THE PHENOMENA OF RUPTURE AND FLOW IN SOLIDS. 
185 
It will be remembered that in the case of the freshly drawn fibres the reduction in 
tenacity required several hours for completion, so that the time taken was large 
compared with the time of cooling. Expressed in terms of molecular motion, this 
means that the molecules resist rotation very much more than they resist translation. 
This is in keeping with the conclusions of Debye,* who found that, on the basis of the 
quantum theory, the phenomena associated with the specific heat of solids could be 
explained only if the thermal vibrations of the molecules were regarded as practically 
irrotational. The same thing is shown more roughly, without introducing the quantum 
theory, by the law of Dtjlong and Petit, which requires that each molecule shall 
have only three degrees of freedom. 
The theory here put forward makes the spontaneous weakening a consequence of 
the attainment of a molecular configuration of stable equilibrium ; it therefore suggests 
that the weakening should be accompanied, in general, by a change in the dimensions 
of the solid. This has been verified by direct observation with a high-power microscope ; 
in the course of half an hour a spontaneously weakening glass fibre increased in length 
by about 0*1 per cent., while the length of a silica fibre decreased by about 0-03 per 
cent. 
On account of the random arrangement of the molecular groups, this spontaneous 
change in unstrained volume must set up internal stresses, which may be sufficiently 
large to start cracks along the directions of least strength. In this connection it may 
be mentioned that irregularly shaped pieces of glass, of which some parts had been 
put into the strong unstable state by heating, have sometimes been observed to break 
spontaneously about an hour after cooling was practically complete. 
It was remarked on p. 184 that cracks could not form spontaneously in a substance 
composed of molecules having spherical fields of force, as the process would involve 
an increase in potential energy. This is no longer true when the attraction is a function 
of orientation, as the surface energy of the cracks may be more than counterbalanced 
by the decrease in potential energy accompanying the molecular rearrangement. 
For this reason, it is impossible to deduce the ratio of the maximum to the minimum 
molecular attractions from the ratio of the maximum and minimum strengths of the 
material, as it is possible that the spontaneous weakening is always accompanied by 
the formation of minute cracks, of the same size as the molecular groups. 
It is probable that, in many cases, the most stable orientation of the molecules at 
a free surface is that in which their maxima of attraction lie along the surface. Such 
an orientation would in turn lead to a similar tendency on the part of the next layer 
of molecules, and so on, the tendency diminishing with increasing distance from the 
surface. There would therefore be a surface layer having the special property that in it 
the “ flaws ” ran parallel to the surface. 
Hence this layer would be of exceptional strength in the direction of the surface. 
This suggests a reason for the experimental fact that the breaking load of wires and 
* ‘ Ann. der Physik,’ 39 (1912), p. 789. 
