their domain of other molecules ; their bounding surfaces were 

 probably kinetic, but they were none the less sharply defined. 

 Another fundamental property of molecules was that they attracted 

 each other strongly ; that mutual attraction gave rise to cohesion, 

 the quality in virtue of which the physical would continue to hold 

 together and exist. In a mass of metal or in any other solid aggre- 

 gate, there was a certain available amount of cohesion which was the 

 product of the specific cohesion of each molecule and the number of 

 molecules present ; the rigidity of any aggregate would be determined 

 by the use to which the available cohesion was put in the actual 

 structure. Taking the simplest of all possible forms for the mole- 

 cular unit, the sphere, it was known that there were various ways in 

 which the spheres might be built up into a homogeneous structure. 

 In the most densely packed form, each sphere within the mass was 

 in contact with twelve others ; in that form there was the condition 

 of greatest rigidity, the molecular centres of attraction being as close 

 to each other as was possible in any unstrained structure. But close 



* Journal of the Iron and Steel Institute, No. 1, 1914, pp. 178-180. 



