186 
MR. A. A. GRIFFITH ON 
fibres consists mainly of two parts, one proportional to the area, and the other to the 
perimeter of the cross-section. The process of drawing, too, might predispose the 
molecules to take up positions with their maxima of attraction parallel to the surface. 
If a perfectly clean glass plate be covered with gelatine and set aside, the gelatine 
gradually contracts, and as it does so it tears from the glass surface thin flakes up to 
about 0-06-inch diameter and shaped like oyster shells.* This tendency to flake at 
the surface is also observed when glass is broken by bending. This was particularly 
well shown in the specially prepared fibres used for the experiments described in the 
present paper. In almost all cases of flexural fracture the crack curled round on 
approaching the compression side, till it was nearly parallel to the surface. On two 
occasions the fracture divided before changing direction, the two branches going 
opposite ways along the fibre and a flake of length several times the diameter of the 
fibre was detached. 
Surface flaking is also observed when some kinds of steel are subjected to repeated 
stress. Here the flakes are usually very small. 
All these facts are evidently in complete agreement with the “ surface layer ” theory 
and, indeed, it is difficult to account for them on any other basis. 
8. Extended Application of the Molecular Orientation Theory. 
On the basis of the present theory, the physical properties of materials must be 
intimately related to the geometrical properties of the molecular sheet-formation. In 
order that a substance may exhibit the characteristic properties of crystals, it is clearly 
necessary for the sheets of molecules to be plane. In this case the crystals are, of 
course, the molecule groups or “ units ” referred to above. In “ amorphous ” materials, 
on the other hand, the sheets are probably curved.f 
In materials of the former type, there must exist planes on which, if they are subjected 
to a sufficiently large shearing stress, the portions on either side of the planes can undergo 
a mutual sliding through a distance equal to any integral multiple of the molecular 
spacing, without fundamentally affecting the structure of the crystal. It is well known 
that the phenomenon of yield in crystals, and especially in metals, is of this nature. 
The planes in question are, of course, the well-known “ gliding planes,” and it is further 
possible that they may be identified also with the •surfaces of least attraction. The 
stress at which gliding occurs in a single crystal must be determined in the following 
manner. The molecules of a crystal are normally in a configuration of stable equilibrium, 
and if two parts of the crystal slide on a gliding plane through one molecular space the 
resulting configuration is also stable. Between these two positions there must, in 
general, be one of higher potential energy, in which the equilibrium is unstable, and the 
shearing stress is determined by the condition that the rate at which work is done, in 
* Lord Rayleigh, ‘ Engineering,’ 1917, vol. 103, p. Ill, and H. E. Head, ‘ Engineering,’ 1917, vol. 103, 
p. 138. 
f See Quincke, ‘ Ann. der Physik,’ (4), 46, 1915, p. 1025. 
